CODEWITHSUNDEEP
python
Duncan
Duncan

Reputation: 21

python: Finding a substring within a string

I'm trying to write a program which counts how many times a substring appears within a string.

word = "wejmfoiwstreetstreetskkjoih"
streets = "streets"
count = 0

if streets in word:
    count += 1

print(count)

as you can see "streets" appears twice but the last s of streets is also the beginning of streets. I can't think of a way to loop this.

Thanks!

Upvotes: 2

Views: 187

Answers (3)

bereal
bereal

Reputation: 34312

A generic (though not as elegant) way would be a loop like this:

def count_substrings(stack, needle):
    idx = 0
    count = 0
    while True:
        idx = stack.find(needle, idx) + 1 # next time look after this idx
        if idx <= 0:
            break
        count += 1
    return count

My measurement shows that it's ~8.5 times faster than the solution with startswith for every substring.

Upvotes: 0

Tim
Tim

Reputation: 43364

Can be done using a regex

>>> import re
>>> text = 'streetstreets'
>>> len(re.findall('(?=streets)', text))
2

From the docs:

(?=...)

Matches if ... matches next, but doesn’t consume any of the string. This is called a lookahead assertion. For example, Isaac (?=Asimov) will match 'Isaac ' only if it’s followed by 'Asimov'.

Upvotes: 4

UltraInstinct
UltraInstinct

Reputation: 44464

Quick and dirty:

>>> word = "wejmfoiwstreetstreetskkjoih"
>>> streets = "streets"
>>> sum(word[start:].startswith(streets) for start in range(len(word)))
2

Upvotes: 2

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