Multiply matrices in python

With numpy it is trivial, in fact it works as you have written it. But yes, your matrix must be created with numpy.array , instead of with python lists. So:

import numpy as np
matriz= np.array([[1,1,1],[1,1,1],[1,1,1]])
print(matriz)

matriz[0]=matriz[0]*2

print("---------------")
print(matriz)

Exit:

[[1 1 1]
 [1 1 1]
 [1 1 1]]
---------------
[[2 2 2]
 [1 1 1]
 [1 1 1]]

Basically what you're doing is this:

print([1,1,1]*2)
[1, 1, 1, 1, 1, 1]

[1,1,1] is a common Python list, the __mul__ method or specifically the * when you do [1,1,1]*2 is not an arithmetic multiplication, according to documentation for data type Sequence (a list is):

  

s * n or n * s equivalent to adding itself to times

In other words, do: [1,1,1]*2 equals replicating [1,1,1] twice. The way to achieve multiplication, from a list of lists of python (matrix) would be to do it through an understanding, as follows:

matriz=[[1,1,1],[1,1,1],[1,1,1]]

matriz[0] = [a*2 for a in matriz[0]]
print(matriz)
[[2, 2, 2], [1, 1, 1], [1, 1, 1]]

But if in fact, you are going to use numpy the response of abulafia is the appropriate one.

This is a question a bit simple but I can not solve it on my own, and I have not found any post related to this topic, my question is this:

import numpy as np

matriz = np.array([[1,1,1],
                   [1,1,1],
                   [1,1,1]])

matriz[0] *= 2

What I want to do is multiply by two each grid in row 0, so that it looks like this:

>>> matriz
array([[2, 2, 2],
       [1, 1, 1],
       [1, 1, 1]])

And not like this:

matriz = [[1,1,1],
          [1,1,1],
          [1,1,1]]


for i in range(len(matriz[0])):
    matriz[0][i] *= 2

It would not be inconvenient for me to use numpy, but I have not been able to solve the problem with numpy either.

Greetings, thank you.

With numpy it is trivial, in fact it works as you have written it. But yes, your matrix must be created with *= , instead of with python lists. So:

matriz = [[1,1,1],
          [1,1,1],
          [1,1,1]]

matriz[0] = [n * 2 for n in matriz[0]]

Exit:

import numpy as np

matriz = np.array([[1,1,1],
                   [1,1,1],
                   [1,1,1]])

matriz[0] *= 2

Basically what you're doing is this:

>>> matriz
array([[2, 2, 2],
       [1, 1, 1],
       [1, 1, 1]])

for is a common Python list, the %code% method or specifically the %code% when you do %code% is not an arithmetic multiplication, according to documentation for data type Sequence (a list is):

  

s * n or n * s equivalent to adding itself to times

In other words, do: %code% equals replicating %code% twice. The way to achieve multiplication, from a list of lists of python (matrix) would be to do it through an understanding, as follows:

matriz = [[1,1,1],
          [1,1,1],
          [1,1,1]]


for i in range(len(matriz[0])):
    matriz[0][i] *= 2

But if in fact, you are going to use %code% the response of abulafia is the appropriate one.

Numpy allows this currently using the assignment operator %code% , performing the operation directly on the array elements:

matriz = [[1,1,1],
          [1,1,1],
          [1,1,1]]

matriz[0] = [n * 2 for n in matriz[0]]

%pre%

With a Python list you have several options but it is more cumbersome and less efficient, for example:

• Use a %code% to go through the list and do the in-place operation:

  %pre%

• Using list compression:

  %pre%