Can you explain me, why does it always output true? I would like to know if appointed substring is present in original string.

Question

//c++
#include <iostream>
#include<string>
using namespace std;

bool is_found(const string str){
if(str.find("am"))
return true;
else
return false;
}

int main() {
string a("12:45:56 pm");
cout << is_found(a);
return 0;
}

Answer 1

The find() method returns the index of the substring's first character if found, or a special constant "npos" otherwise. See:

https://en.cppreference.com/w/cpp/string/basic_string/find

That constant is defined as:

static const size_type npos = -1;

Converted to a bool, that value is still true, hence your incorrect results. The correct condition would be:

if ( str.find( "am" ) != string::npos )
...

Or, when shortening the function:

return ( str.find( "am" ) != string::npos );