How to count occurrences of a list of string inside of a list in python?

Question

For example my list is

lst=['hello','world','this','is','hello','world','world','hello']
subString=['hello','world']

The result I'm looking for is in this case is 2 since the list ['hello','world'] occurs twice with that same order.

I tried doing

list(filter(lambda x : x in substring,lst))

but that returns all of hello and world

What if the array looks like this: `['hello', 'world', 'hello world', 'hello', 'world hello']`? The currently accepted answer falls apart if your goal is to ensure that two consecutive list items create the string `'hello world'`. It may work for the specific example you've provided but it is not a great general-purpose answer. The one I have provided accounts for such a scenario. — Matt, Sep 30 '18 at 00:39

Matt · Answer 1 · 2018-09-30T00:34:13.423

0

You can join the elements into a list of lists and then filter by those that match your substring array.

joinedWords = [lst[n:n + len(subString)] for n in range(0, len(lst), len(subString))]
    # => [['hello', 'world'], ['this', 'is'], ['hello', 'world'], ['world', 'hello']]
filtered = list(filter(lambda x: x == subString, joinedWords))

print(len(filtered))  # 2

edited Sep 30 '18 at 00:34

answered Sep 29 '18 at 23:02

Matt

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1

What if `sub_string` had more than 2 items? – G_M Sep 29 '18 at 23:04

score 0 · Accepted Answer · answered Sep 29 '18 at 23:04

You could use " ".join() on both lists to create a string and then use str.count() to count the number of occurrences of subString in lst

lst=['hello','world','this','is','hello','world','world','hello']
subString=['hello','world']

l = " ".join(lst)
s = " ".join(subString)

count = l.count(s)
print("Joined list:", l)
print("Joined substring:", s)
print("occurrences:", count)

outputs:

Joined list: hello world this is hello world world hello
Joined substring: hello world
occurrences: 2

dvnguyen · Answer 3 · 2018-09-30T02:06:17.617

0

Since all elements are string in this case, I'd create a string from each list, then count the occurrences of the second string in the first string:

lst=['hello','world','this','is','hello','world','world','hello']
subString = ['hello','world']

s = ' '.join(lst)
subs = ' '.join(subString)

print(s.count(subs))

edited Sep 30 '18 at 02:06

answered Sep 29 '18 at 23:06

dvnguyen

2,954
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This will give you false positives if you have an array like `['hel', 'low', 'orld']`. You need to join with a space (in which case your answer would be exactly the same as the accepted answer). – Matt Sep 30 '18 at 00:35

score 0 · Answer 4 · answered Sep 29 '18 at 23:09

Using the window generator from this answer and a Counter, this can be expressed as:

from collections import Counter

lst=['hello','world','this','is','hello','world','world','hello']
subString=('hello','world')

counts = Counter(window(lst, len(subString)))

print(counts[subString])
# 2

If you want to skip the Counter, you could do

print(sum(x == subString for x in window(lst, len(subString))))

How to count occurrences of a list of string inside of a list in python?

4 Answers4