Python counting multiple substrings

Question

I am trying to count the number of times letters show up in a string, so i'm using the count() function which is working fine, however using multiple count() functions seems tedious since i'm trying to count 10+ letters.

def printer_error(s):
 #errorLetters = ('r','s','t','u','v','w','x','y','z')
 errorRate = s.count('o') + s.count('p') + s.count('q')


 print(errorRate)
 return 

printer_error('aaaxbbbbyyhwawiwjjjwwm')

My first attempt was s.count(errorLetters) but that failed because the count() function can't take tuples or lists.

the current code is giving me the result I want but I still have to concatenate the rest of those commented out letters in the errorLetters variable, is there another way to do this that doesn't involve making 20+ concatenations?

The errorRate variable listed currently doesn't count anything because I wanted to keep it alphabetical, but I texted it beforehand with 'w' and 'y' and got the result I wanted. — Bittercold
– Bittercold, Commented May 23, 2017 at 22:02
It's a shame we can't downvote on comments. @ewcz proposal is not good, it's needlessly inefficient to use s.count inside the loop. — wim
– wim, Commented May 23, 2017 at 22:07
@wim fair enough ;) sum([1 if c in errorLetters else 0 for c in s]) — ewcz
– ewcz, Commented May 23, 2017 at 22:19

wim · Accepted Answer · 2017-05-23 22:03:51Z

2

The best way to do this only requires to iterate s once, i.e. O(n).

Standard library collections module provides a Counter object that will be useful and efficient:

>>> s = 'aaaxbbbbyyhwawiwjjjwwm'
>>> errorLetters = ('r','s','t','u','v','w','x','y','z')
>>> from collections import Counter
>>> counts = Counter(s)
>>> sum(counts[k] for k in errorLetters)
8

answered May 23, 2017 at 22:03

wim

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1 Comment

Bittercold Over a year ago

There are a ton of useful libraries this is works great

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