## 1. Introduction

In this blog, you will learn how to implement **logistic regression** with **TensorFlow** and apply it to a **classification** problem. Logistic regression is a simple and powerful machine learning technique that can be used to classify data into two or more classes based on some features. TensorFlow is an open-source framework for building and deploying machine learning models.

Logistic regression is based on the **sigmoid** function, which maps any real number to a value between 0 and 1. The sigmoid function can be used to model the probability of an event occurring, such as whether an email is spam or not, or whether a tumor is benign or malignant. By applying a threshold to the sigmoid function, we can obtain a binary prediction for the class label.

To train a logistic regression model, we need to define a cost function that measures how well the model fits the data. The cost function is usually the negative log-likelihood of the data given the model parameters. We also need to use an optimization algorithm, such as gradient descent, to find the optimal values of the model parameters that minimize the cost function.

In this blog, you will use TensorFlow to perform the following steps:

- Prepare the data for logistic regression.
- Define the logistic regression model using TensorFlow variables and placeholders.
- Train the model using TensorFlow graphs and sessions.
- Evaluate the model using accuracy and confusion matrix.

By the end of this blog, you will have a working logistic regression model that can classify images of handwritten digits from the MNIST dataset. You will also gain a better understanding of the theory and practice of logistic regression and TensorFlow.

Are you ready to get started? Let’s dive in!

## 2. Logistic Regression

In this section, you will learn the basic theory of **logistic regression**, which is a simple and powerful machine learning technique for **classification** problems. Logistic regression can be used to model the probability of an event occurring, such as whether an email is spam or not, or whether a tumor is benign or malignant. You will also learn how to use the **sigmoid** function, the cost function, and the gradient descent algorithm to train a logistic regression model.

What is logistic regression? Logistic regression is a type of linear regression that can handle binary dependent variables. A binary dependent variable is one that can take only two values, such as 0 or 1, yes or no, true or false, etc. For example, if we want to predict whether an email is spam or not, we can use a binary variable to represent the outcome, where 0 means not spam and 1 means spam.

How does logistic regression work? Logistic regression works by finding a linear relationship between the independent variables (the features) and the log-odds of the dependent variable (the outcome). The log-odds is the logarithm of the odds, which is the ratio of the probability of success to the probability of failure. For example, if the probability of an email being spam is 0.8, then the odds are 0.8 / 0.2 = 4, and the log-odds are log(4) = 1.39.

Why do we use the log-odds? We use the log-odds because it can take any real value, from negative infinity to positive infinity, while the probability can only take values between 0 and 1. This makes it easier to find a linear relationship between the log-odds and the features. For example, we can write the log-odds as a linear function of the features, such as:

log_odds = b0 + b1 * x1 + b2 * x2 + ... + bn * xn

where b0, b1, b2, …, bn are the coefficients of the logistic regression model, and x1, x2, …, xn are the features of the data.

How do we convert the log-odds to the probability? We use the **sigmoid** function, which is a special function that maps any real number to a value between 0 and 1. The sigmoid function is defined as:

sigmoid(x) = 1 / (1 + exp(-x))

where exp(x) is the exponential function, which is the inverse of the logarithm function. The sigmoid function has a characteristic S-shaped curve, as shown in the figure below.

By applying the sigmoid function to the log-odds, we can obtain the probability of the outcome, such as:

probability = sigmoid(log_odds) = sigmoid(b0 + b1 * x1 + b2 * x2 + ... + bn * xn)

The probability can then be used to make a binary prediction, by applying a threshold. For example, if the probability is greater than 0.5, we can predict the outcome as 1, otherwise we can predict it as 0.

How do we find the optimal values of the coefficients? We use a cost function that measures how well the logistic regression model fits the data. The cost function is usually the negative log-likelihood of the data given the model parameters, which is the negative of the logarithm of the likelihood. The likelihood is the probability of the data given the model parameters, which is the product of the probabilities of each observation given the model parameters. For example, if we have n observations, the likelihood is:

likelihood = probability_1 * probability_2 * ... * probability_n

The log-likelihood is then:

log_likelihood = log(probability_1) + log(probability_2) + ... + log(probability_n)

And the negative log-likelihood is:

negative_log_likelihood = -log(probability_1) - log(probability_2) - ... - log(probability_n)

The cost function is the average of the negative log-likelihood over all the observations, such as:

cost_function = negative_log_likelihood / n

The cost function can also be written as a function of the model parameters, such as:

cost_function(b0, b1, b2, ..., bn) = -log(probability_1(b0, b1, b2, ..., bn)) - log(probability_2(b0, b1, b2, ..., bn)) - ... - log(probability_n(b0, b1, b2, ..., bn)) / n

The goal of logistic regression is to find the optimal values of the model parameters that minimize the cost function. To do this, we use an optimization algorithm, such as **gradient descent**, which iteratively updates the model parameters in the direction of the steepest descent of the cost function. The gradient descent algorithm works as follows:

- Initialize the model parameters with some random values.
- Compute the cost function and the gradient of the cost function with respect to each model parameter.
- Update each model parameter by subtracting a fraction of the gradient from the current value.
- Repeat steps 2 and 3 until the cost function converges to a minimum value or a maximum number of iterations is reached.

The fraction of the gradient that is subtracted from the model parameters is called the learning rate, which controls how fast the model parameters are updated. A small learning rate can lead to slow convergence, while a large learning rate can lead to overshooting or divergence. The optimal learning rate depends on the data and the model, and it can be tuned using trial and error or other methods.

In summary, logistic regression is a machine learning technique that can be used to classify data into two or more classes based on some features. Logistic regression is based on the sigmoid function, which maps any real number to a value between 0 and 1. The sigmoid function can be used to model the probability of an event occurring, such as whether an email is spam or not, or whether a tumor is benign or malignant. By applying a threshold to the sigmoid function, we can obtain a binary prediction for the class label. To train a logistic regression model, we need to define a cost function that measures how well the model fits the data. The cost function is usually the negative log-likelihood of the data given the model parameters. We also need to use an optimization algorithm, such as gradient descent, to find the optimal values of the model parameters that minimize the cost function.

### 2.1. The Sigmoid Function

In this section, you will learn about the **sigmoid function**, which is a special function that maps any real number to a value between 0 and 1. The sigmoid function is used to model the probability of an event occurring, such as whether an email is spam or not, or whether a tumor is benign or malignant. You will also learn how to implement the sigmoid function in **TensorFlow**, which is an open-source framework for building and deploying machine learning models.

What is the sigmoid function? The sigmoid function is defined as:

sigmoid(x) = 1 / (1 + exp(-x))

where exp(x) is the exponential function, which is the inverse of the logarithm function. The sigmoid function has a characteristic S-shaped curve, as shown in the figure below.

Why do we use the sigmoid function? We use the sigmoid function because it can take any real value, from negative infinity to positive infinity, and map it to a value between 0 and 1. This makes it suitable for modeling the probability of an event occurring, such as whether an email is spam or not, or whether a tumor is benign or malignant. By applying a threshold to the sigmoid function, we can obtain a binary prediction for the class label. For example, if the sigmoid function returns a value greater than 0.5, we can predict the class label as 1, otherwise we can predict it as 0.

