The difference between the two lists with sublists by the first element in each sublist

Question

I have a problem in python. I need to get differences between the two lists with sublists, but I need only comparing first element of each sublists.

Example:

Input:

x=[[1,2],[2,3],[4,5]]

y=[[1,8],[5,1]]

Output:

dif_l=[[5,1]]

Summarizing the problem, I have to subtract the x list from the y list (dif_l=yx), but only check first element from each sublists.

Answer 1

Can use list comprehension:

    x=[[1,2],[2,3],[4,5]]

    y=[[1,8],[5,1]]

    diff_l = [l for l in y if l[0] not in [k[0] for k in x]]
    print(diff_l)

Answer 2

Use dicts as an intermediate step with the first values as keys. Return only those keys not found in the other dict.

A solution can look like this.

x=[[1,2],[2,3],[4,5]]

y=[[1,8],[5,1]]

def custom_sublist_subtract(left, right):
    ''' y - x should be passed as (y, x)
    '''
    dict_left = {item[0]: item for item in left}
    dict_right = {item[0]: item for item in right}
    result = [v for k, v in dict_left.items() if k not in dict_right]
    return result

custom_sublist_subtract(y, x)
#Output:
[[5, 1]]