C++ trouble with while loop addition

Put your statements in curly braces {. Otherwise you only execute the output statement in the cycle and the increment of i will happen outside of the cycle.

Your while loop has no braces.

it's treating your code as:

    while ( i < 11)
        cout << i << '\n';
    i++;

and you really want:

    while ( i < 11)
    {
       cout << i << '\n';
       i++;
    }

You need to write:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    int i = 0;
    while ( i < 11){
        cout << i << '\n';
        i++;
    }

}

note the extra bracing after ( i < 11)

Out of interest, it's stylistically preferable to use a for loop here:

int main()
    {
        for (int i = 0; i < 11; ++i){
            cout << i << '\n';                
        }

    }

Note how I've brought i into the loop scope so it cannot be accessed outside the loop; helping program stability. All the things that happen to i (declaration, definition, termination condition and increment) are all on the same line as well; helping readability.

In C++ you have the choice of putting a single statement into braces or not.

However, if you have multiple statements in a loop-body, you must use braces:

while ( i < 11) {
    cout << i << '\n';
    i++;
}

C++ is not space sensitive like e.g. python, so the following would work, too:

while ( i < 11) {cout << i << '\n';i++;}

while ( i < 11)
{
    cout << i << '\n';
    i++;
}

while ( i < 11)



{
    cout << i << '\n';
    i++;
}

The body of a loop can either be a single statement:

while ( i < 11)
    cout << i << '\n';

or a compound statement; that is, a group of statements surrounded by braces:

while ( i < 11) {
    cout << i << '\n';
    i++;
}

You have written the first form when you want the second. Unlike some other languages, indentation has no meaning in C++, and statements are only grouped into blocks when surrounded with {}.

Loops in C++ only loop the immediately-following statement unless they've been put in a code block. That is, you code is equivalent to:

int main()
{
    int i = 0;
    while ( i < 11)
    {
        cout << i << '\n';
    }
    i++;
}

This this instead:

int main()
{
    int i = 0;
    while ( i < 11)
    {
        cout << i << '\n';
        i++;
    }
}

 while ( i < 11)
    {
       cout << i << '\n';
       i++;
    }