Can't set array values through iterator

Question

using python 3.4

ranks = [[0 for y in range(0,len(graph))] for x in range(0,iterSites)]
for site in ranks[0]:
    site = 2
print(ranks)

Output:

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

Why does "site = 2" not set every value of ranks[0] to 2?

And ... you don't need a comprehension to build the original list: ```ranks = [[0]*len(graph)]*len(iterSites)``` — wwii, May 13 '14 at 14:28

anirudh · Answer 1 · 2014-05-13T14:44:14.920

1

site is a local variable, whose value will not be assigned to the value of the list. Change the code as shown below:

ranks = [[0 for y in range(0,len(graph))] for x in range(0,iterSites)]
for site in range(len(ranks[0])):
    ranks[0][site] = 2
print(ranks)

edited May 13 '14 at 14:44

answered May 13 '14 at 14:17

anirudh

4,116
2
20
35

score 1 · Accepted Answer · answered May 13 '14 at 14:19

1

Because site is a copy of ranks[0][x] not the real value . What you have to do is something like:

for site in range(len(ranks[0])):
    ranks[0][site] = 2
print(ranks)

answered May 13 '14 at 14:19

billpcs

633
10
17

Please look at @anirudh answer it's exactly the same. – Kobi K May 13 '14 at 14:35

Kobi K · Answer 3 · 2014-05-13T14:22:57.730

first of all you are doing site = 2 while you need to update rank

second ranks[0] is a list not a cell inside a list, so your iteration is not right.

You can change you code as follows:

ranks = [[0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0]]

for idx, value in enumerate(ranks[0][:]):
    ranks[0][idx] = 2

print ranks

Output:

[[2, 2, 2, 2, 2, 2, 2], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0]]

Can't set array values through iterator

3 Answers3