How do we implement the sigmoid function in TensorFlow? TensorFlow provides a built-in function for the sigmoid function, which is `tf.math.sigmoid`

. This function takes a tensor as an input and returns a tensor of the same shape and dtype as the input, with the sigmoid function applied element-wise. For example, if we have a tensor `x`

with the values `[1, 2, 3]`

, we can apply the sigmoid function to it as follows:

import tensorflow as tf x = tf.constant([1, 2, 3]) y = tf.math.sigmoid(x) print(y)

This will print:

tf.Tensor([0.7310586 0.88079703 0.95257413], shape=(3,), dtype=float32)

As you can see, the sigmoid function maps the values of `x`

to values between 0 and 1.

In summary, the sigmoid function is a special function that maps any real number to a value between 0 and 1. The sigmoid function is used to model the probability of an event occurring, such as whether an email is spam or not, or whether a tumor is benign or malignant. By applying a threshold to the sigmoid function, we can obtain a binary prediction for the class label. TensorFlow provides a built-in function for the sigmoid function, which is `tf.math.sigmoid`

, that takes a tensor as an input and returns a tensor of the same shape and dtype as the input, with the sigmoid function applied element-wise.

### 2.2. The Cost Function

In this section, you will learn about the **cost function**, which is a function that measures how well the **logistic regression** model fits the data. The cost function is usually the negative log-likelihood of the data given the model parameters, which is the negative of the logarithm of the likelihood. The likelihood is the probability of the data given the model parameters, which is the product of the probabilities of each observation given the model parameters. You will also learn how to implement the cost function in **TensorFlow**, which is an open-source framework for building and deploying machine learning models.

What is the cost function? The cost function is a function that quantifies the difference between the predicted values and the actual values of the dependent variable. The cost function is also known as the loss function or the error function. The goal of logistic regression is to find the optimal values of the model parameters that minimize the cost function. The cost function is usually the negative log-likelihood of the data given the model parameters, which is the negative of the logarithm of the likelihood. The likelihood is the probability of the data given the model parameters, which is the product of the probabilities of each observation given the model parameters. For example, if we have n observations, the likelihood is:

likelihood = probability_1 * probability_2 * ... * probability_n

The log-likelihood is then:

log_likelihood = log(probability_1) + log(probability_2) + ... + log(probability_n)

And the negative log-likelihood is:

negative_log_likelihood = -log(probability_1) - log(probability_2) - ... - log(probability_n)

The cost function is the average of the negative log-likelihood over all the observations, such as:

cost_function = negative_log_likelihood / n

The cost function can also be written as a function of the model parameters, such as:

cost_function(b0, b1, b2, ..., bn) = -log(probability_1(b0, b1, b2, ..., bn)) - log(probability_2(b0, b1, b2, ..., bn)) - ... - log(probability_n(b0, b1, b2, ..., bn)) / n

Why do we use the negative log-likelihood as the cost function? We use the negative log-likelihood as the cost function because it has some desirable properties for logistic regression. First, it is a convex function, which means that it has a single global minimum and no local minima. This makes it easier to find the optimal values of the model parameters using gradient descent or other optimization algorithms. Second, it is a differentiable function, which means that we can compute its gradient with respect to each model parameter. This is useful for applying the gradient descent algorithm, which requires the gradient of the cost function. Third, it is a probabilistic function, which means that it reflects the uncertainty of the data and the model. This is useful for evaluating the performance of the model and comparing it with other models.

How do we implement the cost function in TensorFlow? TensorFlow provides a built-in function for the negative log-likelihood, which is `tf.keras.losses.binary_crossentropy`

. This function takes two tensors as inputs: the true labels and the predicted probabilities. The true labels are the actual values of the dependent variable, which are either 0 or 1. The predicted probabilities are the values returned by the sigmoid function, which are between 0 and 1. The function returns a tensor of the same shape as the inputs, with the negative log-likelihood computed element-wise. For example, if we have a tensor `y_true`

with the values `[0, 1, 0]`

, and a tensor `y_pred`

with the values `[0.1, 0.9, 0.2]`

, we can compute the negative log-likelihood as follows:

import tensorflow as tf y_true = tf.constant([0, 1, 0]) y_pred = tf.constant([0.1, 0.9, 0.2]) nll = tf.keras.losses.binary_crossentropy(y_true, y_pred) print(nll)

This will print:

tf.Tensor([0.10536055 0.10536055 0.22314355], shape=(3,), dtype=float32)

To compute the cost function, we need to take the average of the negative log-likelihood over all the observations, such as:

cost_function = tf.reduce_mean(nll) print(cost_function)

This will print:

tf.Tensor(0.14462154, shape=(), dtype=float32)

As you can see, the cost function is a scalar value that measures how well the logistic regression model fits the data.

In summary, the cost function is a function that measures how well the logistic regression model fits the data. The cost function is usually the negative log-likelihood of the data given the model parameters, which is the negative of the logarithm of the likelihood. The likelihood is the probability of the data given the model parameters, which is the product of the probabilities of each observation given the model parameters. The goal of logistic regression is to find the optimal values of the model parameters that minimize the cost function. TensorFlow provides a built-in function for the negative log-likelihood, which is `tf.keras.losses.binary_crossentropy`

, that takes two tensors as inputs: the true labels and the predicted probabilities. The function returns a tensor of the same shape as the inputs, with the negative log-likelihood computed element-wise. To compute the cost function, we need to take the average of the negative log-likelihood over all the observations.

### 2.3. The Gradient Descent Algorithm

In this section, you will learn about the **gradient descent algorithm**, which is an optimization algorithm that iteratively updates the model parameters in the direction of the steepest descent of the cost function. The gradient descent algorithm is used to find the optimal values of the model parameters that minimize the cost function. You will also learn how to implement the gradient descent algorithm in **TensorFlow**, which is an open-source framework for building and deploying machine learning models.

What is the gradient descent algorithm? The gradient descent algorithm is an iterative algorithm that updates the model parameters in the direction of the steepest descent of the cost function. The gradient descent algorithm works as follows:

- Initialize the model parameters with some random values.
- Compute the cost function and the gradient of the cost function with respect to each model parameter.
- Update each model parameter by subtracting a fraction of the gradient from the current value.
- Repeat steps 2 and 3 until the cost function converges to a minimum value or a maximum number of iterations is reached.

The fraction of the gradient that is subtracted from the model parameters is called the learning rate, which controls how fast the model parameters are updated. A small learning rate can lead to slow convergence, while a large learning rate can lead to overshooting or divergence. The optimal learning rate depends on the data and the model, and it can be tuned using trial and error or other methods.

