Python loop through lists and append according to conditions

Question

I have 3 very lengthy lists (of length 15,000). Lets say for example the three lists are:

A    B    C
0    2    3
0    4    5
0    3    3
1    2    6
1    3    5
0    2    7
1    8    8

I would like to get all those values of B and C that are where the corresponding index of A is 0. For example if A[i] == 0 then I would like to add B[i] to listB_0 and C[i] to listC_0.

I tried

listB_0 = []
listC_0 = []

for a,b,c in zip(A,B,C):
    if a == 0:
        listB_0.append(B)
        listC_0.append(C)

But this seems to put Python through a never ending loop, even after 5 minutes, I see that the program is still running.

What I finally want is, for example the listB and listC for which listA = 0 will be

listB_0 = [2,4,3,2]
listC_0 = [3,5,3,7]

What is the correct way to achieve this?

It looks like you are appending the entire list B instead of the element b. — Brobin
– Brobin, Commented Jul 14, 2015 at 13:21
that was my guess as well!! How do I append according to the above condition? — user3397243
– user3397243, Commented Jul 14, 2015 at 13:21

Mikhail Gerasimov · Accepted Answer · 2015-07-14 13:26:36Z

2

As noted in comments, you should append b instead of B. I want to note, that you can use list comprehension instead of loop to get results "pythonic" way.

A = [0, 0, 0, 1, 1, 0, 1]
B = [2, 4, 3, 2, 3, 2, 8]

listB_0 = [b for a, b in zip(A, B) if a == 0]
print(listB_0)  # [2, 4, 3, 2]

answered Jul 14, 2015 at 13:26

Mikhail Gerasimov

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Comments

Community · Accepted Answer · 2017-05-23 10:28:32Z

1

Brobin already pointed this out in his comment: Instead of b or c, the whole lists B or C get appended.

This should work:

A = [0, 0, 0, 1, 1, 0, 1]
B = [2, 4, 3, 2, 3, 2, 8]
C = [3, 5, 3, 6, 5, 7, 8]

listB_0 = []
listC_0 = []

for a, b, c in zip(A,B,C):
    if a == 0:
        listB_0.append(b)
        listC_0.append(c)

print listB_0
print listC_0

>>> 
[2, 4, 3, 2]
[3, 5, 3, 7]

edited May 23, 2017 at 10:28

CommunityBot

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answered Jul 14, 2015 at 13:25

adrianus

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Comments

ILostMySpoon · Accepted Answer · 2015-07-14 13:26:19Z

1

You want to append the values b for listB_0 and c for listC_0, not the lists themselves.

for a,b,c in zip(A,B,C):
    if a == 0:
        listB_0.append(b)
        listC_0.append(c)

answered Jul 14, 2015 at 13:26

ILostMySpoon

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Comments

kylieCatt · Accepted Answer · 2015-07-14 13:28:06Z

1

There is really no need for zip() here:

# use xrange(len(A)) if in Python 2
for i in range(len(A)):
    if A[i] == 0:
        listB_0.append(B[i])
        listC_0.append(C[i])

answered Jul 14, 2015 at 13:28

kylieCatt

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