Examples of how to find the product of two matrices in python:

Table of contents

### Function dot()

To get the product of two matrices, a solution is to use the numpy function dot():

\begin{equation}

A = \left( \begin{array}{ccc}

1 & 2 & 0 \\

4 & 3 & -1

\end{array}\right)

\end{equation}

\begin{equation}

B = \left( \begin{array}{ccc}

5 & 1 \\

2 & 3 \\

3 & 4

\end{array}\right)

\end{equation}

\begin{equation}

C = A * B =

\left( \begin{array}{ccc}

1 & 2 & 0 \\

4 & 3 & -1

\end{array}\right)

*

\left( \begin{array}{ccc}

5 & 1 \\

2 & 3 \\

3 & 4

\end{array}\right)

=

\left( \begin{array}{ccc}

9 & 7 \\

23 & 9

\end{array}\right)

\end{equation}

`>>> import numpy as np`

`>>> A = np.array([[1,2,0],[4,3,-1]])`

`>>> A.shape`

`(2, 3)`

`>>> B = np.array([[5,1],[2,3],[3,4]])`

`>>> B.shape`

`(3, 2)`

`>>> C = A.dot(B)`

`>>> C`

`array([[ 9, 7],`

`[23, 9]])`

### The operator *

Warning: the * operator does not returns the product of two matrices, it returns the product of elements of same indexes.

Note: The two matrices must have the same size, example:

`>>> import numpy as np`

`>>> A = np.array([[1,2,0],[4,3,-1]])`

`>>> C = np.array([[3,4,5],[1,2,3]])`

`>>> A * C`

`array([[ 3, 8, 0],`

`[ 4, 6, -3]])`

### Multiply by a constant

To multiply all the elements of a matrice by a constant:

`>>> import numpy as np`

`>>> A = np.array([[1,2,0],[4,3,-1]])`

`>>> A * 2`

`array([[ 2, 4, 0],`

`[ 8, 6, -2]])`

### References

Links | Site |
---|---|

numpy.dot | numpy doc |

Produit matriciel | wikipedia |