Why do we use the gradient descent algorithm? We use the gradient descent algorithm because it is a simple and effective way to find the optimal values of the model parameters that minimize the cost function. The gradient descent algorithm can be applied to any differentiable cost function, such as the negative log-likelihood for logistic regression. The gradient descent algorithm can also be modified to improve its performance, such as using momentum, adaptive learning rates, or stochastic gradient descent.

How do we implement the gradient descent algorithm in TensorFlow? TensorFlow provides a built-in function for the gradient descent algorithm, which is `tf.keras.optimizers.SGD`

. This function takes a learning rate as an input and returns an optimizer object that can be used to update the model parameters. The optimizer object has a method called `apply_gradients`

, which takes a list of tuples of gradients and variables as an input and updates the variables by subtracting the gradients multiplied by the learning rate. For example, if we have a tensor `x`

with the value `1`

, and a tensor `grad`

with the value `2`

, we can update `x`

using the gradient descent algorithm with a learning rate of `0.1`

as follows:

import tensorflow as tf x = tf.Variable(1) grad = tf.constant(2) optimizer = tf.keras.optimizers.SGD(learning_rate=0.1) optimizer.apply_gradients([(grad, x)]) print(x)

This will print:

As you can see, the value of `x`

is updated by subtracting `0.1 * 2`

from `1`

, which is `0`

.

In summary, the gradient descent algorithm is an optimization algorithm that iteratively updates the model parameters in the direction of the steepest descent of the cost function. The gradient descent algorithm is used to find the optimal values of the model parameters that minimize the cost function. TensorFlow provides a built-in function for the gradient descent algorithm, which is `tf.keras.optimizers.SGD`

, that takes a learning rate as an input and returns an optimizer object that can be used to update the model parameters. The optimizer object has a method called `apply_gradients`

, which takes a list of tuples of gradients and variables as an input and updates the variables by subtracting the gradients multiplied by the learning rate.

## 3. TensorFlow Basics

In this section, you will learn about the basics of **TensorFlow**, which is an open-source framework for building and deploying machine learning models. TensorFlow provides a high-level API called `tf.keras`

, which simplifies the process of creating, training, and evaluating machine learning models. You will also learn about some of the core concepts and components of TensorFlow, such as tensors, operations, variables, placeholders, graphs, and sessions.

What is TensorFlow? TensorFlow is a framework that allows you to define, create, and run machine learning models using a variety of algorithms and architectures, such as deep neural networks, convolutional neural networks, recurrent neural networks, and more. TensorFlow supports multiple programming languages, such as Python, C++, Java, and Go, and multiple platforms, such as Windows, Linux, MacOS, Android, and iOS. TensorFlow also offers tools and libraries for data processing, visualization, debugging, and optimization.

What is tf.keras? `tf.keras`

is a high-level API that provides a simple and consistent way to create, train, and evaluate machine learning models using TensorFlow. `tf.keras`

allows you to use predefined layers, models, optimizers, losses, metrics, and callbacks, or to define your own custom ones. `tf.keras`

also supports multiple backends, such as TensorFlow, Theano, and CNTK, and multiple modes, such as eager execution and graph execution.

What are tensors? Tensors are the fundamental data structures in TensorFlow. Tensors are multidimensional arrays that can store any type of data, such as numbers, strings, booleans, etc. Tensors have a shape and a dtype, which specify the dimensions and the data type of the tensor. For example, a tensor with the shape `(2, 3)`

and the dtype `tf.float32`

is a two-dimensional array of 32-bit floating-point numbers, with two rows and three columns. Tensors can be created using various methods, such as `tf.constant`

, `tf.Variable`

, `tf.placeholder`

, or `tf.convert_to_tensor`

.

What are operations? Operations are the building blocks of TensorFlow. Operations are functions that take one or more tensors as inputs and produce one or more tensors as outputs. Operations can perform various computations, such as arithmetic, logic, linear algebra, signal processing, etc. Operations can also have attributes, which specify additional parameters or options for the operation. Operations can be created using various methods, such as `tf.math`

, `tf.nn`

, `tf.linalg`

, or `tf.signal`

.

What are variables? Variables are special types of tensors that can store mutable values. Variables are used to store and update the model parameters, such as the weights and biases of a neural network. Variables can be created using the `tf.Variable`

class, which takes an initial value, a dtype, a name, and other optional arguments. Variables can be updated using various methods, such as `assign`

, `assign_add`

, `assign_sub`

, or `apply_gradients`

.

What are placeholders? Placeholders are special types of tensors that can store values that are fed at runtime. Placeholders are used to provide input data to the model, such as the features and labels of a dataset. Placeholders can be created using the `tf.placeholder`

function, which takes a dtype, a shape, and a name as arguments. Placeholders can be fed using various methods, such as `feed_dict`

, `tf.data`

, or `tf.keras`

.

What are graphs? Graphs are the computational models in TensorFlow. Graphs are composed of nodes and edges, where nodes represent operations and edges represent tensors. Graphs can be created implicitly or explicitly, using the `tf.Graph`

class. Graphs can be executed using various methods, such as `tf.Session`

, `tf.function`

, or `tf.keras`

.

What are sessions? Sessions are the runtime environments in TensorFlow. Sessions are used to run graphs and perform computations. Sessions can be created using the `tf.Session`

class, which takes a graph, a config, and other optional arguments. Sessions can be run using various methods, such as `run`

, `eval`

, or `close`

.

In summary, TensorFlow is a framework that allows you to define, create, and run machine learning models using a variety of algorithms and architectures. TensorFlow provides a high-level API called `tf.keras`

, which simplifies the process of creating, training, and evaluating machine learning models. TensorFlow also provides some core concepts and components, such as tensors, operations, variables, placeholders, graphs, and sessions, which are essential for understanding and using TensorFlow.

### 3.1. Tensors and Operations

In this section, you will learn the basics of **TensorFlow**, which is an open-source framework for building and deploying machine learning models. TensorFlow is based on the concept of **tensors** and **operations**, which are the building blocks of any TensorFlow program. You will also learn how to create and manipulate tensors and operations using TensorFlow’s Python API.

What are tensors? Tensors are generalizations of vectors and matrices to higher dimensions. A tensor can be thought of as a multidimensional array of numbers, where each dimension is called an axis or a rank. For example, a scalar is a rank-0 tensor, a vector is a rank-1 tensor, a matrix is a rank-2 tensor, and so on. Tensors can have any number of dimensions, depending on the complexity of the data. For example, an image can be represented as a rank-3 tensor, where the axes are height, width, and color channels.

How do we create tensors in TensorFlow? We can create tensors in TensorFlow using various methods, such as constants, variables, placeholders, or operations. A constant is a tensor whose value cannot be changed, such as a number or a string. A variable is a tensor whose value can be changed, such as a model parameter or a state. A placeholder is a tensor whose value can be fed at runtime, such as an input or an output. An operation is a tensor that results from applying a function to one or more tensors, such as an addition or a multiplication.

