Extending list by adding element to special position

Question

I have to extend list by adding one element. Pos indicates where I should insert new element and also which value should be inserted (the value of element in original list) F.e.

    ([1, 2, 3], 0) => [1, 1, 2, 3]
    ([1, 2, 3], 1) => [1, 2, 2, 3]
    ([1, 2, 3], 2) => [1, 2, 3, 3]

If pos is too high, I had to add 0 element to the end.

([1, 2, 3], 9) => [1, 2, 3, 0]

I have done so far:

def extend_list(numbers, pos):
    if pos > len(numbers) - 1:
        numbers.append(0)
        return numbers
    return numbers[:pos] + numbers[pos] + numbers[pos:]

But I get an error: TypeError: can only concatenate list (not "int") to list What I'm doing wrong?

`numbers[pos]` is a single value from the list, not itself a list. `[numbers[pos]]` would put it back in a list. Or use `numbers[pos:pos + 1]` to slice one element. — Martijn Pieters, Nov 22 '17 at 16:54
If updating the list *in place* is fine too, use `numbers.insert(pos, numbers[pos])`. — Martijn Pieters, Nov 22 '17 at 16:55

Ajax1234 · Accepted Answer · 2017-11-22T17:07:07.883

2

The issue with your code is that numbers[pos] is an integer that you are trying to add to numbers[:pos] and numbers[pos:], which are lists. A simple solution is to replace return numbers[:pos] + numbers[pos] + numbers[pos:] with return numbers[:pos] + [numbers[pos]] + numbers[pos:]

However, a shorter solution would be:

vals = [([1, 2, 3], 0), ([1, 2, 3], 1), ([1, 2, 3], 2)]
new_vals = [[*a[:b], a[b], *a[b:]] for a, b in vals]

Output:

[[1, 1, 2, 3], [1, 2, 2, 3], [1, 2, 3, 3]]

In Python2:

import itertools
new_vals = [list(itertools.chain.from_iterable([[c, c] if c == a[b] else [c] for c in a])) for a, b in vals]

Output:

[[1, 1, 2, 3], [1, 2, 2, 3], [1, 2, 3, 3]]

edited Nov 22 '17 at 17:07

answered Nov 22 '17 at 16:56

Ajax1234

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Do include an explanation of what you are doing, and answer where the OP went wrong. – Martijn Pieters Nov 22 '17 at 16:59
1

what does *a mean @Ajax1234 – prudhvi Indana Nov 22 '17 at 17:13
got it, this might be helpful https://stackoverflow.com/questions/3480184/unpack-a-list-in-python – prudhvi Indana Nov 22 '17 at 17:15

score 1 · Answer 2 · answered Nov 22 '17 at 17:17

You could alternatively use the .insert() method to make the addition to the list, as I have done below. I also used an else statement instead of 2 return statements out of personal preference.

def extend_list(numbers, pos):
    if pos > len(numbers) - 1:
        numbers.append(0)
    else:
        numbers.insert(pos, numbers[pos])
    return numbers

In fact, due to the nature of lists in python as mutable objects, running this function will edit the original list, meaning the return statement here is unnecessary and the code functions fine without it.

def extend_list(numbers, pos):
    if pos > len(numbers) - 1:
        numbers.append(0)
    else:
        numbers.insert(pos, numbers[pos])

lst = [1,2,3]
extend_list(lst, 1)

## will print [1,2,2,3] ##
print(lst)

Extending list by adding element to special position

2 Answers2