How do we make an element in a list go backwards multiple times in python?

Question

here is the code I'm working with but it keeps moving the element backwards once not multiple times

def backwards(input_list):
    total = 0
    while total < 3:
        total +=1
        return input_list[1:] + input_list[:1]

example : a = from ['a', 'b', 'c', 'd'] to ['b', 'c', 'd', 'a'] but multiple times

you `return` that immidiatly leave the loop and the function — Nullman, Feb 04 '21 at 12:34
`Multiple times` , how many times exactly. Your loop does that for 3 times as you have while condition `total<3` — SAI SANTOSH CHIRAG, Feb 04 '21 at 12:35
Does this answer your question? [Efficient way to rotate a list in python](https://stackoverflow.com/questions/2150108/efficient-way-to-rotate-a-list-in-python) — Tomerikoo, Feb 04 '21 at 12:46

score 1 · Accepted Answer · answered Feb 04 '21 at 12:40

Your issue is that you return on the first iteration of the loop so it runs only once. you can fix it like this:

def backwards(input_list):
    total = 0
    while total < 3:
        total +=1
        input_list = input_list[1:] + input_list[:1]
    return input_list

but we can improve that:

def backwards(input_list):
    total = 0
    while total < 3:
        total +=1
        input_list.append(input_list.pop(0))
    return input_list

Alp Aribal · Answer 2 · 2021-02-04T12:41:02.983

0

You can use np.roll() for this. For example:

np.roll(['a', 'b', 'c', 'd'], -1).tolist()

will return:

['b', 'c', 'd', 'a']

edited Feb 04 '21 at 12:41

answered Feb 04 '21 at 12:37

Alp Aribal

370
1
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how do you use np.roll() ? – James Ongkowijoyo Feb 04 '21 at 12:39
It basically rolls your list/array to left (if shift is negative) or to right (if shift is positive). – Alp Aribal Feb 04 '21 at 12:40
2

Its a function from the module [Numpy](https://numpy.org/doc/stable/reference/generated/numpy.roll.html). So for this example to work you would have to add `import numpy as np` – DJSchaffner Feb 04 '21 at 12:44

score -1 · Answer 3 · answered Feb 04 '21 at 12:35

-1

I'm not 100% sure what you're trying to achieve, but you might like:

a[::-1]

Which would return your list, but "backwards" using slicing:

['d', 'c', 'b', 'a']

answered Feb 04 '21 at 12:35

Thomas Kimber

10,601
3
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If you don't understand the question - don't answer! Comment and ask for clarifications instead. Anyway, this is not at all related to the question. OP is trying to make a cyclic shift of the list, not reverse it – Tomerikoo Feb 04 '21 at 12:44

How do we make an element in a list go backwards multiple times in python?

3 Answers3