Here are some examples of how to create tensors in TensorFlow using Python:

# Import TensorFlow import tensorflow as tf # Create a rank-0 tensor (a scalar) t0 = tf.constant(42) # Create a rank-1 tensor (a vector) t1 = tf.constant([1, 2, 3]) # Create a rank-2 tensor (a matrix) t2 = tf.constant([[1, 2], [3, 4]]) # Create a rank-3 tensor (a cube) t3 = tf.constant([[[1, 2], [3, 4]], [[5, 6], [7, 8]]]) # Create a variable tensor v = tf.Variable(tf.random.normal(shape=(2, 2))) # Create a placeholder tensor p = tf.placeholder(tf.float32, shape=(None, 2)) # Create an operation tensor o = tf.add(t1, t2)

How do we manipulate tensors in TensorFlow? We can manipulate tensors in TensorFlow using various operations, such as arithmetic, logical, linear algebra, or neural network operations. Operations can take one or more tensors as inputs and produce one or more tensors as outputs. Operations can also have attributes, such as name, type, or shape, that describe their properties. Operations can be composed to form complex expressions or graphs, which represent the computational logic of a TensorFlow program.

Here are some examples of how to manipulate tensors in TensorFlow using Python:

# Import TensorFlow import tensorflow as tf # Create some tensors t1 = tf.constant([1, 2, 3]) t2 = tf.constant([4, 5, 6]) # Perform some arithmetic operations t3 = tf.add(t1, t2) # t3 = [5, 7, 9] t4 = tf.subtract(t1, t2) # t4 = [-3, -3, -3] t5 = tf.multiply(t1, t2) # t5 = [4, 10, 18] t6 = tf.divide(t1, t2) # t6 = [0.25, 0.4, 0.5] # Perform some logical operations t7 = tf.equal(t1, t2) # t7 = [False, False, False] t8 = tf.not_equal(t1, t2) # t8 = [True, True, True] t9 = tf.greater(t1, t2) # t9 = [False, False, False] t10 = tf.less(t1, t2) # t10 = [True, True, True] # Perform some linear algebra operations t11 = tf.constant([[1, 2], [3, 4]]) t12 = tf.constant([[5, 6], [7, 8]]) t13 = tf.matmul(t11, t12) # t13 = [[19, 22], [43, 50]] t14 = tf.transpose(t11) # t14 = [[1, 3], [2, 4]] t15 = tf.linalg.inv(t11) # t15 = [[-2, 1], [1.5, -0.5]] # Perform some neural network operations t16 = tf.constant([0.1, 0.2, 0.3]) t17 = tf.nn.relu(t16) # t17 = [0.1, 0.2, 0.3] t18 = tf.nn.sigmoid(t16) # t18 = [0.524, 0.549, 0.574] t19 = tf.nn.softmax(t16) # t19 = [0.300, 0.332, 0.367]

In summary, tensors and operations are the building blocks of any TensorFlow program. Tensors are generalizations of vectors and matrices to higher dimensions, and they can be created using various methods, such as constants, variables, placeholders, or operations. Operations are functions that take one or more tensors as inputs and produce one or more tensors as outputs, and they can be used to manipulate tensors in various ways, such as arithmetic, logical, linear algebra, or neural network operations.

### 3.2. Variables and Placeholders

In this section, you will learn how to use **variables** and **placeholders** in TensorFlow, which are two types of tensors that can store and feed data to your model. Variables and placeholders are essential for training and testing your model, as they allow you to update and pass data to your model parameters and inputs. You will also learn how to initialize, assign, and save variables, and how to feed data to placeholders using feed dictionaries.

What are variables in TensorFlow? Variables are tensors whose values can be changed, such as model parameters or states. Variables are usually initialized with some random values, and then updated during the training process using gradient descent or other optimization algorithms. Variables can also be saved and restored using checkpoints, which allow you to resume your training from a previous state or transfer your learning to a new model.

How do we create and use variables in TensorFlow? We can create variables in TensorFlow using the `tf.Variable`

function, which takes an initial value and an optional name as arguments. The initial value can be a tensor, a numpy array, or a Python object that can be converted to a tensor. The name can be used to identify the variable in the graph and to save and restore it later. For example, we can create a variable tensor with a shape of (2, 2) and a name of “v” as follows:

# Import TensorFlow import tensorflow as tf # Create a variable tensor v = tf.Variable(tf.random.normal(shape=(2, 2)), name="v")

We can use variables in TensorFlow as inputs or outputs of operations, just like any other tensor. For example, we can create an operation that adds a constant tensor to a variable tensor as follows:

# Create a constant tensor c = tf.constant([[1, 2], [3, 4]]) # Create an operation that adds a constant tensor to a variable tensor o = tf.add(v, c)

We can also assign new values to variables using the `tf.assign`

function, which takes a variable and a value as arguments. The value can be a tensor, a numpy array, or a Python object that can be converted to a tensor. The `tf.assign`

function returns a tensor that holds the new value of the variable. For example, we can assign a new value to the variable “v” as follows:

# Assign a new value to the variable "v" v = tf.assign(v, [[5, 6], [7, 8]])

We can also save and restore variables using checkpoints, which are files that store the values of the variables in the graph. To save variables, we need to create a `tf.train.Saver`

object, which can save and restore all or some of the variables in the graph. We can then use the `save`

method of the `tf.train.Saver`

object, which takes a session and a file name as arguments. The file name can also include a path and a prefix. For example, we can save the variables in the graph to a file named “model.ckpt” as follows:

# Create a saver object saver = tf.train.Saver() # Save the variables in the graph to a file named "model.ckpt" saver.save(sess, "model.ckpt")

To restore variables, we need to use the `restore`

method of the `tf.train.Saver`

object, which takes a session and a file name as arguments. The file name must match the one used to save the variables. For example, we can restore the variables in the graph from a file named “model.ckpt” as follows:

# Create a saver object saver = tf.train.Saver() # Restore the variables in the graph from a file named "model.ckpt" saver.restore(sess, "model.ckpt")

What are placeholders in TensorFlow? Placeholders are tensors whose values can be fed at runtime, such as inputs or outputs. Placeholders are useful for passing data to your model, such as training or testing data, without having to store them in the graph. Placeholders can also be used to change the shape or type of the data, such as batching or casting.

