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]]

Answer 1

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.