Iterating in a area contained in a multidimensional array in python, why it doesn't work? [duplicate]

Question

I need to fill a area in a 10*10 matrix with a real number and for this i have made this code in python:

# 'x' and 'x1' are points in a surface that delimit the area
[x, x1, y, y1] = [0, 5, 0, 2]
surfaceXY = [[0]*10]*10

for i in range(x, x1):
  for j in range(y, y1):
    surfaceXY[i][j] = 5

for k in range(10):
  for l in range(10):
    print surfaceXY[k][l],
  print ""

i want to output this:

5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 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 
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 0 0

but the code will output is:

5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0

Can someone explain me why, and how is the right way to solve this problem in python?

Many thanks everyone, i have spent 3 hours in this problem and you guys discover answer in seconds =] — Glauco Amigo do Jonas
– Glauco Amigo do Jonas, Commented Dec 12, 2013 at 5:36

Community · Accepted Answer · 2017-05-23 10:32:18Z

1

Note that [[0]*10]*10 does not create a list of ten lists with ten zeroes each. It creates a list with one list, with ten zeroes in it.

Since there is only one list, changes the whole column in your grid.

Use

[[0]*10 for _ in range(10)]

See 2d array of zeros for additional discussion.

edited May 23, 2017 at 10:32

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answered Dec 12, 2013 at 5:26

Paul Draper

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Comments

Matt Williamson · Accepted Answer · 2013-12-12 05:34:35Z

For me it looks like each of the rows have the same id (memory location) meaning they are the same data structure.

>>> id(surfaceXY[0])
4548151416
>>> id(surfaceXY[1])
4548151416

So this means that the mult operator of a list makes a copy of the pointer and not a copy of the data.

Try this initializer:

surfaceXY = [[0]*10 for i in range(10)]

Test:

>>> [x, x1, y, y1] = [0, 5, 0, 2]
>>> surfaceXY = [[0]*10 for i in range(10)]
>>> for i in range(x, x1):
...   for j in range(y, y1):
...     surfaceXY[i][j] = 5
... 
>>> for k in range(10):
...   for l in range(10):
...     print surfaceXY[k][l],
...   print ""
... 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 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 
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 0 0 
>>>

aIKid · Accepted Answer · 2013-12-12 05:32:07Z

The problem is with your list creation. This:

surfaceXY = [[0]*10]*10

Creates ten references to a same list, or here, all rows you're making is actually pointing to one row only. When you change one of it, all of them is modified. Thus the result you are having.

To solve this, one would use this:

surfaceXY = [[0 for _ in range(10)] for _ in range(10)]

Or alternatively:

surfaceXY = [[0]*10 for _ in range(10)]

Here's my run with your modified code :)

>>> surfaceXY = [[0 for _ in range(10)] for _ in range(10)]
>>> for i in range(x, x1):
  for j in range(y, y1):
    surfaceXY[i][j] = 5


>>> for k in range(10):
  for l in range(10):
    print (surfaceXY[k][l], end=' ')
  print('', end='\n')


5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 0 0 0 0 0 0 0 0 
5 5 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 
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 0 0

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