How do we create and use placeholders in TensorFlow? We can create placeholders in TensorFlow using the `tf.placeholder`

function, which takes a data type and an optional shape and name as arguments. The data type can be any valid TensorFlow data type, such as `tf.float32`

or `tf.int32`

. The shape can be a tuple or a list that specifies the dimensions of the placeholder, or `None`

to allow any shape. The name can be used to identify the placeholder in the graph and to feed data to it later. For example, we can create a placeholder tensor with a data type of `tf.float32`

, a shape of `(None, 2)`

, and a name of “p” as follows:

# Import TensorFlow import tensorflow as tf # Create a placeholder tensor p = tf.placeholder(tf.float32, shape=(None, 2), name="p")

We can use placeholders in TensorFlow as inputs or outputs of operations, just like any other tensor. For example, we can create an operation that multiplies a placeholder tensor by a constant tensor as follows:

# Create a constant tensor c = tf.constant([[1, 2], [3, 4]]) # Create an operation that multiplies a placeholder tensor by a constant tensor o = tf.matmul(p, c)

We can feed data to placeholders using feed dictionaries, which are Python dictionaries that map placeholders to values. The values can be tensors, numpy arrays, or Python objects that can be converted to tensors. The feed dictionaries can be passed as arguments to the `run`

or `eval`

methods of a session or a tensor, respectively. For example, we can feed data to the placeholder “p” as follows:

# Create some data data = [[1, 2], [3, 4], [5, 6]] # Feed data to the placeholder "p" using a feed dictionary feed_dict = {p: data} # Run or evaluate the operation "o" using the feed dictionary o_value = sess.run(o, feed_dict=feed_dict) # or o_value = o.eval(feed_dict=feed_dict)

In summary, variables and placeholders are two types of tensors that can store and feed data to your model. Variables are tensors whose values can be changed, such as model parameters or states. Variables can be initialized, assigned, and saved using various methods. Placeholders are tensors whose values can be fed at runtime, such as inputs or outputs. Placeholders can be created and fed using various methods.

### 3.3. Graphs and Sessions

In this section, you will learn how to use **graphs** and **sessions** in TensorFlow, which are two concepts that enable the execution of your model. Graphs and sessions are essential for running your model, as they define the computational logic and the runtime environment of your model. You will also learn how to create and use graphs and sessions using TensorFlow’s Python API.

What are graphs in TensorFlow? Graphs are data structures that represent the computational logic of your model. Graphs are composed of nodes and edges, where nodes are tensors or operations, and edges are data flows between nodes. Graphs can be visualized using tools such as TensorBoard, which can help you debug and optimize your model. Graphs can also be serialized and deserialized using protocols such as Protocol Buffers, which can help you save and load your model.

How do we create and use graphs in TensorFlow? We can create graphs in TensorFlow using the `tf.Graph`

class, which provides methods for creating and manipulating nodes and edges. We can also use the default graph, which is automatically created when we import TensorFlow. The default graph can be accessed using the `tf.get_default_graph`

function. We can use graphs in TensorFlow by adding nodes and edges to them, either explicitly or implicitly. For example, we can create and use a graph in TensorFlow as follows:

# Import TensorFlow import tensorflow as tf # Create a new graph g = tf.Graph() # Use the graph as the default graph with g.as_default(): # Create some nodes and edges in the graph a = tf.constant(1) b = tf.constant(2) c = tf.add(a, b)

What are sessions in TensorFlow? Sessions are objects that manage the runtime environment of your model. Sessions are responsible for allocating and releasing resources, such as memory and devices, and executing the operations in the graph. Sessions can also store and retrieve the values of the variables in the graph, using checkpoints or other methods.

How do we create and use sessions in TensorFlow? We can create sessions in TensorFlow using the `tf.Session`

class, which takes an optional graph and a configuration as arguments. The graph specifies which graph to execute, and the configuration specifies how to execute the graph, such as which devices to use or how to optimize the graph. We can use sessions in TensorFlow by calling the `run`

or `eval`

methods, which take one or more tensors or operations as arguments, and return the values or outputs of those tensors or operations. We can also use the `close`

method to release the resources used by the session. For example, we can create and use a session in TensorFlow as follows:

# Import TensorFlow import tensorflow as tf # Create a graph g = tf.Graph() # Use the graph as the default graph with g.as_default(): # Create some nodes and edges in the graph a = tf.constant(1) b = tf.constant(2) c = tf.add(a, b) # Create a session sess = tf.Session(graph=g) # Run or evaluate the node "c" using the session c_value = sess.run(c) # or c_value = c.eval(session=sess) # Close the session sess.close()

In summary, graphs and sessions are two concepts that enable the execution of your model. Graphs are data structures that represent the computational logic of your model, and they are composed of nodes and edges. Sessions are objects that manage the runtime environment of your model, and they are responsible for allocating and releasing resources, and executing the operations in the graph.

## 4. Logistic Regression with TensorFlow

In this section, you will learn how to implement **logistic regression** with **TensorFlow** and apply it to a **classification** problem. You will use the theory and practice of logistic regression, TensorFlow basics, and the **sigmoid** function that you learned in the previous sections. You will also use the MNIST dataset, which is a collection of images of handwritten digits, as an example of a binary classification problem.

What are the steps of logistic regression with TensorFlow? The steps of logistic regression with TensorFlow are as follows:

- Data preparation: Load and preprocess the data, such as splitting, scaling, and batching.
- Model definition: Define the model parameters, inputs, outputs, and predictions using variables, placeholders, and operations.
- Model training: Define the cost function, the optimization algorithm, and the accuracy metric using operations. Train the model using sessions and feed dictionaries.
- Model evaluation: Evaluate the model performance on the test data using sessions and feed dictionaries.

How do we prepare the data for logistic regression with TensorFlow? We can prepare the data for logistic regression with TensorFlow using the following steps:

- Load the data: We can use the
`tf.keras.datasets.mnist.load_data`

function to load the MNIST dataset, which returns two tuples of numpy arrays, one for the training data and one for the test data. Each tuple contains the images and the labels of the digits, where the images are arrays of shape (28, 28) and the labels are integers from 0 to 9. - Preprocess the data: We can use the
`tf.reshape`

,`tf.cast`

, and`tf.divide`

functions to preprocess the data, such as flattening the images, converting the data types, and scaling the pixel values. We can also use the`tf.equal`

and`tf.cast`

functions to convert the labels to binary values, where 0 means not 5 and 1 means 5. This will create a binary classification problem, where the goal is to predict whether a digit is 5 or not. - Split the data: We can use the
`tf.split`

function to split the training data into a training set and a validation set, which can be used to tune the model parameters and avoid overfitting. - Batch the data: We can use the
`tf.data.Dataset.from_tensor_slices`

and`tf.data.Dataset.batch`

functions to create batches of data, which can be fed to the model in each iteration of the training process.

Here is an example of how to prepare the data for logistic regression with TensorFlow using Python:

# Import TensorFlow import tensorflow as tf # Load the data (x_train, y_train), (x_test, y_test) = tf.keras.datasets.mnist.load_data() # Preprocess the data x_train = tf.reshape(x_train, shape=(-1, 784)) # flatten the images x_train = tf.cast(x_train, dtype=tf.float32) # convert to float32 x_train = tf.divide(x_train, 255.0) # scale the pixel values y_train = tf.equal(y_train, 5) # convert to binary labels y_train = tf.cast(y_train, dtype=tf.float32) # convert to float32 x_test = tf.reshape(x_test, shape=(-1, 784)) # flatten the images x_test = tf.cast(x_test, dtype=tf.float32) # convert to float32 x_test = tf.divide(x_test, 255.0) # scale the pixel values y_test = tf.equal(y_test, 5) # convert to binary labels y_test = tf.cast(y_test, dtype=tf.float32) # convert to float32 # Split the data x_train, x_val = tf.split(x_train, [50000, 10000]) # split into train and val sets y_train, y_val = tf.split(y_train, [50000, 10000]) # split into train and val sets # Batch the data train_data = tf.data.Dataset.from_tensor_slices((x_train, y_train)) # create a dataset from train data train_data = train_data.batch(32) # create batches of size 32 val_data = tf.data.Dataset.from_tensor_slices((x_val, y_val)) # create a dataset from val data val_data = val_data.batch(32) # create batches of size 32 test_data = tf.data.Dataset.from_tensor_slices((x_test, y_test)) # create a dataset from test data test_data = test_data.batch(32) # create batches of size 32

### 4.1. Data Preparation

In this section, you will learn how to prepare the data for logistic regression with TensorFlow. You will use the MNIST dataset, which is a collection of 70,000 images of handwritten digits from 0 to 9. Each image is 28 by 28 pixels, and each pixel has a grayscale value between 0 and 255. The MNIST dataset is a popular benchmark for image classification problems, and you can download it from here.

