Matrices
A matrix is set of Numbers.
A matrix is an Rectangular Array.
A matrix is arranged in Rows and Columns.
Matrix Dimensions
This Matrix has 1 row and 3 columns:
The Dimension of the matrix is (1x3).
This matrix has 2 rows and 3 columns:
The dimension of the matrix is (2x3).
Square Matrices
A Square Matrix is a matrix with the same number of rows and columns.
An nbyn matrix is known as a square matrix of order n.
A 2by2 matrix (Square matrix of order 2):
A 4by4 matrix (Square matrix of order 4):
C = 
1 
2 
3 
4 
5 
6 
7 
8 
4 
3 
2 
1 
8 
7 
6 
5 

Diagonal Matrices
A Diagonal Matrix has values on the diagonal entries, and zero on the rest:
Scalar Matrices
A Scalar Matrix has equal diagonal entries and zero on the rest:
C = 
3 
0 
0 
0 
0 
3 
0 
0 
0 
0 
3 
0 
0 
0 
0 
3 

The Identity Matrix
The Identity Matrix has 1 on the diagonal and 0 on the rest.
This is the matrix equivalent of 1. The symbol is I.
I = 
1 
0 
0 
0 
0 
1 
0 
0 
0 
0 
1 
0 
0 
0 
0 
1 

If you multiply any matrix with the identity matrix, the result equals the original.
The Zero Matrix
The Zero Matrix (Null Matrix) has only zeros.
Equal Matrices
Matrices are Equal if each element correspond:
Negative Matrices
The Negative of a matrix is easy to understand:
Linear Algebra in JavaScript
In linear algebra, the most simple math object is the Scalar:
Another simple math object is the Array:
const array = [ 1, 2, 3 ];
Matrices are 2dimensional Arrays:
const matrix = [ [1,2],[3,4],[5,6] ];
Vectors can be written as Matrices with only one column:
const vector = [ [1],[2],[3] ];
Vectors can also be written as Arrays:
const vector = [ 1, 2, 3 ];
JavaScript Matrix Operations
Programming matrix operations in JavaScript, can easily become a spaghetti of loops.
Using a JavaScript library will save you a lot of headache.
One of the most common libraries to use for matrix operations is called math.js.
It can be added to your web page with one line of code:
Using math.js
<script src="https://cdnjs.cloudflare.com/ajax/libs/mathjs/9.3.2/math.js"></script>
Adding Matrices
If two matrices have the same dimension, we can add them:
Example
const mA = math.matrix([[1, 2], [3, 4], [5, 6]]);
const mB = math.matrix([[1,1], [2,2], [3,3]]);
// Matrix Addition
const matrixAdd = math.add(mA, mB);
// Result [ [2, 1], [5, 2], [8, 3] ]
Try it Yourself »
Subtracting Matrices
If two matrices have the same dimension, we can subtract them:
Example
const mA = math.matrix([[1, 2], [3, 4], [5, 6]]);
const mB = math.matrix([[1,1], [2,2], [3,3]]);
// Matrix Subtraction
const matrixSub = math.subtract(mA, mB);
// Result [ [0, 3], [1, 6], [2, 9] ]
Try it Yourself »
To add or subtract matrices, they must have the same dimension.
Scalar Multiplication
While numbers in rows and columns are called Matrices, single numbers are called Scalars.
It is easy to multiply a matrix with a scalar. Just multiply each number in the matrix with the scalar:
Example
const mA = math.matrix([[1, 2], [3, 4], [5, 6]]);
// Matrix Multiplication
const matrixMult = math.multiply(2, mA);
// Result [ [2, 4], [6, 8], [10, 12] ]
Try it Yourself »
Example
const mA = math.matrix([[0, 2], [4, 6], [8, 10]]);
// Matrix Division
const matrixDiv = math.divide(mA, 2);
// Result [ [0, 1], [2, 3], [4, 5] ]
Try it Yourself »
Transpose a Matrix
To transpose a matrix, means to replace rows with columns.
When you swap rows and columns, you rotate the matrix around it's diagonal.
Multiplying Matrices
Multiplying matrices is more difficult.
We can only multiply two matrices if the number of colums in matrix A is the same as the number of rows in matrix B.
Then, we need to compile a "dot product":
We need to multiply the numbers in each column of A with the numbers in each row of B, and then add the products:
Example
const mA = math.matrix([1, 2, 3]);
const mB = math.matrix([[1, 4, 7], [2, 5, 8], [3, 6, 9]]);
// Matrix Multiplication
const matrixMult = math.multiply(mA, mB);
// Result [14, 32, 50]
Try it Yourself »
Explained:
(1,2,3) * (1,2,3) = 1x1 + 2x2 + 3x3 =  14 
(1,2,3) * (4,5,6) = 1x4 + 2x5 + 3x6 =  32 
(1,2,3) * (7,8,9) = 1x7 + 2x8 + 3x9 =  50 
If you know how to multiply matrices, you can solve many complex equations.
Example
You sell roses.
 Red roses are $3 each
 White roses are $4 each
 Yellow roses are $2 each
 Monday you sold 260 roses
 Tuesday you sold 200 roses
 Wednesday you sold 120 roses
What was the value of all the sales?

$3 
$4 
$2 
Mon  120  80  60 
Tue  90  70  40 
Wed  60  40  20 
Example
const mA = math.matrix([3, 4, 2]);
const mB = math.matrix([[120, 90, 60], [80, 70, 40], [60, 40, 20]);
// Matrix Multiplication
const matrixMult = math.multiply(mA, mB);
// Result [800, 630, 380]
Try it Yourself »
Explained:
A 

B 





x 
120 
90 
60 
80 
70 
40 
60 
40 
20 

= 

= 

(3,4,2) * (120,80,60)  = 3x120 + 4x80 + 2x60  = 800 
(3,4,2) * (90,70,40)  = 3x90 + 4x70 + 2x40  = 630 
(3,4,2) * (60,40,20)  = 3x60 + 4x40 + 2x20  = 380 
Matrix Factorization
With AI, you need to know how to factorize a matrix.
Matrix factorization is a key tool in linear algebra, especially in Linear Least Squares.
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