CODEWITHSUNDEEP
c++string
pisoir
pisoir

Reputation: 226

Weird behaviour in string comparison C++

I am using standard std::string variables which I try to compare. Quite strangely however, I am getting weird results:

string str1 = "44", str2 = "111";
bool smaller1 = str1.compare(str2) > 0;   // <- returns true, although should be false

bool smaller2 = "44" < "111";  // <- returns true, CORRECT

string str3(2, '4'), str4(3, '1');
bool small3 = str3 < str4;     // <- returns false, although should be true

I would expect since one string is shorter than the other, the lexacographical ordering should always give me true for "44" < "111".

The whole code looks like:

#include <iostream>
#include <string>

using namespace std;

int main() {

    string str1 = "44", str2 = "111";
    string str3(2, '4'), str4(3, '1');

    bool cmp1 = str1 < str2,
         cmp2 = "44" < "111",
         cmp3 = str3 < str4,
         cmp4 = str3.compare(str4) < 0;

    std::cout << "1: " << cmp1 << "\n";
    std::cout << "2: " << cmp2 << "\n";
    std::cout << "3: " << cmp3 << "\n";
    std::cout << "4: " << cmp4 << "\n";


    return 0;
}

and returns:

1: 0
2: 1
3: 0
4: 0

I am using g++ (GCC) 8.2.0 from MinGW 16.1 on Windows10. Am I missing something here? How can I force it to give me the correct results (shorter strings are smaller than longer). Thanks.

Upvotes: 1

Views: 384

Answers (1)

selbie
selbie

Reputation: 104474

bool smaller2 = "44" < "111";  // <- returns true, CORRECT

No. You're just getting lukcy. That's comparing the addresses of string literals, not the same thing as an instance of an std::string. As a matter of fact, you could replace that expression in your code as "111" < "44" and it will likely return true if the compiler aligns the strings in memory in the same order as declared.

Correct way to compare strings lexicographical:

std::string str1 = "44";
std::string str2 = "111";
bool cmp1 = (str1 < str2);  // false, since '4' is greater than '1'

Correct way to compare strings as integers:

int val1 = std::stoi(str1);  // 44
int val2 = std::stoi(str2);  // 111
bool cmp2 = (val1 < val2);   // true since 44 is less than 111

Upvotes: 1

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