Sort string array with sort, error reported

Question

Both problems are solved when I change "<=" to "<": it worked! I don't know why, can someone answer me? Thanks!!!

The first problem

code:

#include <bits/stdc++.h>
using namespace std;

int main(){
    string s[30];
    int n = 20;

    for(int i = 0; i < n; i++){
        s[i] = "3";
    }
    sort(s, s + n, [](string a, string b){
        return a <= b;
    });

    return 0;
}

error:

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc

The second problem

code:

#include <bits/stdc++.h>
using namespace std;

int main(){
    string s[30];
    int n = 20;

    for(int i = 0; i < n; i++){
        s[i] = "3";
    }
    sort(s, s + n, [](const string& a, const string& b){
        return a <= b;
    });

    return 0;
}

error:

Segmentation fault

Answer 1

You should use '<' for ascending sort, but you can use <=. The key is:

comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

std::sort

So <= will satisfy the requirement, (if and only if there are differences in all values being sorted) but it was not intended for that purpose and the = part is simply superfluous in the case all elements are different and likely undefined where all elements are the same. The < is evaluated and controls the sort. Don't use <= the sort compare function requires < or >. See C++ named requirements: Compare

Do not #include <bits/stdc++.h>. Use the proper headers that provide the prototypes and functionality necessary for your program. See also Why is “using namespace std;” considered bad practice?

Taking both into consideration and cleaning up the example a bit, you could do:

#include <iostream>
#include <string>
#include <algorithm>

#define MAXS 30         /* if you need a constant, #define one (or more) */

int main () {
    
    std::string s[MAXS] {};

    for(int i = 0; i < MAXS; i++) {         /* loop assigning to all strings */
        s[i] = std::to_string (i % 10);     /* convert (i % 10) to string */
    }
    /* sort the array of std::string */
    std::sort (s, s + MAXS, [](const std::string& a, const std::string& b){
                               return a < b; });
    
    for (const auto& st : s)                /* output results */
        std::cout << st << '\n';
}

Look things over and let me know if you have further questions.