modifying one element modifies the whole column in python [duplicate]

Question

x = raw_input("")
y = raw_input("")
a = []  
b = []
count = 1

for i in range(0, int(y)):
    b.append(count)
    count+=1

for i in range(0, int(x)):
    a.append(b)

for i in a:
    print ""
    for j in i:
        print j, 

a[1][1] = 0



for i in a:
    print ""
    for j in i:
        print j,

a has been created by appending list "b" n time to it Now when i modify a[1][1] the whole column that is a[0][1] - a[n][1] gets modified to that value

Can anyone explain why this is happening

BrenBarn · Accepted Answer · 2014-05-16 06:26:02Z

1

Every time you append b, you are appending the same list -- not copies of the list, but multiple references to the same object. If you want each row to be a different list, you need to append a new list each time, by doing a.append(b[:]).

answered May 16, 2014 at 6:26

BrenBarn

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sshashank124 · Accepted Answer · 2014-05-16 06:25:49Z

1

That is because by appending b, you are creating pointers to the same objects. Instead, make a copy as follows:

for i in range(0, int(x)):
    a.append(b[:])

You can see it working as expected, here

answered May 16, 2014 at 6:25

sshashank124

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