Find sublists with common starting elements - python

Question

I have a nested list:

lists =[['a','b','c'],
        ['a','b','d'],
        ['a','b','e'],
        ['с','с','с','с']]

I need to find sublists with 2 or more common (2 or more occurrences) first elements, make a single string from this elements, and make a single string from sublists, which does not contain common first elements. Sublists can go in different order, so just checking next or previous element is the wrong way, I suppose. Desired output:

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

I tried some for loops, but with no success. I currently don't know, where to start, so any help would be appreciated. Thank you for your time!

Answer 1

Probably not the most efficient way, but you could try something like this:

def foo(l,n):
    #Get all of the starting sequences
    first_n = [list(x) for x in set([tuple(x[:n]) for x in l])]

    #Figure out which of those starting sequences are duplicated
    duplicates = []
    for starting_sequence in first_n:
        if len([x for x in l if x[:n] == starting_sequence])>2:
            duplicates.append(starting_sequence)

    #make changes
    result = []
    for x in l:
        if x[:n] in duplicates:
            result.append([" ".join(x[:n])]+x[n:])
        else:
            result.append([" ".join(x)])

    return result

Set's have no repeats, but elements of sets must be hashable. Since lists are unhashable, that is why I have converted them into tuples and then back into lists.