Finding a substring of multiple occurences in a string [C++]

Question

is there any STL algorithm or a standard way of finding how many occurences of particular substring are there in a string? For example in string:

'How do you do at ou'

the string "ou" appears twice. I tried some STL algorithms with and without predicates but what I found is that those algorithms from STL want to compare components of string which in my case is char but cannot? compare substrings. I come up with something like this:

str - string

obj - substring we're looking for

std::string::size_type count_subs(const std::string& str, const std::string& obj)
{
std::string::const_iterator beg = str.begin();
std::string::const_iterator end = str.end();
std::string::size_type count = 0;
while ((beg + (obj.size() - 1)) != end)
{
    std::string tmp(beg, beg + obj.size());
    if (tmp == obj)
    {
        ++count;
    }
    ++beg;
}
return count;
}

thank you.

anon · Accepted Answer · 2009-12-03 10:44:11Z

5

#include <string>
#include <iostream>

int Count( const std::string & str, 
           const std::string & obj ) {
    int n = 0;
    std::string ::size_type pos = 0;
    while( (pos = obj.find( str, pos )) 
                 != std::string::npos ) {
        n++;
        pos += str.size();
    }
    return n;
}

int main() {
    std::string s = "How do you do at ou";
    int n = Count( "ou", s );
    std::cout << n << std::endl;
}

answered Dec 3, 2009 at 10:44

anon

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5 Comments

yves Baumes Over a year ago

Question: In the string "oooo" Would you count the pattern "oo" twice or three times. (Personnaly I would count it three time, and therefore fo ++pos instead of pos += str.size().)

anon Over a year ago

It would depend what I wanted to use the function for. From the questioner's own code, he seems to want non-overlapping occurences.

There is nothing we can do Over a year ago

Thank you for your answer - although I think that there is logical error - it should be str.find(obj,pos). Anyway, so basically there is no way in which one can use STL alg. like count_if or one similar to it to avoid explicit loop. Pity. I wonder if guys from boost have any recipe for that problem.

anon Over a year ago

There is no error - my version searches for str in obj - I slightly misread your code & so got them the other way round from you. This indicates the neeed for better names, such as 'needle' and 'haystack'. As for avoiding the explicit loop - anything that used an algorithm and a predicate would actually be more code. Sometimes the simplest old skool solution may be the best.

There is nothing we can do Over a year ago

Thanks, love your names - definitely going to use them in my code.

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