To prepare the data, you will need to perform the following steps:

- Load the MNIST dataset and split it into training and testing sets.
- Normalize the pixel values to be between 0 and 1.
- Reshape the images to be one-dimensional vectors of length 784.
- Encode the labels as one-hot vectors of length 10.

Let’s start by loading the MNIST dataset and splitting it into training and testing sets. You can use the **TensorFlow** function `tf.keras.datasets.mnist.load_data()`

to do this. This function returns two tuples, one for the training set and one for the testing set. Each tuple contains two arrays, one for the images and one for the labels. For example, you can write:

import tensorflow as tf # Load the MNIST dataset and split it into training and testing sets (x_train, y_train), (x_test, y_test) = tf.keras.datasets.mnist.load_data()

Next, you need to normalize the pixel values to be between 0 and 1. This will make the data more suitable for the logistic regression model, as the sigmoid function outputs values between 0 and 1. To normalize the pixel values, you can divide them by 255, which is the maximum possible value. For example, you can write:

# Normalize the pixel values to be between 0 and 1 x_train = x_train / 255.0 x_test = x_test / 255.0

Then, you need to reshape the images to be one-dimensional vectors of length 784. This will make the data compatible with the logistic regression model, which expects the input to be a vector of features. To reshape the images, you can use the **NumPy** function `np.reshape()`

, which takes an array and a new shape as arguments. For example, you can write:

import numpy as np # Reshape the images to be one-dimensional vectors of length 784 x_train = np.reshape(x_train, (x_train.shape[0], 784)) x_test = np.reshape(x_test, (x_test.shape[0], 784))

Finally, you need to encode the labels as one-hot vectors of length 10. A one-hot vector is a vector that has only one element equal to 1 and the rest equal to 0. For example, the label 3 can be encoded as [0, 0, 0, 1, 0, 0, 0, 0, 0, 0]. This will make the data compatible with the logistic regression model, which outputs a probability for each class. To encode the labels as one-hot vectors, you can use the **TensorFlow** function `tf.one_hot()`

, which takes an array of indices and a depth as arguments. For example, you can write:

# Encode the labels as one-hot vectors of length 10 y_train = tf.one_hot(y_train, depth=10) y_test = tf.one_hot(y_test, depth=10)

Now, you have prepared the data for logistic regression with TensorFlow. You have loaded the MNIST dataset and split it into training and testing sets. You have normalized the pixel values to be between 0 and 1. You have reshaped the images to be one-dimensional vectors of length 784. And you have encoded the labels as one-hot vectors of length 10. You are ready to define the logistic regression model in the next section.

### 4.2. Model Definition

In this section, you will learn how to define the logistic regression model with TensorFlow. You will use the TensorFlow variables and placeholders to represent the model parameters and the input data. You will also use the TensorFlow operations to implement the sigmoid function, the cost function, and the gradient descent algorithm.

What are TensorFlow variables and placeholders? TensorFlow variables are objects that store the values of the model parameters, such as the coefficients of the logistic regression model. TensorFlow placeholders are objects that store the values of the input data, such as the features and the labels of the MNIST dataset. TensorFlow variables and placeholders are different from regular Python variables and constants, as they are part of the TensorFlow computation graph, which is a symbolic representation of the mathematical operations that define the model.

How do you create TensorFlow variables and placeholders? You can use the TensorFlow functions `tf.Variable()`

and `tf.placeholder()`

to create TensorFlow variables and placeholders. The `tf.Variable()`

function takes an initial value and a name as arguments, and returns a TensorFlow variable object. The `tf.placeholder()`

function takes a data type and a shape as arguments, and returns a TensorFlow placeholder object. For example, you can write:

import tensorflow as tf # Create a TensorFlow variable for the bias term of the logistic regression model b = tf.Variable(tf.zeros([10]), name="b") # Create a TensorFlow variable for the coefficient matrix of the logistic regression model W = tf.Variable(tf.random_normal([784, 10]), name="W") # Create a TensorFlow placeholder for the input features of the MNIST dataset x = tf.placeholder(tf.float32, [None, 784], name="x") # Create a TensorFlow placeholder for the input labels of the MNIST dataset y = tf.placeholder(tf.float32, [None, 10], name="y")

How do you use TensorFlow operations to implement the logistic regression model? You can use the TensorFlow operations to perform the mathematical operations that define the logistic regression model. TensorFlow operations are functions that take one or more TensorFlow variables or placeholders as inputs, and return one or more TensorFlow variables or placeholders as outputs. For example, you can use the TensorFlow operation `tf.matmul()`

to perform matrix multiplication, and the TensorFlow operation `tf.nn.sigmoid()`

to perform the sigmoid function. To implement the logistic regression model, you can write:

# Compute the log-odds as a linear function of the input features and the model parameters log_odds = tf.matmul(x, W) + b # Compute the probability as the sigmoid function of the log-odds probability = tf.nn.sigmoid(log_odds)

How do you use TensorFlow operations to implement the cost function and the gradient descent algorithm? You can use the TensorFlow operations to perform the mathematical operations that define the cost function and the gradient descent algorithm. For example, you can use the TensorFlow operation `tf.reduce_mean()`

to compute the average of an array, and the TensorFlow operation `tf.train.GradientDescentOptimizer()`

to create an optimizer object that can perform the gradient descent algorithm. To implement the cost function and the gradient descent algorithm, you can write:

# Define the cost function as the negative log-likelihood of the data given the model parameters cost_function = -tf.reduce_mean(y * tf.log(probability) + (1 - y) * tf.log(1 - probability)) # Define the learning rate as a constant value learning_rate = 0.01 # Create an optimizer object that can perform the gradient descent algorithm optimizer = tf.train.GradientDescentOptimizer(learning_rate) # Define the training operation as the minimization of the cost function training_operation = optimizer.minimize(cost_function)

In summary, you have defined the logistic regression model with TensorFlow. You have used the TensorFlow variables and placeholders to represent the model parameters and the input data. You have also used the TensorFlow operations to implement the sigmoid function, the cost function, and the gradient descent algorithm. You are ready to train the model in the next section.

