Why does left justification not work in the first iteration of a loop?

#include <iostream>

int main(){
       using namespace std;

       string s1("Hello1");
       string s2("Hello2");
       
       for(int i = 0; i < 3; i++){
           
           cout.width(20); cout<<"Some String:"<<left<<s1<<endl;
           cout.width(20); cout<<"Another String:"<<left<<s2<<endl;
           
           cout<<endl;
       }
       return 0;
 }

Here is my code. It, to my knowledge, should print s1 and s2 20 characters from the very left of the screen. However, it prints

        Some String:Hello1
Another String:     Hello2

Some String:        Hello1
Another String:     Hello2

Some String:        Hello1
Another String:     Hello2

I am using onlineGDB to compile, as per instruction of my teacher. What error am I making?

std::left is a "sticky" manipulator, meaning you just set it once. By default, padded strings will be right-justified, which is what happens when you output "Some String:" before ever applying the std::left manipulator.

See the documentation which states:

The initial default for standard streams is equivalent to right.

Fixing your code, and tidying it up a bit:

#include <iostream>
#include <iomanip>

int main()
{
    using namespace std;

    string s1("Hello1");
    string s2("Hello2");

    cout << left;       
    for(int i = 0; i < 3; i++) {
        cout << setw(20) << "Some String:" << s1 << endl;
        cout << setw(20) << "Another String:" << s2 << endl;
        cout << endl;
    }
    return 0;
}

Output:

Some String:        Hello1
Another String:     Hello2

Some String:        Hello1
Another String:     Hello2

Some String:        Hello1
Another String:     Hello2

Note that I used std::setw I/O manipulator from <iomanip> instead of the call to cout.width(). This makes the code easier to read and follow.

See the documentation for std::setw