Modifying a matrix using nested list comprehensions in Python

Question

I am assigning values to a nested matrix in a traditional for loop.

    matrix= [[0 for j in range(3)]for i in range(3)]
    value = 10
    # Setting a value to a particular row in the matrix
    for i in range(3):
        if i == 2:
            for j in range(3):
                matrix[i][j] = 10
    # Setting a value to a particular column in the matrix
    val  = 20
    for i in range(3):
        for j in range(3):
            if j == 1:
                matrix[i][j] = 20

Can assigning of the values to the matrix be done in a nested list comprehension? I did try this :

                matrix = [[value for j in i if j == col ]for i in matrix]

But it isn't modifying the matrix, instead creates a new one.How can I accomplish this with nested list comprehensions?

matrix[:] = [[value for j in i if j == col ]for i in matrix] would change the original list but I don't think your examples are the same — Padraic Cunningham
– Padraic Cunningham, Commented Jun 10, 2015 at 14:46

asimoneau · Accepted Answer · 2015-06-10 14:58:24Z

4

Your example can easily be done using numpy arrays.

import numpy as np
matrix = np.zeros((3,3))

# Setting a row
matrix[1] = 10

# Setting a column
matrix[:,1] = 20

answered Jun 10, 2015 at 14:58

asimoneau

6927 silver badges18 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

lukkol · Accepted Answer · 2015-06-10 15:19:38Z

0

The following comprehension does the job:

matrix= [
    [
        20 if j == 1 else 10 if i == 2 else 0
        for j in range(3)
    ]
    for i in range(3)
]

It will produce:

[[0, 20, 0], [0, 20, 0], [10, 20, 10]]

In your solution you don't have to do go through all cells twice.

matrix = [[0 for j in range(3)] for i in range(3)]
for i in range(3):
    matrix[2][i] = 10
    matrix[i][1] = 20

answered Jun 10, 2015 at 15:19

lukkol

644 bronze badges

2 Comments

Arun Kumar Mohan Over a year ago

What if I would like to modify the existing matrix by assigning a value to a particular row? As far as i know,your solution(list comprehension ) just creates a new list.If I am wrong,please explain.

lukkol Over a year ago

List comprehension I proposed is the only thing you should do (without matrix= [[0 for j in range(3)]for i in range(3)]). List comprehension will create list but that would be the only list you have to create. If you want just modification than look at the second part of the answer (with for loop).

Collectives™ on Stack Overflow

Modifying a matrix using nested list comprehensions in Python

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related