Strings and Nested For Loops in Python

Question

I have a problem where I want to have a nested for loop concatenate a string. For some reason I'm not getting the correct output. Can someone give me some advice? This should be simple.

newRow = []
matrix = []
for i in range(0,3):
    for j in range(0,5):
        newRow.append(j)
    matrix.append(newRow)
print matrix

python test.py

[[0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4], 
[0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4], 
[0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4]]

I want it to print...

[[0, 1, 2, 3, 4],
[0, 1, 2, 3, 4],
[0, 1, 2, 3, 4]]

That's not a string, that's not concatenating, and your indentation is off. — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Feb 19, 2012 at 2:59

rid · Accepted Answer · 2012-02-19 03:11:52Z

5

You keep adding to the same newRow. If you want to get a clean row, you need to create an empty one on each iteration:

matrix = []                                      newRow = [] <- the same array
for i in range(0,3):                             matrix = []
    newRow = [] <- a new array                   for i in range(0,3):
    for j in range(0,5):         instead of          for j in range(0,5):
        newRow.append(j)                                 newRow.append(j)
    matrix.append(newRow)                            matrix.append(newRow)

There are, of course, more pythonic ways of achieving this, but I'm trying to point out what the problem with the code is.

edited Feb 19, 2012 at 3:11

answered Feb 19, 2012 at 2:59

rid

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Comments

lpostula · Accepted Answer · 2012-02-19 03:09:07Z

3

You can make this if you want nested loop:

matrix = [ [  i for i in range(5)] for j in range(3)]
print matrix 
[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

edited Feb 19, 2012 at 3:09

answered Feb 19, 2012 at 3:00

lpostula

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Matthew Flaschen · Accepted Answer · 2012-02-19 03:00:21Z

1

matrix = []
for i in range(0,3):
    matrix.append(list(range(0, 5)))

or even:

matrix = [list(range(0, 5)) for i in range(0, 3)]

answered Feb 19, 2012 at 3:00

Matthew Flaschen

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Stedy · Accepted Answer · 2012-02-19 03:02:26Z

1

You are close, its just a matter of properly scoping:

matrix = []
for i in range(0,3):
    newRow = []
    for j in range(0,5):
        newRow.append(j)
    matrix.append(newRow)
print matrix

answered Feb 19, 2012 at 3:02

Stedy

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machine yearning · Accepted Answer · 2012-02-20 06:10:39Z

0

Whatever you do,

DON'T DO THIS

DON'T DO THIS:

>>> x = [range(0,5)]*3
>>> x
[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]
>>> x[0][0] = 'LOL'
>>> x
[['LOL', 1, 2, 3, 4], ['LOL', 1, 2, 3, 4], ['LOL', 1, 2, 3, 4]]

use a list comprehension.

edited Feb 20, 2012 at 6:10

answered Feb 19, 2012 at 4:32

machine yearning

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1 Comment

Matthew Flaschen Over a year ago

That gives you three references to the same row. If you set matrix[0][0], you're also effectively setting matrix[1][0].

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