Problem with a User Define Function in Python

Ok so here's my problem, I defined a function that's suppose to count the number of times a certain fragment occurs in a string. The function starts searching at Index[0] and subsequent searches start at the index of the last match in the string.

HERE'S THE CODE

def function(fragment, string):
    count = -1
    last_match = 0
    while last_match != -1:
        count += 1
        last_match = string.find(fragment, last_match)
    return count

Now here's the problem, When pass the parameters to the fu开发者_运维技巧nction with a fragment that's obviously a match in the string I get an infinite loop. When I fix the infinite loop, If a pass fragment that located in Index[0] the function doesn't return a the right count???

ex:

function('gtg' , 'gttacgtggatg' ) This gives me an infinite loop

function('gtt' , 'gttacgtggatg' ) This doesn't return a count..

Just looking for some help and input..

Thanks

You should use existing functionality, if possible:

"aabbccaabb".count("aa")

The count method does exactly what you want. The above example returns 2.

You have two mistakes:

1. String documentation states that find will return -1 if the fragment is not found
2. The last_match returns the index of the beginning of the match and so if you search again from that index you will find the same match again and again.

So you would probably have to use this :

def function(fragment, string):
    count = -1
    last_match = 0
    while last_match != -1:
        count += 1
        last_match = string.find(fragment, last_match)
        if last_match != -1:
            last_match += 1
    return count

>>> function('gtt' , 'gttacgtggatg' )
1
>>> function('gtg' , 'gttacgtggatg' )
1

Of course you could also rely on string.count(fragment) if you're not interested in overlapping matches... or regular expressions as mentionned by previous answers...

>>> 'gttacgtggatg'.count('ctt')
0
>>> 'gttacgtggatg'.count('gtt')
1

Have you considered using regular expressions? It's well adapted to your use. Python re module

Update: To expand off that, if you do something like this...

import re

...

def countOccurrences(myFragment, myString): expr = re.compile(myFragment) return len(expr.findall(myString))

Logically, your problem is that you're simply finding the same match over and over again:

last_match = string.find(fragment, last_match)

...will start from where you found the last match, which will simply give you the same position back! You need to use string.find(fragment, last_match + 1), which will require you to jump through hoops to use a sentinel for your initial value of last_match.

Achim's answer answer is spot on: you should use existing functionality to do this.