How can you split a list every x elements and add those x amount of elements to an new list?

Question

I have a list of multiple integers and strings

['-200', ' 0', ' 200', ' 400', ' green', '0', '0', '200', '400', ' yellow', '200', '0', '200', '400', ' red']

I'm having difficulty separating the list every 5 elements and creating a new list with just 5 elements inside.

However, I don't want 3 different lists, i just want one that changes every time a new 5 elements goes through.

Solution 1

You want something like:

composite_list = [my_list[x:x+5] for x in range(0, len(my_list),5)]

print (composite_list)

Output:

[['-200', ' 0', ' 200', ' 400', ' green'], ['0', '0', '200', '400', ' yellow'], ['200', '0', '200', '400', ' red']]

What do you mean by a "new" 5 elements?

If you want to append to this list you can do:

composite_list.append(['200', '200', '200', '400', 'bluellow'])

Solution 2

I feel that you will have to create 1 new list, but if I understand correctly, you want a nested list with 5 elements in each sublist.

You could try the following:

l = ['-200', ' 0', ' 200', ' 400', ' green', '0', '0', '200', '400', ' yellow', '200', '0', '200', '400', ' red']

new = []
for i in range(0, len(l), 5):
    new.append(l[i : i+5])

This will step through your first list, 'l', and group 5 elements together into a sublist in new. Output:

[['-200', ' 0', ' 200', ' 400', ' green'], ['0', '0', '200', '400', ' yellow'], ['200', '0', '200', '400', ' red']]

Hope this helps

Solution 3

You could do it in a single sentence like

>>> import math
>>> s = ['-200', ' 0', ' 200', ' 400', ' green', '0', '0', '200', '400', ' yellow', '200', '0', '200', '400', ' red']
>>> [s[5*i:5*i+5] for i in range(0,math.ceil(len(s)/5))]

Then the output should be :

[['-200', ' 0', ' 200', ' 400', ' green'], ['0', '0', '200', '400', ' yellow'], ['200', '0', '200', '400', ' red']]

Solution 4

There is a slightly different approach that I detailed in a previous answer using zip. The previous answer was closed as a duplicate of this answer so I add my answer here for reference.

You can chunk up a list using iter and zip.

def chunk(lst, n):
    return zip(*[iter(lst)]*n)

Example:

In []:
data = ['-200', ' 0', ' 200', ' 400', ' green', '0', '0', '200', '400',
        ' yellow', '200', '0', '200', '400', ' red']

for l in chunk(data, 5):
    print(l)

Out[]:
('-200', ' 0', ' 200', ' 400', ' green')
('0', '0', '200', '400', ' yellow')
('200', '0', '200', '400', ' red')

if you want them in a list then:

In []:
list(chunk(data, 5))

Out[]:
[('-200', ' 0', ' 200', ' 400', ' green'), ('0', '0', '200', '400', ' yellow'),  
 ('200', '0', '200', '400', ' red')]

Explanation:

def chunk(lst, n):
    it = iter(lst)       # creates an iterator
    its = [it]*n         # creates a list of n it's (the same object)
    return zip(*its)     # unpack the list as args to zip(), e.g. when `n=3` - zip(it, it, it)

Because the zip() is operating on the same iterator n times then it will chunk the original lst into groups of n.

In Py3, this is lazy (zip() returns an iterator), so will only create the chunks of n as you need them, which could be important if you have a big original list.

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