find last occurence of multiple characters in a string in Python

Question

I would like to find the last occurrence of a number of characters in a string.

str.rfind() will give the index of the last occurrence of a single character in a string, but I need the index of the last occurrence of any of a number of characters. For example if I had a string:

test_string = '([2+2])-[3+4])'

I would want a function that returns the index of the last occurence of {, [, or { similar to

test_string.rfind('(', '[', '{')

Which would ideally return 8. What is the best way to do this?

max(test_string.rfind('('), test_string.rfind('['), test_string.rfind('{'))

seems clunky and not Pythonic.

I don't like where this question is going.... are you trying to write a math parser using this type of character matching? You should use [`pyparsing`](https://pythonhosted.org//pyparsing/) it is a far more robust library for parsing strings with arbitrarily nested enclosing character pairs — Cory Kramer, Aug 11 '15 at 19:49
You could use a regex to find one of those characters with as much as possible before that: `re.match(r".*[\(\[\}]", test_string).end() - 1` Not pretty, but seems to work. — tobias_k, Aug 11 '15 at 19:52
@CoryKramer yes this is for math parsing. In normal circumstances I would use pyparsing, but I'm doing this more for educational purposes and trying to implement something on my own. — capt-calculator, Aug 12 '15 at 19:49

michaelpri · Accepted Answer · 2015-08-11T20:32:13.437

9

You can use generator expression to do this in a Pythonic way.

max(test_string.rfind(i) for i in "([{")

This iterates through the list/tuple of characters that you want to check and uses rfind() on them, groups those values together, and then returns the maximum value.

edited Aug 11 '15 at 20:32

answered Aug 11 '15 at 19:51

michaelpri

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you have a typo (one extra `)`) – Pynchia Aug 11 '15 at 19:55
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`'([{'` is a shorter alternative for `('(', '[', '{')`. – RemcoGerlich Aug 11 '15 at 19:55
what happens when none of the keys is found in the string? explain it in the answer, it would be helpful – Pynchia Aug 11 '15 at 19:56
@Pynchia Thanks, that was because I copy/pasted carelessly (had an extra `)` from a `print` statement) – michaelpri Aug 11 '15 at 19:58
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@Pynchia: rfind returns -1 if the needle isn't found, and the max of three -1s would still be -1. – RemcoGerlich Aug 11 '15 at 19:58
yes, ok. mine was just a suggestion to explain how to use the solution. An excellent one. Upvoted – Pynchia Aug 11 '15 at 20:04
1

That is a generator expression not a list comp – Padraic Cunningham Aug 11 '15 at 20:28
@PadraicCunningham Ahh, I knew something seemed off. I still don't have a great understanding of generators, but this definitely isn't list comp. Thanks :) – michaelpri Aug 11 '15 at 20:30
if you see `(for i ...)` you have a gen exp, a list comp has `[]` – Padraic Cunningham Aug 11 '15 at 20:32

score 2 · Answer 2 · answered Aug 11 '15 at 19:54

2

This is pretty concise, and will do the trick.

max(map(test_string.rfind, '([{'))

answered Aug 11 '15 at 19:54

pzp

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Padraic Cunningham · Answer 3 · 2015-08-11T20:28:02.703

You can use reversed to start at the end of the string getting the first match, using the length of the string -1 - the index i to get the index counting from the start, doing at worst a single pass over the string:

test_string = '([2+2])-[3+4])'
st = {"[", "(", "{"}
print(next((len(test_string) - 1 - i 
            for i, s in enumerate(reversed(test_string)) if s in st),-1))
8

If there is no match, you will get -1 as the default value. This is a lot more efficient if you a large amount of substrings to search for than doing an O(n) rfind for every substring you want to match and then getting the max of all those

score 0 · Answer 4 · answered Aug 11 '15 at 19:51

0

>>> def last_of_many(string, findees):
...     return max(string.rfind(s) for s in findees)
... 
>>> test_string = '([2+2])-[3+4])'
>>> last_of_many(test_string, '([{')
8
>>> last_of_many(test_string, ['+4', '+2'])
10
>>>

answered Aug 11 '15 at 19:51

Cyphase

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find last occurence of multiple characters in a string in Python

4 Answers4