how to modify this python for loop?

Question

i'm having strange behavior from the for loop in python. The problem is not exactly this one but very similar to :

a = []
b = [1,2,3,4]
for i in xrange (0,10):
     a.append(b)
     b[3] += 1

And the result is :

a = [[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14],[1,2,3,14]]

The result i'm expecting is

a =  [[1,2,3,4],[1,2,3,5],[1,2,3,6],[1,2,3,7],.....,[1,2,3,14]]

I do not know why at each iteration, b[3] is added up to 14 and then the list [1,2,3,14] is added to a. I think b[3] should only increase by 1 at each iteration

This is because you are appending a reference to b to a at each iteration of the loop, not a copy of b. Each reference points to the same b, which, at the end of your loop, has b[3] = 14. — Chris
– Chris, Commented Jun 20, 2012 at 11:16
Try pasting the code in this python code visualizer to see exactly what is happening. — Lauritz V. Thaulow
– Lauritz V. Thaulow, Commented Jun 20, 2012 at 11:18
@lazyr wow, there are even such things, but seems it is problematic, because even for simple code segments as such, it crashes. — zinking
– zinking, Commented Jun 20, 2012 at 12:21

bdew · Accepted Answer · 2012-06-20 11:17:53Z

7

Your problem is that every iteration you append a reference to the same array, and keep changing it.

The simplest fix is to change the append to

 a.append(list(b))

This will make every iteration append a (shallow) copy to the target array, instead of a reference.

answered Jun 20, 2012 at 11:17

bdew

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mVChr · Accepted Answer · 2012-06-20 11:18:52Z

4

b is accessed by reference, so when you modify b[3] it's affecting every reference of it that you've appended to a over and over again. To fix this you just need to create a new copy of b each time:

a = []
b = [1,2,3,4]
for i in xrange (0,10):
     a.append(b[:])
     b[3] += 1

answered Jun 20, 2012 at 11:18

mVChr

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1 Comment

Maria Zverina Over a year ago

list(b) is much more novice friendly syntax for b[:]

Ashwini Chaudhary · Accepted Answer · 2012-06-20 11:33:25Z

2

you can use deepcopy:

from copy import deepcopy
a = []
b = [1,2,3,4]
for i in xrange (0,10):
     a.append(deepcopy(b))
     b[3] += 1

answered Jun 20, 2012 at 11:33

Ashwini Chaudhary

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