Python while loop return Nth letter

I have a list of strings

X=['kmo','catlin','mept']

I was trying to write a loop that would return a list that contains lists of every Nth letter of each word:

[['k','c','m'], ['m','a','e'],['o','t','p']]

But all the methods I tried only returned a list of all the letters returned consecutively in one list:

['k','m','o','c','a','t','l','i'.....'t'] 

Here is one version of my code:

def letters(X):
    prefix=[]
    for i in X:
      j=0
      while j < len(i):
        while j < len(i):
          prefix.append(i[j])
          break
        j+=1
    return prefix

I know I'm looping within each word, but I'm not sure how to correct it.

It seems that the length of the resulting list is dictated by the length of the smallest string in the original list. If that is indeed the case, you could simply do it like this:

X = ['kmo','catlin','mept']

l = len(min(X, key=len))

res = [[x[i] for x in X] for i in range(l)]

which returns:

print(res)  # -> [['k', 'c', 'm'], ['m', 'a', 'e'], ['o', 't', 'p']]

or the even simpler (kudos @JonClemens):

res = [list(el) for el in zip(*X)]

with the same result. Note that this works because zip automatically stops iterating as soon as one of its elements is depleted.

If you want to fill the blanks so to speak, itertools has got your back with its zip_longest method. See this for more information. The fillvalue can be anything you chose; here, '-' is used to demonstrate the use. An empty string '' might be a better option for production code.

res = list(zip_longest(*X, fillvalue = '-'))
print(res)  # -> [('k', 'c', 'm'), ('m', 'a', 'e'), ('o', 't', 'p'), ('-', 'l', 't'), ('-', 'i', '-'), ('-', 'n', '-')]

You can use zip.

output=list(zip(*X))
print(output)

*X will unpack all the elements present in X.

After unpacking I'm zipping all of them together. The zip() function returns a zip object, which is an iterator of tuples where the first item in each passed iterator is paired together, and then the second item in each passed iterator are paired together etc. Finally, I wrapped everything in a list using list.

output

[('k', 'c', 'm'), ('m', 'a', 'e'), ('o', 't', 'p')]

If you want output to be a list of lists. Then use map.

output=list(map(list,zip(*X)))
print(output)

output

[['k', 'c', 'm'], ['m', 'a', 'e'], ['o', 't', 'p']]

try this

def letters(X):
    prefix=[]
    # First lets zip the list element
    zip_elemets =  zip(*X)

    for element in zip_elemets:
        prefix.append(list(element))
    
    return prefix