why cout is not printing this string?

Question

So, I'm new to C++ and I can't figure why this is happening. I have a string with all the alphabets and I copied 10 characters from it into a new string s2, character by character using a for loop as follows and when I execute it the cout function is printing a blank line.

#include <iostream>

using namespace std;

int main(){
    string s = "abcdefghijklmnopqrstuvwxyz";
    string s2;
    for(int i=0; i<10; i++){
        s2[i] = s[i];
    }
    cout << s2 << endl;
    return 0;
}

But when I print this string s2 character by character I got the correct output

#include <iostream>

using namespace std;

int main(){
    string s = "abcdefghijklmnopqrstuvwxyz";
    string s2;
    for(int i=0; i<10; i++){
        s2[i] = s[i];
    }
    
    for(int i=0; i<10; i++){
        cout << s2[i];
    }
    return 0;
}

Any help would be appreciated!

Answer 1

Both your code have undefined behavior. string s2; just initializes s2 as an empty std::string which contains no elements. Trying to access element as s2[i] leads to UB, means anything is possible. (Even it appears to give the correct output in your 2nd code snippet.)

You can use push_back to append element as

for(int i=0; i<10; i++){
    s2.push_back(s[i]);
}

Or

for(int i=0; i<10; i++){
    s2 += s[i];
}

Or you can make s2 containing 10 elements in advance.

string s2(10, '\0'); // or... string s2; s2.resize(10);
for(int i=0; i<10; i++){
    s2[i] = s[i];
}