### 4.3. Model Training

In this section, you will learn how to train the logistic regression model with TensorFlow. You will use the TensorFlow graphs and sessions to execute the computation graph that defines the model. You will also use the TensorFlow functions to monitor the training process and evaluate the model performance.

What are TensorFlow graphs and sessions? TensorFlow graphs are objects that represent the computation graph that defines the model. TensorFlow graphs contain nodes and edges, where nodes are TensorFlow operations and edges are TensorFlow variables or placeholders. TensorFlow graphs are created implicitly when you define the model using TensorFlow operations. For example, when you write:

probability = tf.nn.sigmoid(tf.matmul(x, W) + b)

You are creating a TensorFlow graph that contains four nodes: `tf.nn.sigmoid`

, `tf.matmul`

, `+`

, and `probability`

. The graph also contains four edges: `x`

, `W`

, `b`

, and `probability`

.

TensorFlow sessions are objects that execute the computation graph that defines the model. TensorFlow sessions allocate the resources, such as CPU and memory, that are needed to run the graph. TensorFlow sessions also initialize the variables and placeholders, and perform the operations that are specified in the graph. For example, when you write:

with tf.Session() as sess: sess.run(tf.global_variables_initializer()) sess.run(training_operation, feed_dict={x: x_train, y: y_train})

You are creating a TensorFlow session that executes the computation graph that defines the model. The session initializes the variables `W`

and `b`

with random and zero values, respectively. The session also performs the training operation, which updates the variables `W`

and `b`

using the gradient descent algorithm. The session feeds the training data `x_train`

and `y_train`

to the placeholders `x`

and `y`

, respectively.

How do you train the logistic regression model with TensorFlow? To train the logistic regression model with TensorFlow, you need to perform the following steps:

- Create a TensorFlow graph that defines the model, the cost function, and the training operation.
- Create a TensorFlow session that executes the graph.
- Initialize the variables and placeholders.
- Loop over a number of epochs and batches.
- Feed the training data to the graph and perform the training operation.
- Print the cost function value at each epoch.

Let’s start by creating a TensorFlow graph that defines the model, the cost function, and the training operation. You have already done this in the previous section, so you can reuse the same code. For example, you can write:

# Create a TensorFlow graph that defines the model, the cost function, and the training operation graph = tf.Graph() with graph.as_default(): # Create a TensorFlow variable for the bias term of the logistic regression model b = tf.Variable(tf.zeros([10]), name="b") # Create a TensorFlow variable for the coefficient matrix of the logistic regression model W = tf.Variable(tf.random_normal([784, 10]), name="W") # Create a TensorFlow placeholder for the input features of the MNIST dataset x = tf.placeholder(tf.float32, [None, 784], name="x") # Create a TensorFlow placeholder for the input labels of the MNIST dataset y = tf.placeholder(tf.float32, [None, 10], name="y") # Compute the log-odds as a linear function of the input features and the model parameters log_odds = tf.matmul(x, W) + b # Compute the probability as the sigmoid function of the log-odds probability = tf.nn.sigmoid(log_odds) # Define the cost function as the negative log-likelihood of the data given the model parameters cost_function = -tf.reduce_mean(y * tf.log(probability) + (1 - y) * tf.log(1 - probability)) # Define the learning rate as a constant value learning_rate = 0.01 # Create an optimizer object that can perform the gradient descent algorithm optimizer = tf.train.GradientDescentOptimizer(learning_rate) # Define the training operation as the minimization of the cost function training_operation = optimizer.minimize(cost_function)

Next, you need to create a TensorFlow session that executes the graph. You can use the TensorFlow function `tf.Session()`

to create a TensorFlow session object. You can also use the Python context manager `with`

to ensure that the session is closed automatically after the execution. For example, you can write:

# Create a TensorFlow session that executes the graph with tf.Session(graph=graph) as sess: # The rest of the code goes here

Then, you need to initialize the variables and placeholders. You can use the TensorFlow function `tf.global_variables_initializer()`

to create an operation that initializes all the variables in the graph. You can also use the TensorFlow method `sess.run()`

to execute the operation in the session. For example, you can write:

# Initialize the variables and placeholders sess.run(tf.global_variables_initializer())

After that, you need to loop over a number of epochs and batches. An epoch is a complete pass over the entire training data, and a batch is a subset of the training data that is used for one iteration of the gradient descent algorithm. Looping over epochs and batches can help to speed up the convergence and avoid overfitting. You can use the Python functions `range()`

and `len()`

to create loops over the epochs and batches. You can also use the NumPy function `np.random.permutation()`

to shuffle the training data at each epoch. For example, you can write:

# Define the number of epochs and the batch size epochs = 10 batch_size = 100 # Loop over the epochs for epoch in range(epochs): # Shuffle the training data at each epoch permutation = np.random.permutation(x_train.shape[0]) x_train = x_train[permutation] y_train = y_train[permutation] # Loop over the batches for i in range(0, x_train.shape[0], batch_size): # Extract a batch of features and labels from the training data x_batch = x_train[i:i+batch_size] y_batch = y_train[i:i+batch_size] # The rest of the code goes here

Inside the loop over the batches, you need to feed the training data to the graph and perform the training operation. You can use the TensorFlow method `sess.run()`

to execute the training operation in the session. You can also use the `feed_dict`

argument to pass the values of the placeholders to the graph. For example, you can write:

# Feed the training data to the graph and perform the training operation sess.run(training_operation, feed_dict={x: x_batch, y: y_batch})

Finally, you need to print the cost function value at each epoch. You can use the TensorFlow method `sess.run()`

to execute the cost function operation in the session. You can also use the `feed_dict`

argument to pass the values of the placeholders to the graph. You can use the Python function `print()`

to display the cost function value on the screen. For example, you can write:

# Compute the cost function value on the entire training data cost = sess.run(cost_function, feed_dict={x: x_train, y: y_train}) # Print the cost function value at each epoch print("Epoch", epoch, "Cost", cost)

Now, you have trained the logistic regression model with TensorFlow. You have used the TensorFlow graphs and sessions to execute the computation graph that defines the model. You have also used the TensorFlow functions to monitor the training process and evaluate the model performance. You are ready to evaluate the model in the next section.

### 4.4. Model Evaluation

In this section, you will learn how to evaluate the performance of your logistic regression model with TensorFlow. You will use two metrics to measure the accuracy and the confusion matrix to visualize the results. You will also plot the decision boundary to see how well the model separates the classes.

What is accuracy? Accuracy is the ratio of the number of correct predictions to the total number of predictions. It is a simple and intuitive way to evaluate the performance of a classification model. For example, if you have 100 predictions and 80 of them are correct, then your accuracy is 80 / 100 = 0.8 or 80%.

How do you calculate accuracy with TensorFlow? You can use the `tf.equal`

function to compare the predicted labels with the true labels. This will return a boolean tensor of the same shape as the labels, where each element is either `True`

or `False`

depending on whether the prediction is correct or not. For example, if your predicted labels are `[0, 1, 1, 0]`

and your true labels are `[0, 1, 0, 1]`

, then the result of `tf.equal`

will be `[True, True, False, False]`

.

