Trouble understanding break and scope of for loop.

[Olysold]

After managing to complete problem 7 of the Euler's project (finding the 10001st prime number), I looked in the forums to find a quicker solution that doesn't use cmath. I stumbled on this:

//Parts of it has been edited. Originally bool was used in place of variable pri.

#include <iostream>
using std::cin;
using std::cout; using std::endl;

int main() 
{
	int x = 1, counter = 1;
	int pri = 0;

	while (counter < 10000)
	{
		++x;
	
		for (int priCheck = 2; priCheck < x; ++priCheck)
		
			if (x % priCheck == 0)
			{
				pri = 0;
				break;
			}
			else  
				++pri; 
		
		if (pri >= 2)
	
			++counter;
		
	}
	cout << x << endl;
}

What I don't understand is, once the if condition is met, where does it break to?

Also the original author of this code didn't include curly braces after declaring the for loop. Does the compiler determine the scope of the for loop? I don't get what's going on here. Please, can I get some help understanding what's happening?

[Chervil]

I'm looking purely at the syntax here, not at the deeper meaning of what the code is trying to achieve.
Below, I've added extra braces which don't affect the meaning, but may help to clarify.

#include <iostream>
using std::cin;
using std::cout; using std::endl;

int main() 
{
    int x = 1, counter = 1;
    int pri = 0;

    while (counter < 10000)
    {
        ++x;
    
        for (int priCheck = 2; priCheck < x; ++priCheck)
        {
            if (x % priCheck == 0)
            {
                pri = 0;
                break;
            }
            else
            {            
                ++pri;
            }
        }                
        
        if (pri >= 2)
        {
            ++counter;
        }
        
    }
    cout << x << endl;
}

When the break statement at line 19 is executed, it exits from the for loop which began at line 14.

Also the original author of this code didn't include curly braces after declaring the for loop. Does the compiler determine the scope of the for loop?

If there are no curly braces, then the for loop will control just a single statement. The important thing is how to determine what is a single statement?

Here, this if-else is considered as a single statement,

    if (x % priCheck == 0)
    {
        pri = 0;
        break;
    }
    else  
        ++pri;

Notice how the first part uses braces, that's because there are two statements:

        pri = 0;
        break;

You can see there are two statements, as each one ends in a semicolon. But wrap them in a pair of braces { } it becomes a single compound statement.
Similarly, the whole of the if-else is regarded as a single statement for the purposes of determining the scope of the for loop.

This is one reason why indentation is so very important. It doesn't matter to the compiler. It just sees some extra whitesplace which is ignored. But to the human reader, indentation is used as a way of indicating the scope. You can see all the statements controlled by the for loop are indented, and the next statement aligned vertically with the for itself is line 27 if (pri >= 2) and this is not indented because it does not belong to the scope of the for loop.

[Olysold]

Thanks a lot Chervil, that cleared things up nicely. Appreciate the advice on the indentation too!