Issue changing single item in a list of lists - Python

Question

Okay. So I'm in Programing fundamentals and for our final project we have to design a game with a map. I created a program that creates a list of ten lists with ten items in each. However when I try to change a single item in one list (for example mapGrid[4][5] = "i") it changes all of the 6th items in all of the lists. Here is the code I'm using to test it:

import random

def createMap():

    mapGrid = []
    col = []
    randx = 0
    randy = 0
    i = 0

    for x in range(0,10):
        col += "#"

    for y in range(0,10):
        mapGrid.append(col)

    while i < 10:
        randx = random.randint(0,9)
        randy = random.randint(0,9)
        if mapGrid[randx][randy] != "i":
            mapGrid[randx][randy] = "i"
            i += 1

    return mapGrid

def printMap(x,y,mapGrid):

    mapGrid[x][y] = "0"

    print("",mapGrid[x-1][y+1],mapGrid[x][y+1],mapGrid[x+1][y+1],"\n",
          mapGrid[x-1][y],mapGrid[x][y],mapGrid[x+1][y],"\n",
          mapGrid[x-1][y-1],mapGrid[x][y-1],mapGrid[x+1][y-1])

examp = createMap()
print(examp)
print("")
printMap(4,4,examp)

and the result I keep getting is:

[['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i'], ['i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i', 'i']]

 i i i
 0 0 0 
 i i i

instead of there being ten single i's and one '0' it becomes all i's and every 5th item in each list is a 0.

How do I fix it so I only change a single item instead of an item in every list?

Scis · Accepted Answer · 2015-04-10 14:50:19Z

1

You're appending the same col list several times, what you want is to create a new copy each time:

mapGrid.append(col[:]) # a copy of col

See live example

answered Apr 10, 2015 at 14:50

Scis

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Comments

aleju · Accepted Answer · 2015-04-10 14:50:27Z

You are adding 10 times the identical list col to your map grid, it's not a copy each time. So every change in your while loop affects every column/sublist in your mapGrid in the same way - not just one of them.

Changing

for y in range(0,10):
        mapGrid.append(col)

To

for y in range(0,10):
        mapGrid.append(list(col))

Outputs

[['i', '#', '#', '#', '#', '#', '#', '#', '#', '#'], ['#', '#', '#', '#', 'i', '
#', 'i', '#', '#', '#'], ['#', '#', '#', '#', '#', '#', '#', '#', '#', '#'], ['#
', '#', '#', '#', '#', '#', '#', '#', '#', '#'], ['#', '#', '#', '#', '#', '#',
'i', '#', '#', '#'], ['#', '#', '#', '#', 'i', '#', '#', 'i', '#', '#'], ['#', '
#', '#', '#', '#', 'i', '#', '#', '#', '#'], ['#', '#', 'i', '#', '#', '#', '#',
 '#', '#', '#'], ['#', 'i', '#', '#', '#', '#', '#', '#', '#', '#'], ['#', '#',
'#', '#', '#', 'i', '#', '#', '#', '#']]

('', '#', '#', '#', '\n', '#', '0', 'i', '\n', '#', '#', '#')

declension · Accepted Answer · 2015-04-10 14:51:43Z

0

The problem is you've created a list with ten references to the same list, in this:

for y in range(0,10):
    mapGrid.append(col)

You need to construct a new list each time - you can do this using col[:], as per this FAQ entry, or by constructing a new one explicitly with list(col).

answered Apr 10, 2015 at 14:51

declension

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