Python list basic manipulation

Question

So I was attempting to write a very basic function that can move every element in the list one index earlier. And I think I am actually pretty close to the result I want.

For example if a list is

l = [1, 2, 4, 5, 'd']

I want it to be like that afterwards

l = [2, 4, 5, 'd', 1]

The reality of my code

l = [2, 4, 5, 1, 1]

Here is my code, I just don't know what is happening here after a lot of random attempts on changing the code...

Thank you guys in advance!

def cycle(input_list):
count = 0
while count < len(input_list):
     tmp = input_list[count - 1]
     input_list[count - 1] = input_list[count]
     count+=1

You never use the tmp-variable. You need to define it outside the while-loop and then actually use it — Metareven, Apr 01 '16 at 10:56

jDo · Answer 1 · 2016-04-02T07:19:15.647

9

You could do this (in place):

l.append(l.pop(0))

In function form (makes a copy):

def cycle(l):
    ret = l[:]
    ret.append(ret.pop(0))
    return ret

edited Apr 02 '16 at 07:19

answered Apr 01 '16 at 10:57

jDo

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This does not make a copy of `l`, you need `ret = l[:]` if you want to create a copy. – Holt Apr 01 '16 at 11:58
@Holt or alternatively `ret = list(l)`. – marcelm Apr 01 '16 at 12:23
@Holt Thanks. I forgot that it just assigns a new handle/variable. Changed it. – jDo Apr 02 '16 at 07:23

score 2 · Answer 2 · edited Apr 02 '16 at 07:44

2

As a python developer, I really cant resist typing this one liner

newlist = input[start:] + input[:start]

where start is the amount by which you have to rotate the list

Ex :

input = [1,2,3,4]

you want to shift array by 2 , start = 2

input[2:] = [3,4]

input[:2] = [1,2]

newlist = [3,4,1,2]

edited Apr 02 '16 at 07:44

formatkaka

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answered Apr 01 '16 at 10:57

ffff

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Mark Skelton · Answer 3 · 2016-04-01T10:59:05.107

1

Here is what I would do. Get the first item of the list, delete it, then add it back to the end.

def cycle(input_list):
    first_item = input_list.pop(0)  #gets first element then deletes it from the list 
    input_list.append(first_item)  #add first element to the end

edited Apr 01 '16 at 10:59

answered Apr 01 '16 at 10:56

Mark Skelton

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`pop(0)` already returns the item, there's no need to get first_item separately. – Daniel Roseman Apr 01 '16 at 10:57

score 0 · Answer 4 · answered Apr 01 '16 at 15:58

You can do this using the while or the for statements. Using for:

    newList = []
    for index in range(1, len(list)):
        newList.append(list[index])
    newList.append(list[0])

Using while:

newList = []
index = 1
while index < len(list):
    newList.append(list[index])
    index += 1
newList.append(list[0])

You'll be able to move any list one element to the left :)

Example:

def move(list):
    newList = []
    for index in range(1, len(list)):
        newList.append(list[index])
    newList.append(list[0])
    return newList

list = [1, 2, 3, 4, 5, 'd']
list = move(list)

print(list)
>>>[2, 3, 4, 5, 'd', 1]

Python list basic manipulation

4 Answers4