Basic Mathematics for Neural Networks | Vectors and Matrices with Matplotlib

Ranvir Singh · 12 mins read ·
machinelearning python datascience matplotlib

Vector and Matrices are at the heart of all Neural Networks. Today we are going to learn about vector and Matrix mathematics with the help of Matplotlib and numpy.

First, we are going to understand different analogies in Neural Networks which correspond to Vectors and Matrices.

Also, we are going to plot some vectors and see how vector operations work.

Vector in Machine Learning

A vector represents one of the data points in the whole dataset.

For example, let’s say you are working with a YouTube video creator who makes unboxing videos of different phones and other electronic stuff.

Just for the simplicity, your task is to predict which phone to review next so that your channel can get maximum views and maximum social media shares.

You already have the historical data of the channel. You also have a list of 50 upcoming phones but you are only allowed to review 5 out of them.

The features available to you are:

| Company  | Price | Number of Cameras | Screen length | Traction |
| Apple    | 1000  | 3                 | 5.5           | 85%      |
This one data point can be called a vector with a number of features being the dimension of the plane in which this vector can be represented.

Matrix representation in Machine Learning

A Matrix can be represented as a set of a few vectors. The collection of data points is a Matrix.

For example, a few data points for predicting the next phone to review.

| Company  | Price | Number of Cameras | Screen length | Traction |
| Apple    | 1000  | 3                 | 5.5           | 85%      |
| Samsung  | 800   | 1                 | 6             | 70%      |

Vectors in Simple Mathematics and its operations

In general, a vector is defined as something having both magnitude and direction. If we can plot this in the 2-D graph, it will be represented as a line pointing in a direction.

From now on, we will be talking about vectors in a 2-D planes to make things simple.

In mathematics, a vector is represented as,

Let’s represent a simple vector in 2-D plane using matplotlib and numpy.

0, 0, 5, 3
$\begin{vmatrix}0\\ 0\end{vmatrix}\ and\ \begin{vmatrix}5\\ 3\end{vmatrix}$
To represent this vector in a plane, we need to import NumPy and Matplotlib.

Import required packages

import numpy as np
import matplotlib.pyplot as plt
We are going to use Matplotlib’s quiver method to plot the vector on the 2D plane.

plt.quiver(0, 0, 5, 3,  scale_units='xy', angles='xy', scale=1, color="red")

# Just to make the plot look better.
plt.xlim(-5, 10)
plt.ylim(-5, 10)
Single vector on matplotlib
Let’s write a function that can help us to plot vectors whenever we want to,

def plot_2D_vectors(vectors, colors=('red', 'blue', 'green')):
    # random numbers
    min_x = 0
    max_x = 0
    min_y = 0
    max_y = 0
    i = -1
    for points in vectors:
        for a in range(0, 4):
            if a % 2 == 0:
                if points[a] <= min_x:
                    min_x = points[a] - 1
                if points[a] >= max_x:
                    max_x = points[a] + 1
                if points[a] <= min_y:
                    min_y = points[a] - 1
                if points[a] >= max_y:
                    max_y = points[a] + 1
        i = i + 1
        plt.quiver(points[0], points[1], points[2], points[3], scale_units='xy', angles='xy', scale=1, color=colors[i])
    plt.xlim(min_x, max_x)
    plt.ylim(min_y, max_y)
This function takes two arguments, first is the vectors and the other is the colors, whose default value is red, blue and green. We are also trying to figure out what should be the boundary of the graph by running a loop to find minimum and maximum of the graph by subtracting 1 from the minimum and adding 1 to the maximum values of x and y.

This function will help us to apply various mathematical operations on vectors.

Addition and Subtraction of Vectors

Before doing any operation on vectors, we will have to represent them using Numpy.

np_vector_1 = np.array([0, 0, 5, 3])
np_vector_2 = np.array([0, 0, 6, 8])

plot_2D_vectors([np_vector_1, np_vector_2])
Two vectors on matplotlib
Now, let’s add these two vectors and plot the new result vector.

np_vector_3 = np_vector_1 + np_vector_2

plot_2D_vector(np_vector_2, 'b')
plot_2D_vector(np_vector_3, 'g')
Adding two vectors using numpy
If we add two vectors having an obtuse angle in between them then the magnitude of the resultant vector decreases.

np_vector_1 = np.array([0, 0, -5, -3])
np_vector_2 = np.array([0, 0, 6, 8])
np_vector_3 = np_vector_1 + np_vector_2

plot_2D_vectors((np_vector_1, np_vector_2, np_vector_3), ['r', 'b', 'g'])
Adding two obtuse vectors using numpy
Subtraction is also similar, we can plot the subtracted vector simply using the function.

np_vector_1 = np.array([0, 0, -5, -3])
np_vector_2 = np.array([0, 0, 6, 8])
np_vector_3 = np_vector_2 - np_vector_1

plot_2D_vectors((np_vector_1, np_vector_2, np_vector_3), ['r', 'b', 'g'])
Subtracting two vectors using numpy

Dot product of two vectors

The dot product of two vectors gives just the magnitude by multiplying the point-wise values in the vector.[3, 4]), np.array([5, -1]))
Mathematically, it is given by,

$\overrightarrow{x} . \overrightarrow{y} = x_i y_i + x_j y_j$

Projection of a vector onto another vector

Projection of a vector onto another vector is the image that it makes when the vector image is seen when there is a light source right at the top of the vector whose projection is being calculated.

np_vector_1 = np.asarray([0, 0, 3, 4])
np_vector_2 = np.asarray([0, 0, 4, -1])
dot_product =, np_vector_2)
proj_vector = (dot_product/np.power(np.linalg.norm(np_vector_2), 2))*np_vector_2

plot_2D_vectors((np_vector_1, np_vector_2, proj_vector), ['r', 'green', 'red'])
Projection with vectors on matplotlib
Projection of vector having 90 degrees between them is 0.

np_vector_1 = np.array([0, 0, 4, 0])
np_vector_2 = np.array([0, 0, 0, 4])
dot_product =, np_vector_2)
proj_vector = (dot_product/np.power(np.linalg.norm(np_vector_2), 2))*np_vector_2

plot_2D_vectors((np_vector_1, np_vector_2, proj_vector), ['r', 'green', 'red'])
Projection with orthogonal vectors
Mathematically, projection of one vector on another is given by,

$\frac{\overrightarrow{x} . \overrightarrow{y}}{\begin{Vmatrix}\overrightarrow{y}\end{Vmatrix}^2}\ \overrightarrow{y}$

Matrices in Mathematics

Matrices are the collection of vectors.

Mathematically they are represented as,

\begin{vmatrix}x_1 & x_2 & x_3\ x_4 & x_5 & x_6\ x_7 & x_8 & x_9\end{vmatrix}

Matrix Multiplication.

Matrix multiplication is done by multiplying and adding every row to the corresponding columns.

Read the following post to understand it thoroughly.

We can do the same using NumPy dot product as well.

vector_1 = np.array([(1, 2, 3), (4, 5, 6), (7, 8, 9)])
vector_2 = np.array([(1, 4), (2, 5), (3, 6)])

> array([[ 14,  32],
       [ 32,  77],
       [ 50, 122]])
That’s it for the basic version of mathematics using numpy and matplotlib. We will talk about a more extended version of mathematics in different models in later versions.

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About Author

Ranvir Singh

Greetings! Ranvir is an Engineering professional with 3+ years of experience in Software development.

Original Source: Original Post
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