Nested List and For Loop

Question

Consider this:

list = 2*[2*[0]]
for y in range(0,2):
  for x in range(0,2):
    if x ==0:
      list[x][y]=1
    else:
      list[x][y]=2
print list

Result:

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

Why doesn't the result be [[1,1],[2,2]]?

John La Rooy · Accepted Answer · 2011-04-21 06:12:59Z

12

Because you are creating a list that is two references to the same sublist

>>> L = 2*[2*[0]]
>>> id(L[0])
3078300332L
>>> id(L[1])
3078300332L

so changes to L[0] will affect L[1] because they are the same list

The usual way to do what you want would be

>>> L = [[0]*2 for x in range(2)]
>>> id(L[0])
3078302124L
>>> id(L[1])
3078302220L

notice that L[0] and L[1] are now distinct

answered Apr 21, 2011 at 6:12

John La Rooy

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2 Comments

James Khoury Over a year ago

yep! try list = [[0,0],[0,0]] instead.

Blender Over a year ago

Whoa. I was debugging this thing for a while, and it didn't seem to make sense. I added print statements, and it outputted the right values, but I didn't think that it would be like that!

Martijn Pieters · Accepted Answer · 2012-05-26 15:33:24Z

0

Alternatively to save space:

>>> [[x,x] for x in xrange(1,3)]

edited May 26, 2012 at 15:33

Martijn Pieters

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answered Apr 21, 2011 at 22:34

marty

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