Check if one word follows the other one in the list with python

Question

I want to count the occurrence of words which are followed by each other in a given list using python. E.g.

my_list = ['ABC', 'DEF', 'PQR', 'ASD', 'ABC', 'DEF', 'ZXC', 'ABC', 'PQR']

In the above list, the count of times when 'ABC' is followed by 'DEF' is 2.

Kindly help me out. Thanks.

@Jean-FrançoisFabre wrongly typed 3. the actual count is 2. now its not allowing me to edit. — SubodhD, Jan 13 '17 at 15:05

Willem Van Onsem · Accepted Answer · 2017-01-13T15:07:44.310

3

A not very efficient way to do this might be:

count = 0
for first,second in zip(my_list,my_list[1:]):
    if first == 'ABC' and second == 'DEF':
        count += 1

Or in a one-liner using sum(..):

count = sum(1 for first,second in zip(my_list,my_list[1:]) if first == 'ABC' and second == 'DEF')

Or as @khelwood says, you can exploit the fact that int(..) of a boolean returns 0 and 1:

count = sum(first == 'ABC' and second == 'DEF' for first,second in zip(my_list,my_list[1:]))

edited Jan 13 '17 at 15:07

answered Jan 13 '17 at 15:03

Willem Van Onsem

@Jean-FrançoisFabre: I was just thinking the same ;) – Willem Van Onsem Jan 13 '17 at 15:04
You can use `sum` better than that. `sum(first=='ABC' and second=='DEF' for first, second in zip(my_list, my_list[1:]))` – khelwood Jan 13 '17 at 15:06
@khelwood: that's actually a nice hack, I like it :) – Willem Van Onsem Jan 13 '17 at 15:07

score 3 · Answer 2 · edited May 23 '17 at 11:53

A straightforward way that does not involve creating an extra slice for pairwise iteration would be to simply use indexes:

sum(1 for index in range(len(my_list) - 1) 
    if my_list[index] == 'ABC' and my_list[index + 1] == 'DEF')

Or, a bit shorter (thanks to @khelwood):

sum(my_list[index] == 'ABC' and my_list[index + 1] == 'DEF' 
    for index in range(len(my_list) - 1))

Or, without sum() and expanded:

c = 0
for index in range(len(my_list) - 1):
    if my_list[index] == 'ABC' and my_list[index + 1] == 'DEF':
        c += 1

Or:

c = 0
for index in range(len(my_list) - 1):
    c += my_list[index] == 'ABC' and my_list[index + 1] == 'DEF'

Related materials for one versus the other:

2 Answers2