python: FOR loop, Iterating assignment not assigning correct value

Question

I am making a list, with a list nested inside. This second list I need to iterate through, and assign a value to a position. So I wrote this

matrix=[]
list=[0,1,2,3,4,5,6,7,8,9]
def board():
    for x in range(10):
        for y in range(10):
            matrix.append([[x,y],1]) #A grid is made, with a spare element
    for i in range(100):
        matrix[i][1]=list            #The grid is two elements, co-ords...
                                     #and a list of attributes


board()
for i in range(100):
    matrix[i][2][0]=i        #The first attribute,(in 2nd of2elements),
print(matrix)                ##SHOULD be its own position

All variations of this code result in the same thing:

Error or the number "99" being placed in EVERY INSTANCE of that location. That is to say my final FOR loop seems to be assigning "99" NOT i...

I have tried several variations of this (generating the numbers inside the FOR loop was my preference, alas) I have spent about 6 hours staring at this. All my logic tells me that the computer should KNOW that I want the first element, of that second element, to be a count 0,1,2... I know this can't be because of a limit on the language, so tell me what I am not seeing.

Do not use built-in names as variable names. I suggest you use a name different from list for your variable, like numbers = [...]. — Bakuriu
– Bakuriu, Commented May 2, 2016 at 13:06
You could use an if statement in the x y loop to assign list to a matrix element couldn't you? — OneCricketeer
– OneCricketeer, Commented May 2, 2016 at 13:07
Plus, matrix[i][2][0]=i raises an IndexError as your matrix is filled by lists of two elements... — mvelay
– mvelay, Commented May 2, 2016 at 13:13

R Singh · Accepted Answer · 2016-05-02 13:13:15Z

2

matrix[i][1]=list

When you do this the same array is being assigned in the matrix array. Hence if you change one item, all the items get changed. Instead assign a copy of the list like this:

matrix[i][1] = list[:]

As pointed out by @Bakuriu , in python 3 you can use list.copy() method. In Python 2 you can use the copy module to copy containers with l2 = copy.copy(l1)

Also, you should change the name of the variable 'list' as 'list' is an inbuilt type in python.

edited May 2, 2016 at 13:13

answered May 2, 2016 at 13:04

R Singh

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3 Comments

Bakuriu Over a year ago

Note: in python3 all containers have a copy method so you could write: matrix[i][1] = list.copy(). this is, IMO, clearer than the [:] syntax.

scarecrow Over a year ago

OOOH, ok so if i understand: that particular item in the list was being updated to be the same as every other time i updated that item in another location. I knew this happened with variables, so that the original variable changed like... a=6...b=a.....b=7 and then a suddenly equals 7...(I cant imagine when that would be useful(yet))

scarecrow Over a year ago

Thanks to everyone who helped, and pointed that one style gaffe

mvelay · Accepted Answer · 2016-05-02 13:41:38Z

0

You should replace matrix[i][2][0]=i assignement as it raises an IndexError to:

for i in range(100):
    matrix[i][1][0]=i        #The first attribute,(in 2nd of2elements),
print(matrix)                ##SHOULD be its own position

answered May 2, 2016 at 13:41

mvelay

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1 Comment

scarecrow Over a year ago

Quite right. I had left that in from another version attempt wherein the list WAS the third item i.e. matrix =[x,y,list]

yourstruly · Accepted Answer · 2016-05-03 11:00:40Z

Lists are faster in for loop, but here You have solution with dicts:

from pprint import pprint as ppr

matrix={}
list=[0,1,2,3,4,5,6,7,8,9]
def board():
    for x in range(10):
        for y in range(10):
            matrix[(x,y)]=1 
    for k in matrix.keys():
        matrix[k]=list

board()
ppr(matrix)

for k in matrix.iterkeys():
    v=matrix[k][:]   
    v[2]=k[0]*10+k[1]
    matrix[k]=v
ppr(matrix)

Input:

{(0, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (1, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (2, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (3, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (4, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (5, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (6, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (7, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (8, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 0): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 1): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 3): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 4): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 5): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 6): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 7): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 8): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (9, 9): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]}

Output:

