List inserting and mutation

Question

def new_insert(lst, i, e):
    lst = lst[:i] + [e] + lst[i:]
    
a = [0, 1, 2, 3, 4, 6, 7]
new_insert(a, 5, 5)
#Expected output: a = [0, 1, 2, 3, 4, 5, 6, 7]
    
b = ['0', '1', '0']
new_insert(b, 0, '1')
#Expected output: b = ['1', '0', '1', '0']

    
new_insert([1,2,3], 1, 0)
#Expected output = None

For the above code, I am unable to get the answer for the newly modified list when adding e into the list. When e = 0, the output should return None. How can I change the code, so that I can get my expected output?

I think you mean that `lst[:] = lst[:i] + [e] + lst[i:]` if you can't return the list — Jean-François Fabre, Oct 04 '20 at 16:17
thanks @Jean-FrançoisFabre, your answer worked and answers all the output. Why is there a need to declare as lst[:]? — kachiever, Oct 04 '20 at 16:20

SpiderPig1297 · Answer 1 · 2020-10-04T16:29:16.370

0

Option 1:

You need to return the modified list from new_insert:

def new_insert(lst, i, e):
    return lst[:i] + [e] + lst[i:]

And then do:

a = [0, 1, 2, 3, 4, 6, 7]
a = new_insert(a, 5, 5)

Option 2:

Or just do:

def new_insert(lst, i, e):
    lst[:] = list[:i] + [e] + lst[i:]

if you don't want to return it from the function.

edited Oct 04 '20 at 16:29

answered Oct 04 '20 at 16:19

SpiderPig1297

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1
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changing to lst[:] works perfectly fine as it answers all the expected output. why is there a need to put lst[:]? – kachiever Oct 04 '20 at 16:24
WIth the code you did, the function `new_insert` creates a _local_ variable named `lst`, which is destroyed when the function exits, without saving you changes. Doing `lst[:]` actually tells python to change the list you passed to the function and not to create a copy of it. – SpiderPig1297 Oct 04 '20 at 16:27
there's a much better option here actually: `lst[i:i] = e` when e is a list – Jean-François Fabre Oct 04 '20 at 16:36
@Jean-FrançoisFabre it fails with `TypeError: can only assign an iterable` – SpiderPig1297 Oct 04 '20 at 16:38
ah then just `lst.insert(i,e)`. This is the fastest way – Jean-François Fabre Oct 04 '20 at 16:38
Yeah, I didn't know if he can use functions of list... anyway, all options will work :) – SpiderPig1297 Oct 04 '20 at 16:39

score 0 · Answer 2 · answered Oct 04 '20 at 16:22

You need to return lst but also have an if condition and return none

def new_insert(lst, i, e):
    if (e == 0):
        return None

    lst = lst[:i] + [e] + lst[i:]

    return lst

a = [0, 1, 2, 3, 4, 6, 7]
print(new_insert(a, 5, 5))
#Expected output: a = [0, 1, 2, 3, 4, 5, 6, 7]

b = ['0', '1', '0']
print(new_insert(b, 0, '1'))
#Expected output: b = ['1', '0', '1', '0']


print(new_insert([1,2,3], 1, 0))
#Expected output = None

List inserting and mutation

2 Answers2