How to multiply matrices in JavaScript

by Janeth Kent Date: 30-04-2021 javascript coding functions webdev

It may seem strange to want to know how to multiply matrices in JavaScript. But we will see some examples where it is useful to know how to perform this operation and how useful it is when handling coordinates as matrices, which we will see in a following article.

But let's go step by step and the first thing to do is to create a matrix in JavaScript. We have to know that an array is an array where each of the positions is an array of elements, for example numbers. In this way we can initialize an array in JavaScript as follows:

m1 = [[1,2,3],[4,5,6]];  
m2 = [[7,8],[9,10],[11,12]];

In order to understand what the columns and rows of the matrix would be, we will take into account that the elements of the main array are the rows and each of the elements of the internal array will be the columns. In this way the two arrays that we have instantiated in these lines of code would correspond to the matrices:

Matrix 2x3  
|1   2    3|    
|4   5   6|    

Matrix 3x2  
|7    8|  
|9   10|  
|11  12|

If we want to know how many rows and columns the matrix has, we can calculate it in the following way:

fil_m1 = m1.length;  
col_m1 = m1[0].length;    
fil_m2 = m2.length;  
col_m2 = m2[0].length;

We can see that the rows are obtained through the .length property of the array and the columns by asking the first element of the matrix, again with the .length property of the array.

This is very important because in order to be able to multiply matrices, the columns of the first matrix must be the same as the columns of the second matrix. We will make this check in the following way:

if (col_m1 != fil_m2)    
throw "Matrices cannot be multiplied";

The next step is to create the matrix with the result. The matrix will have a size equal to as many rows as matrix 1 as columns of matrix 2. So we create the matrix for the result of the multiplication as follows:

let multiplication = new Array(fil_m1);  
for (x=0; x<multiplication.length;x++)      
multiplication[x] = new Array(col_m2).fill(0);

We see that we first create an array and then for each element of the array we create a new element. We rely on the .fill() method that allows us to fill the array with a number. In this case we are going to initialise it to 0.

The next step is to perform the multiplication. To do this we go through the result matrix and in each x,y position we assign the result of multiplying each element of the row of the first matrix with each of the columns of the second matrix.

The scheme would be as follows

multiply matrices

And the code that implements it would be:

for (x=0; x < multiplication.length; x++) {      
for (y=0; y < multiplication[x].length; y++) {   
    for (z=0; z<col_m1; z++) {              
               multiplication [x][y] = multiplication [x][y] + m1[x][z]*m2[z][y]; 

In this way we will have managed to perform the multiplication and the result will be stored in the multiplication matrix. This way we will have managed to multiply matrices in JavaScript.

by Janeth Kent Date: 30-04-2021 javascript coding functions webdev hits : 12291  
Janeth Kent

Janeth Kent

Licenciada en Bellas Artes y programadora por pasión. Cuando tengo un rato retoco fotos, edito vídeos y diseño cosas. El resto del tiempo escribo en MA-NO WEB DESIGN AND DEVELOPMENT.


Related Posts

Mastering array sorting in JavaScript: a guide to the sort() function

In this article, I will explain the usage and potential of the sort() function in JavaScript.   What does the sort() function do?   The sort() function allows you to sort the elements of…

Infinite scrolling with native JavaScript using the Fetch API

I have long wanted to talk about how infinite scroll functionality can be implemented in a list of items that might be on any Web page. Infinite scroll is a technique…

Sorting elements with SortableJS and storing them in localStorage

SortableJS is a JavaScript extension that you will be able to use in your developments to offer your users the possibility to drag and drop elements in order to change…

What is a JWT token and how does it work?

JWT tokens are a standard used to create application access tokens, enabling user authentication in web applications. Specifically, it follows the RFC 7519 standard. What is a JWT token A JWT token…

Template Literals in JavaScript

Template literals, also known as template literals, appeared in JavaScript in its ES6 version, providing a new method of declaring strings using inverted quotes, offering several new and improved possibilities. About…

How to use the endsWith method in JavaScript

In this short tutorial, we are going to see what the endsWith method, introduced in JavaScript ES6, is and how it is used with strings in JavaScript. The endsWith method is…

What are javascript symbols and how can they help you?

Symbols are a new primitive value introduced by ES6. Their purpose is to provide us unique identifiers. In this article, we tell you how they work, in which way they…

Callbacks in JavaScript

Callback functions are the same old JavaScript functions. They have no special syntax, as they are simply functions that are passed as an argument to another function. The function that receives…

How to create PDF with JavaScript and jsPDF

Creating dynamic PDF files directly in the browser is possible thanks to the jsPDF JavaScript library. In the last part of this article we have prepared a practical tutorial where I…

How to make your own custom cursor for your website

When I started browsing different and original websites to learn from them, one of the first things that caught my attention was that some of them had their own cursors,…

Node.js and npm: introductory tutorial

In this tutorial we will see how to install and use both Node.js and the npm package manager. In addition, we will also create a small sample application. If you…

How to connect to MySQL with Node.js

Let's see how you can connect to a MySQL database using Node.js, the popular JavaScript runtime environment. Before we start, it is important to note that you must have Node.js installed…