CODEWITHSUNDEEP
pythonpython-3.x
regishumphy
regishumphy

Reputation: 23

How do I remove a value from a list, and remove a value in the same position from another list

Lets say I have two lists:

x=[1,0,0,3,2,5,6,0,4,2]

y=[e,r,g,d,e,w,t,y,t,r]

I want to remove all the zeros from the "x" list, and all values correspond to the position of the zeros from the "y" list, such that in the end:

x=[1,3,2,5,6,4,2]

y=[e,d,e,w,t,t,r]

I tried:

for i in range(len(x)):
    if x[i]==0:
        del x[i]
        del y[i]
return x
return y

However, I realized that as the element in the i-th index position gets deleted, the range changes. Is there a better loop or method I can implement.

Upvotes: 2

Views: 1077

Answers (4)

ufoxDan
ufoxDan

Reputation: 609

You can do this in one line with a list comprehension:

result = [[x[i], y[i]] for i in range(len(x)) if x[i] != 0]

Then you have a list of the pairs, which you can quickly get back into your original format with numpy:

import numpy as np
resultX = np.array(result)[:, 0]
resultY = np.array(result)[:, 1]

Upvotes: 0

Kenivia
Kenivia

Reputation: 384

aha, I spent a super long time trying to figure this out when I needed it. Glad you dont have to go through it.

def list_del(inp,poss):
'''delete items at indexes given in the list poss from list inp'''
try:
    assert(isinstance(inp,list))
except AssertionError:
    raise(AssertionError('input has to be a list, if numpy, using np.delete'))
inpu = inp

pos = 0
for i in list(sorted(poss)):
    del inpu[i-pos]
    pos += 1
return inpu

example:

list_del([0,1,2,3,4],[0,2,-1])

returns 

[1, 2]

In your case:

for i in list(x):
    if i == 0:
        deleting.append(i)
list_del(x,deleting)
list_del(y,deleting)

Upvotes: 0

Alex Hall
Alex Hall

Reputation: 36033

Instead of deleting, you can construct new lists:

x = [1, 0, 0, 3, 2, 5, 6, 0, 4, 2]

y = 'e,r,g,d,e,w,t,y,t,r'.split(',')

x, y = zip(*[
    (xe, ye)
    for xe, ye in zip(x, y)
    if xe != 0
])

print(x, y)

Upvotes: 3

kaya3
kaya3

Reputation: 51063

The problem is that when you delete an element from a list, all the later indices get shifted backwards by 1 so that there's no gap left by what you deleted. The earlier indices, however, remain untouched; so the simple fix is to iterate over the indices backwards.

def remove_zeros(x, y):
    for i in reversed(range(len(x))):
        if x[i] == 0:
            del x[i]
            del y[i]
    return x, y

Note that if you want to return two results, you have to return them as a tuple; the second return statement won't be reached otherwise.

Upvotes: 3

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