{(0, 0): [0, 1, 0, 3, 4, 5, 6, 7, 8, 9],
 (0, 1): [0, 1, 1, 3, 4, 5, 6, 7, 8, 9],
 (0, 2): [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
 (0, 3): [0, 1, 3, 3, 4, 5, 6, 7, 8, 9],
 (0, 4): [0, 1, 4, 3, 4, 5, 6, 7, 8, 9],
 (0, 5): [0, 1, 5, 3, 4, 5, 6, 7, 8, 9],
 (0, 6): [0, 1, 6, 3, 4, 5, 6, 7, 8, 9],
 (0, 7): [0, 1, 7, 3, 4, 5, 6, 7, 8, 9],
 (0, 8): [0, 1, 8, 3, 4, 5, 6, 7, 8, 9],
 (0, 9): [0, 1, 9, 3, 4, 5, 6, 7, 8, 9],
 (1, 0): [0, 1, 10, 3, 4, 5, 6, 7, 8, 9],
 (1, 1): [0, 1, 11, 3, 4, 5, 6, 7, 8, 9],
 (1, 2): [0, 1, 12, 3, 4, 5, 6, 7, 8, 9],
 (1, 3): [0, 1, 13, 3, 4, 5, 6, 7, 8, 9],
 (1, 4): [0, 1, 14, 3, 4, 5, 6, 7, 8, 9],
 (1, 5): [0, 1, 15, 3, 4, 5, 6, 7, 8, 9],
 (1, 6): [0, 1, 16, 3, 4, 5, 6, 7, 8, 9],
 (1, 7): [0, 1, 17, 3, 4, 5, 6, 7, 8, 9],
 (1, 8): [0, 1, 18, 3, 4, 5, 6, 7, 8, 9],
 (1, 9): [0, 1, 19, 3, 4, 5, 6, 7, 8, 9],
 (2, 0): [0, 1, 20, 3, 4, 5, 6, 7, 8, 9],
 (2, 1): [0, 1, 21, 3, 4, 5, 6, 7, 8, 9],
 (2, 2): [0, 1, 22, 3, 4, 5, 6, 7, 8, 9],
 (2, 3): [0, 1, 23, 3, 4, 5, 6, 7, 8, 9],
 (2, 4): [0, 1, 24, 3, 4, 5, 6, 7, 8, 9],
 (2, 5): [0, 1, 25, 3, 4, 5, 6, 7, 8, 9],
 (2, 6): [0, 1, 26, 3, 4, 5, 6, 7, 8, 9],
 (2, 7): [0, 1, 27, 3, 4, 5, 6, 7, 8, 9],
 (2, 8): [0, 1, 28, 3, 4, 5, 6, 7, 8, 9],
 (2, 9): [0, 1, 29, 3, 4, 5, 6, 7, 8, 9],
 (3, 0): [0, 1, 30, 3, 4, 5, 6, 7, 8, 9],
 (3, 1): [0, 1, 31, 3, 4, 5, 6, 7, 8, 9],
 (3, 2): [0, 1, 32, 3, 4, 5, 6, 7, 8, 9],
 (3, 3): [0, 1, 33, 3, 4, 5, 6, 7, 8, 9],
 (3, 4): [0, 1, 34, 3, 4, 5, 6, 7, 8, 9],
 (3, 5): [0, 1, 35, 3, 4, 5, 6, 7, 8, 9],
 (3, 6): [0, 1, 36, 3, 4, 5, 6, 7, 8, 9],
 (3, 7): [0, 1, 37, 3, 4, 5, 6, 7, 8, 9],
 (3, 8): [0, 1, 38, 3, 4, 5, 6, 7, 8, 9],
 (3, 9): [0, 1, 39, 3, 4, 5, 6, 7, 8, 9],
 (4, 0): [0, 1, 40, 3, 4, 5, 6, 7, 8, 9],
 (4, 1): [0, 1, 41, 3, 4, 5, 6, 7, 8, 9],
 (4, 2): [0, 1, 42, 3, 4, 5, 6, 7, 8, 9],
 (4, 3): [0, 1, 43, 3, 4, 5, 6, 7, 8, 9],
 (4, 4): [0, 1, 44, 3, 4, 5, 6, 7, 8, 9],
 (4, 5): [0, 1, 45, 3, 4, 5, 6, 7, 8, 9],
 (4, 6): [0, 1, 46, 3, 4, 5, 6, 7, 8, 9],
 (4, 7): [0, 1, 47, 3, 4, 5, 6, 7, 8, 9],
 (4, 8): [0, 1, 48, 3, 4, 5, 6, 7, 8, 9],
 (4, 9): [0, 1, 49, 3, 4, 5, 6, 7, 8, 9],
 (5, 0): [0, 1, 50, 3, 4, 5, 6, 7, 8, 9],
 (5, 1): [0, 1, 51, 3, 4, 5, 6, 7, 8, 9],
 (5, 2): [0, 1, 52, 3, 4, 5, 6, 7, 8, 9],
 (5, 3): [0, 1, 53, 3, 4, 5, 6, 7, 8, 9],
 (5, 4): [0, 1, 54, 3, 4, 5, 6, 7, 8, 9],
 (5, 5): [0, 1, 55, 3, 4, 5, 6, 7, 8, 9],
 (5, 6): [0, 1, 56, 3, 4, 5, 6, 7, 8, 9],
 (5, 7): [0, 1, 57, 3, 4, 5, 6, 7, 8, 9],
