Nested for loop updates incorrect array values python

Asked Aug 17 '22 at 17:06

Active Aug 17 '22 at 17:06

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Apologies in advance for for the noob question

My input code is

mat = [[0,0,1], [1,0,0], [0,0,0]]
arr = [[float('inf')]*3]*3
for i in range(3):
    for j in range(3):
         if mat[i][j] == 0: 
               arr[i][j] = 0
print(arr)

Desired output:

[[0, 0, inf], [inf, 0, 0], [0, 0, 0]]

Actual output:

[[0,0,0], [0,0,0], [0,0,0]]

What am I doing wrong, since the conditional is seemingly being ignored?

asked Aug 17 '22 at 17:06

dpbrewer

2

`arr = [[float('inf')]*3]*3` creates 3 references to the same list. – Code-Apprentice Aug 17 '22 at 17:07
In your particular case, you can use a list comprehension: `arr = [[0 if x == 0 else math.inf for x in row] for row in mat]`. Also you can use `math.inf` instead of `float('inf')`. – Code-Apprentice Aug 17 '22 at 17:10

Nested for loop updates incorrect array values python

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