Question

I am very curious to learn why the below code does not run in a continuous loop. And I’m also looking for some ways to achieve what I want to achieve–which is resetting a loop inside of the loop. I need to do this because I need to account for each element in a container. The reason why this is because I might start off in the middle, and need to loop back around to check the others / and need to recheck other information too. So on with my little test example:

for ( int i = 0; i != 10; i++ ) {
    std::cout << std::endl << "TEST: " << i << std::endl;
    if ( i++ == 10 ) {
        i = 0;
    } else {
        i--;
    }
}

Is there any particular reason why the above does not work? I am very interested in knowing why, so I can learn how everything works. This also leads into a much bigger problem I am facing. Which is the below code. I am using MSVC++ 2010 Express. Also, this is one thread, so other data is not accessing it. It is an unordered_map using STL. its size if 2 (i checked).

for (game_player_client_map::const_iterator it = gpc_map_ptr->begin(); it != gpc_map_ptr->end(); ++it) {
    if ( it++ == gpc_map_ptr->end() ) {
        cout << endl << "IT == gpc_map_ptr->end()" << endl;
        it = gpc_map_ptr->begin();
    } else {
        it--;
    }
}

I appreciate any feedback SO has to offer, and any new things to learn 🙂 If further information is needed I will provide. Thank you for your time.

Answer 1

Because the condition is checked before the body of the loop is entered. When i == 10, the loop is broken, before your code can execute at the time that i++ would evaluate to 10.

Remember that postincrement increments the variable and returns the old value. So if i is 9, i++ evaluates to 9 also, but the next time you use i, it will be 10.

If you want the variable to be incremented and use the new value in an expression, use preincrement:

if (++i == 10) // changes i to i + 1 and checks if the new value of i is 10

You could completely ditch the increment however, and just use i + 1. That way you don’t have to de-increment i in the else block.

Your misunderstanding of postincrement is probably also the source of the bug in the second block of code you posted. You can change it to preincrement, or if it is a random-access iterator, you can do the same thing as mentioned above and check if it + 1 == gpc_map_ptr->end() and not have to de-increment it in the else block.