 (5, 8): [0, 1, 58, 3, 4, 5, 6, 7, 8, 9],
 (5, 9): [0, 1, 59, 3, 4, 5, 6, 7, 8, 9],
 (6, 0): [0, 1, 60, 3, 4, 5, 6, 7, 8, 9],
 (6, 1): [0, 1, 61, 3, 4, 5, 6, 7, 8, 9],
 (6, 2): [0, 1, 62, 3, 4, 5, 6, 7, 8, 9],
 (6, 3): [0, 1, 63, 3, 4, 5, 6, 7, 8, 9],
 (6, 4): [0, 1, 64, 3, 4, 5, 6, 7, 8, 9],
 (6, 5): [0, 1, 65, 3, 4, 5, 6, 7, 8, 9],
 (6, 6): [0, 1, 66, 3, 4, 5, 6, 7, 8, 9],
 (6, 7): [0, 1, 67, 3, 4, 5, 6, 7, 8, 9],
 (6, 8): [0, 1, 68, 3, 4, 5, 6, 7, 8, 9],
 (6, 9): [0, 1, 69, 3, 4, 5, 6, 7, 8, 9],
 (7, 0): [0, 1, 70, 3, 4, 5, 6, 7, 8, 9],
 (7, 1): [0, 1, 71, 3, 4, 5, 6, 7, 8, 9],
 (7, 2): [0, 1, 72, 3, 4, 5, 6, 7, 8, 9],
 (7, 3): [0, 1, 73, 3, 4, 5, 6, 7, 8, 9],
 (7, 4): [0, 1, 74, 3, 4, 5, 6, 7, 8, 9],
 (7, 5): [0, 1, 75, 3, 4, 5, 6, 7, 8, 9],
 (7, 6): [0, 1, 76, 3, 4, 5, 6, 7, 8, 9],
 (7, 7): [0, 1, 77, 3, 4, 5, 6, 7, 8, 9],
 (7, 8): [0, 1, 78, 3, 4, 5, 6, 7, 8, 9],
 (7, 9): [0, 1, 79, 3, 4, 5, 6, 7, 8, 9],
 (8, 0): [0, 1, 80, 3, 4, 5, 6, 7, 8, 9],
 (8, 1): [0, 1, 81, 3, 4, 5, 6, 7, 8, 9],
 (8, 2): [0, 1, 82, 3, 4, 5, 6, 7, 8, 9],
 (8, 3): [0, 1, 83, 3, 4, 5, 6, 7, 8, 9],
 (8, 4): [0, 1, 84, 3, 4, 5, 6, 7, 8, 9],
 (8, 5): [0, 1, 85, 3, 4, 5, 6, 7, 8, 9],
 (8, 6): [0, 1, 86, 3, 4, 5, 6, 7, 8, 9],
 (8, 7): [0, 1, 87, 3, 4, 5, 6, 7, 8, 9],
 (8, 8): [0, 1, 88, 3, 4, 5, 6, 7, 8, 9],
 (8, 9): [0, 1, 89, 3, 4, 5, 6, 7, 8, 9],
 (9, 0): [0, 1, 90, 3, 4, 5, 6, 7, 8, 9],
 (9, 1): [0, 1, 91, 3, 4, 5, 6, 7, 8, 9],
 (9, 2): [0, 1, 92, 3, 4, 5, 6, 7, 8, 9],
 (9, 3): [0, 1, 93, 3, 4, 5, 6, 7, 8, 9],
 (9, 4): [0, 1, 94, 3, 4, 5, 6, 7, 8, 9],
 (9, 5): [0, 1, 95, 3, 4, 5, 6, 7, 8, 9],
 (9, 6): [0, 1, 96, 3, 4, 5, 6, 7, 8, 9],
 (9, 7): [0, 1, 97, 3, 4, 5, 6, 7, 8, 9],
 (9, 8): [0, 1, 98, 3, 4, 5, 6, 7, 8, 9],
 (9, 9): [0, 1, 99, 3, 4, 5, 6, 7, 8, 9]}

You could use numpy here, with fast indexing (faster is cython memoryview so look at it too):

matrix=np.zeros((100,10),dtype=int)
list=[0,1,2,3,4,5,6,7,8,9]
def board(matrix):
    for x in range(10):
        for y in range(10):
            matrix[x*10+y]=1 #but here You populate whole sub-array of shape (10) with 1, so it will be: [1,1,1,1...,1] 
    print matrix
    for k in range(matrix.shape[0]):
        matrix[k]=list

board(matrix)
ppr(matrix)

for k in range(matrix.shape[0]):
    matrix[k,2]=k
ppr(matrix)

I trust that this is useful too, but I haven't worked with dicts so I will use your example to experiment with them, to learn them. Thanks for it :)

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