You can then use the `tf.cast`

function to convert the boolean tensor to a numeric tensor, where each element is either 1 or 0 depending on whether the prediction is correct or not. For example, the result of `tf.cast`

will be `[1, 1, 0, 0]`

.

Finally, you can use the `tf.reduce_mean`

function to compute the average of the numeric tensor, which will give you the accuracy. For example, the result of `tf.reduce_mean`

will be `(1 + 1 + 0 + 0) / 4 = 0.5`

.

The following code shows how to calculate the accuracy of your logistic regression model with TensorFlow:

# Define the predicted labels as the ones with the highest probability predicted_labels = tf.argmax(probability, axis=1) # Define the true labels as the ones in the test set true_labels = y_test # Compare the predicted labels with the true labels correct_predictions = tf.equal(predicted_labels, true_labels) # Convert the boolean tensor to a numeric tensor correct_predictions = tf.cast(correct_predictions, tf.float32) # Compute the accuracy accuracy = tf.reduce_mean(correct_predictions) # Evaluate the accuracy with the test data accuracy_value = sess.run(accuracy, feed_dict={X: X_test, y: y_test}) # Print the accuracy print("Accuracy: {:.2f}%".format(accuracy_value * 100))

What is a confusion matrix? A confusion matrix is a table that shows the number of true positives, false positives, true negatives, and false negatives for a classification model. A true positive is a correct prediction of a positive class, a false positive is an incorrect prediction of a positive class, a true negative is a correct prediction of a negative class, and a false negative is an incorrect prediction of a negative class. For example, if you have two classes, 0 and 1, and your predictions and true labels are as follows:

Predicted | True |

0 | 0 |

1 | 1 |

1 | 0 |

0 | 1 |

Then your confusion matrix will be:

Predicted 0 | Predicted 1 | |

True 0 | 1 | 1 |

True 1 | 1 | 1 |

The confusion matrix can help you understand the strengths and weaknesses of your classification model. For example, you can see how many false positives and false negatives your model makes, which can affect the precision and recall of your model. Precision is the ratio of true positives to the total number of positive predictions, and recall is the ratio of true positives to the total number of positive labels. For example, in the above confusion matrix, the precision and recall are both 0.5.

How do you create a confusion matrix with TensorFlow? You can use the `tf.math.confusion_matrix`

function to create a confusion matrix with TensorFlow. This function takes the true labels and the predicted labels as inputs, and returns a tensor of shape `[num_classes, num_classes]`

where each element is the number of occurrences of the corresponding pair of labels. For example, the following code shows how to create a confusion matrix for your logistic regression model with TensorFlow:

# Define the predicted labels as the ones with the highest probability predicted_labels = tf.argmax(probability, axis=1) # Define the true labels as the ones in the test set true_labels = y_test # Create the confusion matrix confusion_matrix = tf.math.confusion_matrix(true_labels, predicted_labels) # Evaluate the confusion matrix with the test data confusion_matrix_value = sess.run(confusion_matrix, feed_dict={X: X_test, y: y_test}) # Print the confusion matrix print("Confusion matrix:") print(confusion_matrix_value)

How do you plot the decision boundary? The decision boundary is the line that separates the classes based on the model’s predictions. The decision boundary can help you visualize how well the model separates the classes and identify any outliers or misclassifications. To plot the decision boundary, you need to create a grid of points that cover the range of the features, and then use the model to predict the labels for each point. You can then use a color map to show the different classes and the decision boundary. For example, the following code shows how to plot the decision boundary for your logistic regression model with TensorFlow:

# Import matplotlib for plotting import matplotlib.pyplot as plt # Define the grid of points that cover the range of the features x_min, x_max = X_test[:, 0].min() - 1, X_test[:, 0].max() + 1 y_min, y_max = X_test[:, 1].min() - 1, X_test[:, 1].max() + 1 xx, yy = np.meshgrid(np.arange(x_min, x_max, 0.1), np.arange(y_min, y_max, 0.1)) # Flatten the grid of points and reshape it to match the input shape of the model grid = np.c_[xx.ravel(), yy.ravel()] grid = grid.reshape(-1, 2) # Use the model to predict the labels for each point in the grid grid_labels = sess.run(predicted_labels, feed_dict={X: grid}) # Reshape the grid labels to match the shape of the grid grid_labels = grid_labels.reshape(xx.shape) # Plot the grid points with different colors for different classes plt.contourf(xx, yy, grid_labels, cmap=plt.cm.Spectral, alpha=0.8) # Plot the test data points with different markers for different classes plt.scatter(X_test[:, 0], X_test[:, 1], c=y_test, s=40, cmap=plt.cm.Spectral) # Show the plot plt.show()

In conclusion, you have learned how to evaluate the performance of your logistic regression model with TensorFlow. You have used two metrics, accuracy and confusion matrix, to measure how well the model fits the data. You have also plotted the decision boundary to see how well the model separates the classes. You have completed the fourth and final step of building a logistic regression model with TensorFlow.

## 5. Conclusion

In this blog, you have learned how to implement **logistic regression** with **TensorFlow** and apply it to a **classification** problem. You have followed the four steps of building a logistic regression model with TensorFlow:

- Data preparation: You have loaded and normalized the MNIST dataset, which contains images of handwritten digits. You have also split the data into training and test sets, and converted the labels into one-hot vectors.
- Model definition: You have defined the logistic regression model using TensorFlow variables and placeholders. You have also defined the probability, the cost function, and the optimizer.
- Model training: You have trained the model using TensorFlow graphs and sessions. You have also used mini-batch gradient descent to optimize the model parameters.
- Model evaluation: You have evaluated the performance of the model using accuracy and confusion matrix. You have also plotted the decision boundary to visualize the results.

By completing this blog, you have gained a better understanding of the theory and practice of logistic regression and TensorFlow. You have also learned how to use TensorFlow to build and deploy machine learning models. TensorFlow is an open-source framework that offers many features and functionalities for developing and deploying machine learning applications. You can use TensorFlow to create and train various types of models, such as neural networks, convolutional networks, recurrent networks, and more. You can also use TensorFlow to perform data analysis, visualization, debugging, and testing. TensorFlow is compatible with multiple platforms, such as Windows, Linux, Mac OS, Android, and iOS. You can also use TensorFlow with different programming languages, such as Python, C++, Java, and more.

We hope you enjoyed this blog and learned something new and useful. If you want to learn more about logistic regression and TensorFlow, you can check out the following resources:

- TensorFlow official website: Here you can find the documentation, tutorials, guides, and examples of TensorFlow.
- Logistic Regression with TensorFlow course: This is a free online course that teaches you how to implement logistic regression with TensorFlow and apply it to various classification problems.
- TensorFlow Examples: This is a GitHub repository that contains many examples of TensorFlow applications, such as image recognition, natural language processing, reinforcement learning, and more.

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