CODEWITHSUNDEEP
listpython-2.7append
AK9309
AK9309

Reputation: 791

For loop, list append

I have a simple code

a_list=[1,2,3,4,5]
a2_list=[]
for x in a_list:
    a2_list.append(x*2)

and I get a2_list=[2,4,6,8,10]

If I write code like

a_list=[1,2,3,4,5]
a2_list=[]
for x in a_list:
    a2_list.append(x*2)
    print a2_list

I get

[2]
[2,4]
[2,4,6]
[2,4,6,8]
[2,4,6,8,10]

I want to make a list of lists to record each step

a_list=[1,2,3,4,5]
a2_list=[]
b_list=[]
for x in a_list:
    a2_list.append(x*2)
    b_list.append(a2_list)

I would like to get b_list = [[2],[2,4],[2,4,6],[2,4,6,8],[2,4,6,8,10]]

but I get b_list=[[2,4,6,8,10],[2,4,6,8,10],[2,4,6,8,10],[2,4,6,8,10],[2,4,6,8,10]]

It seems like a very easy problem, but I can't figure what I am doing wrong

Upvotes: 0

Views: 85

Answers (3)

Trent
Trent

Reputation: 36

While the solutions above work fine, list.copy() is not only more readable, but also faster as of Python 3.7. See this discussion on all the ways to copy a list. Therefore, this solution is the most intuitive to me.

a_list=[1,2,3,4,5]
a2_list=[]
b_list=[]
for x in a_list:
    a2_list.append(x*2)
    b_list.append(a2_list.copy())
b_list

Upvotes: 0

Mohammad Mustaqeem
Mohammad Mustaqeem

Reputation: 1084

This is because list is mutable object. You should learn about it. The possible solution is to copy the list as others suggested. e.g.

a_list = [1,2,3,4,5]
a_list = list(a_list)
or
a_list = a_list[:]

There are other methods too.

Upvotes: 0

AK9309
AK9309

Reputation: 791

That works

a_list=[1,2,3,4,5]
a2_list=[]
b_list=[]
for x in a_list:
    a2_list.append(x*2)
    b = a2_list[:]
    b_list.append(b)

Upvotes: 2

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