I am writing a function that searches for a substring in a given string and returns how many times the substring occurs in the string. For example, if the functions parameters were ("hellohello", "hel") the function would return 2. I apologize if the answer is obvious, I am relatively new to c++.
The if loop that i've created hasn't been working, and I'm not sure why. My function returns 0 when I call it in the main() function with its parameters set to ("hellohello", "hel").
int countMatches(string input, string substring)
{
if(input == "" || substring == "")
{
return -1;
}
else
{
int matches;
int substringLength = substring.length() - 1;
for(int i = 0; i < input.length(); i++)
{
if(input.substr(i, substringLength) == substring)
{
matches++;
return matches;
}
}
}
}
int main(){
//test 1
//expected output 2
cout << countMatches("hellohello", "hel") << endl;
}
Your problem specification is slightly ambiguous. Can the substrings overlap? For example, if you're looking for the number of times AA occurs in AAAA, do you consider the correct answer to be 2 or 3?
Either way, we can use std::string::find to find the substrings. find takes a parameter to specify where it should start looking for a match. So, if we want to allow overlapping sub-strings, we search from the beginning, then each time we find a sub-string we re-start the search from one position after the beginning of the match we found.
If we only want to allow non-overlapping sub-strings, we start the same (search from the beginning) but when we find a substring we restart the search from just past the end of the substring we just find.
So, here's a version that'll count overlapping substrings:
size_t count_substrings(std::string const &needle, std::string const &haystack) {
size_t count = 0;
for (size_t pos =0; (pos=haystack.find(needle, pos)) != std::string::npos; ++pos, ++count)
;
return count;
}
The modification necessary to count non-overlapping substrings is left as an exercise for the reader (but it honestly is pretty simple).
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