How can i return the second element in the list that starts with “b”

Question

I have this function with lists that has strings in it and I have to find the second element in this list that starts with \"b\".

for example:

second_e         


        

Answer 1

It would be more efficient to use a generator, rather than build lists of all strings starting with 'b' by iterating over the whole initial list, then only keep the second one.

def second_element_starting_with_b(lst):
    # g is a generator, it will produce items lazily when we call 'next' on it 
    g = (item for item in lst if item.startswith('b'))
    next(g)  # skip the first one
    return next(g)



second_element_starting_with_b(["b", "a", "bb"]) 
# 'bb'

This way, the code stops iterating on the initial list as soon as the string we are looking for is found.

As suggested by @Chris_Rands, it is also possible to avoid repeated calls to next by using itertools.islice. This way, an extended version looking for the nth item starting with 'b' would look like:

from itertools import islice

def nth_element_starting_with_b(lst, n):
    "Return nth item of lst starting with 'b', n=1 for first item"
    g = (item for item in lst if item.startswith('b'))
    return next(islice(g, n-1, n))



nth_element_starting_with_b(["b", "a", "bb"], 2) 
# 'bb'