Question

How to find occurrences of a string in string in C++?

Here is the Scenario.

string base_string = "BF;1;2;3;FF;10;20;30;BF;11;;22;33;FF;100;200;300;BF;110;;220;330;FF;1000;2000;3000";
string to_find_occurances_of = "BF";

Answer 1

int occurrences = 0;
string::size_type start = 0;

while ((start = base_string.find(to_find_occurrences_of, start)) != string::npos) {
    ++occurrences;
    start += to_find_occurrences_of.length(); // see the note
}

string::find takes a string to look for in the invoking object and (in this overload) a character position at which to start looking, and returns the position of the occurrence of the string, or string::npos if the string is not found.

The variable start starts at 0 (the first character) and in the condition of the loop, you use start to tell find where to start looking, then assign the return value of find to start. Increment the occurrence count; now that start holds the position of the string, you can skip to_find_occurrences_of.length()¹ characters ahead and start looking again.

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¹ drhirsch makes the point that if to_find_occurrences_of contains a repeated sequence of characters, doing start += to_find_occurrences_of.length() may skip some occurrences. For instance, if base_string was "ffff" and to_find_occurrences_of was "ff", then only 2 occurrences would be counted if you add to_find_occurrences_of.length() to start. If you want to avoid that, add 1 instead of to_find_occurrences_of.length() to start, and in that example, 3 occurrences would be counted instead of just 2.