why finding an empty string in non-empty string returns 0

Question

I just noticed this strange behavior of string::find. I have a non-empty string b and another empty string a. When I call b.find(a) it should return npos but returning 0.

#include <iostream>
#include <string>
using namespace std;

int main() {
    // your code goes here
    string a , b("ABC");
    if ( string::npos == b.find(a) ) std::cout << std::endl << "TRUE" << std::endl;
    return 0;
}

Above code doesn't print true. Can someone please explain me what this means ? Since a is empty and b is non-empty finding a empty string in non-empty doesn't make sense and hence error. So it should return npos

Thanks

The empty string is indeed contained in any other string, so the result 0 is correct — Vitor
– Vitor, Commented Dec 1, 2015 at 11:49
"When I call b.find(a) it should return npos" - where do you get that idea from? In future, check cppreference docs before presuming such things - it explains it very clearly. — Tony Delroy
– Tony Delroy, Commented Dec 1, 2015 at 11:51
If you read e.g. this find reference you will see all conditions that needs to be true for the sub-string to be found, and if you carefully check them you will see that the empty string indeed causes all to be true. — Some programmer dude
– Some programmer dude, Commented Dec 1, 2015 at 11:51
@noname It is not non-empty string. It is contained in it, as everyone is telling you. — Daniel Daranas
– Daniel Daranas, Commented Dec 1, 2015 at 11:53
"how come empty string can be non-empty string?" - are you misunderstanding what b.find(a) does? It searches for a in b, not the other way around.... — Tony Delroy
– Tony Delroy, Commented Dec 1, 2015 at 11:54

eerorika · Accepted Answer · 2015-12-01 11:58:04Z

11

Empty string is a substring of all strings. The first position where an empty substring exists is the first index. find returns the first index where it finds the first occurrence of the substring.

If the definition of empty substring confuses you, consider the algorithm that checks if string is a substring. The algorithm checks each character in the potential substring and compares it to the corresponding character in the other string. If any character does not match, then it is not a substring. If the end of the searched string is reached, then it is a match. In the case of an empty string, no character can differ because there are no characters. The end is reached immediately and the conclusion is that the empty string is a substring.

edited Dec 1, 2015 at 11:58

answered Dec 1, 2015 at 11:52

eerorika

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2 Comments

Tony Delroy Over a year ago

"In fact, all strings contain infinitely many empty substrings." - that's a bit OOT - I'd count them as one per position in the string, though it's true that naive find-at-pos / pos += x.size() logic could get stuck in an infinite loop....

eerorika Over a year ago

@TonyD whether either of our claims is correct seems very much a philosophical question. I'll remove it since it's not essential.

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