Remove item in a list based on index

Question

I'm trying to solve a little puzzle, where I need to delete the number 13, and the number after that in a list (its an exercise on CodingBat).

This is my code:

n = [1, 2, 3, 13, 5, 13] 

for i in n:
    if i == 13:
        n.remove(i) and n.remove(n.index(i+1))

print n

Desired output: [1, 2, 3]

However, my incorrect output is: [1, 2, 3, 5] #the item after 13 (i.e. 5) did not get deleted

I thought that this n.remove(n.index(i+1)) would remove the item after 13, but it doesn't.

After you remove `13`, what's the index of `5`? Don't change the length of a list while iterating over it. — jonrsharpe, Jun 28 '16 at 19:29
`n.index(x)` is defined as "Return the smallest `i` such that `i` is the index of the first occurrence of `x` in the array." — Eli Sadoff, Jun 28 '16 at 19:29
Your use of the `and` keyword is wrong. You cannot string together commands on a single line using it. `and` is a replacement for the logical operator `&&`, so youre trying to compare two none returning operations instead of executing two — m_callens, Jun 28 '16 at 19:31
@m_callens he is also mixing his use of `i` as an iterated value and index — Assimilater, Jun 28 '16 at 19:32
@m_callens actually that will work just fine although it's considered a bit of a hack. — jonrsharpe, Jun 28 '16 at 19:45
@m_callens ...the thing you said was wrong. Chaining two operations with `and`. It's lazily evaluated, so the second half won't happen if the first half returns a falsy value. — jonrsharpe, Jun 28 '16 at 19:49
@jonrsharpe yes I understand that the FIRST will always execute, but I don't think that was on purpose....syntactically correct, semantically wrong — m_callens, Jun 28 '16 at 19:50
@apples-oranges The issue with the way that you're using `i` is that you're using it as an `iterator` AND an `indexer` in the same situation. When you do `for i in n`... `i` "becomes" each value within the list `(i.e. 1, 2, 3, 13, 5, 15)`, so when you do `n.index(i + 1)`, you're trying to find the index of `14` when `i = 13` and `14` is not in the list — m_callens, Jun 28 '16 at 19:53
`new_n = [n[i] for i,x in enumerate(n) if not 13 in n[max(0,i-1):i+1]]` — ml-moron, Jun 28 '16 at 20:44

Psidom · Accepted Answer · 2016-06-28T19:58:03.743

2

This should work:

n = [1, 2, 3, 13, 5, 13] 

for i in n:
    if i == 13:
        n.remove(n[n.index(i)+1]) # remove the element after `i` first
        n.remove(i) 

print n

A while loop for the problem:

n = [1, 2, 3, 13, 5, 13] 

i = 0
while i < len(n):
    if n[i] == 13:
        n.pop(i)
        if i < len(n):
            n.pop(i)
    else:
        i = i + 1

print n

# [1, 2, 3]

edited Jun 28 '16 at 19:58

answered Jun 28 '16 at 19:35

Psidom

209,562
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`if i == 13` --> `i` is used as a value....`n.remove(i)` --> `i` is used as an index....problem? ;) – Assimilater Jun 28 '16 at 19:36
@Assimilater `n.remove(i)` will remove the first element which has the value `i` in the list. `i` is used as a value not index. – Psidom Jun 28 '16 at 19:38
Ah, nvm, you're right :P – Assimilater Jun 28 '16 at 19:39
@Psidom Thanks for the input! However, when I run this, it outputs `[1, 2, 3, 13]` – apples-oranges Jun 28 '16 at 19:41
Probably because of the post your question is marked as a duplicate of – Assimilater Jun 28 '16 at 19:42
Do you want to remove all `13` and the element after it? – Psidom Jun 28 '16 at 19:44
I think what OP wants is `13` to be like a flag for removal of the next element...kind of odd, but say `n` was `[1, 2, 13, 5, 3, 4, 13, 6, 5]` output to be like `[1, 2, 3, 4, 5]` – Assimilater Jun 28 '16 at 19:45
Yes exactly. Essentially what @Assimilater described. – apples-oranges Jun 28 '16 at 19:47
If that is the case, I would write a while loop for the problem, since you can not really change the iterating variable in python. Where we use `pop` to remove the element by index instead of value. – Psidom Jun 28 '16 at 19:59

score 0 · Answer 2 · answered Jun 28 '16 at 19:29

0

After your n.remove(), your array is [1, 2, 3, 5, 13] so the n.remove(n.index(i+1)) refers to 13 not to 5

answered Jun 28 '16 at 19:29

Florian Cordillot

55
10

Remove item in a list based on index

2 Answers2