Python 2.7, how can I find the position of group of letters in a string?

Question

I need to know how can I find ALL the position of a group of letters in a string. For example, in the string "Canyoucanacanasacannercancanacan" the letters "an" recur 7 times. I want to know the exact position of each one, possibly in a list. How can I do?

Thank you!

You start by writing code and showing us what you have don't expect us to do for you — Mike Tung, Sep 19 '17 at 22:02

score 2 · Answer 1 · answered Sep 19 '17 at 22:12

I would use re.finditer(), like so:

import re

s = "Canyoucanacanasacannercancanacan"
pattern = "an"

result = [m.start() for m in re.finditer(pattern, s)]

assert result == [1, 7, 11, 17, 23, 26, 30]

Note that this only finds the non-overlapping instances, which in your specific case is all of them.

Jeremy McGibbon · Accepted Answer · 2017-09-19T22:21:30.997

0

Surprisingly I couldn't find a duplicate of this question! You can do this using str.index, while updating the starting position that you're looking from to exclude parts of the string you've already checked.

s = 'Canyoucanacanasacannercancanacan'
position_list = []
i = 0
while True:
    try:
        position = s.index('an', i)
    except ValueError:
        break
    position_list.append(position)
    i = position + 1
print(position_list)

As someone suggested, you could also use str.find, like this:

s = 'Canyoucanacanasacannercancanacan'
position_list = []
i = s.find('an')
while i != -1:
    position_list.append(i)
    i = s.find('an', i+1)
print(position_list)

edited Sep 19 '17 at 22:21

answered Sep 19 '17 at 22:07

Jeremy McGibbon

3,527
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1

you could use `.find`, which returns `-1` if not found and then loop over `while found != -1` – juanpa.arrivillaga Sep 19 '17 at 22:18
Good suggestion! I don't know which one I prefer. – Jeremy McGibbon Sep 19 '17 at 22:22
well, the one good thing about using `.index` is that it works for *all sequences*, so `tuple`, `list`, `range`, etc – juanpa.arrivillaga Sep 19 '17 at 22:24

score 0 · Answer 3 · answered Sep 19 '17 at 22:12

Try the following, you can modify the output as you wish (start and end):

import re

text = "Canyoucanacanasacannercancanacan"
for m in re.finditer(r"an", text):
    print('%02d-%02d: %s' % (m.start(), m.end(), m.group(0)))

The output am getting:

01-03: an
07-09: an
11-13: an
17-19: an
23-25: an
26-28: an
30-32: an

score 0 · Answer 4 · answered Sep 19 '17 at 22:21

You can create a small generator to find all positions of letters in a text:

def find_positions(text, letters):
    curr = text.find(letters)
    while curr >= 0:
        yield curr
        curr = text.find(letters, curr + len(letters))

Usage:

positions = list(find_positions("Canyoucanacanasacannercancanacan", "an"))
print(positions)

You get:

[1, 7, 11, 17, 23, 26, 30]

Python 2.7, how can I find the position of group of letters in a string?

4 Answers4