#include <iostream>

// This example demonstrates that the loop variable
// in a for loop can be modified both by the update
// expression and within the body of the loop.
//
// It also demonstrates why the scope of a variable
// can matter after a loop has terminated.

// Function declarations
int main();

// Function definitions
int main() {
    int k{};

    // In this for loop, 'k' is incremented by the
    // update expression ++k at the end of each
    // iteration. However, the user may also change
    // 'k' within the body of the loop.
    //
    // Because 'k' was declared before the for loop,
    // it remains in scope after the loop terminates.
    // We can therefore print the value of 'k' that
    // caused the condition k < 20 to become false.
    //
    // Notice that 'k' may increase by more than one
    // during an iteration:
    //
    //   1. the user enters a value that is added
    //      to 'k', and
    //   2. ++k is then executed before the loop
    //      condition is tested again.

    for ( k = 0; k < 20; ++k ) {
        std::cout << "k = " << k << std::endl;

        int value{};
        std::cout << "Add what to 'k'? ";
        std::cin  >> value;

        k += value;

        std::cout << "The value of 'k' is now "
                  << k << "." << std::endl
                  << std::endl;
    }

    std::cout << "The final value of 'k' is "
              << k << "." << std::endl;

    return 0;
}



























// Exercises:
//
// 1. Suppose the user always enters 0. What values
//    of 'k' are printed, and what is the final value
//    of 'k' after the loop terminates?
//
// 2. Remember that ++k is still executed after the
//    statements in the loop body. If the user enters
//    5 when k == 10, what will the value of 'k' be
//    when the condition k < 20 is next tested?
//
// 3. What happens if you replace
//
//        for ( k = 0; k < 20; ++k ) {
//
//    with
//
//        for ( int k{ 0 }; k < 20; ++k ) {
//
//    Before compiling the program, predict what will
//    happen. Will it compile? If it does, what value
//    will the final std::cout statement print?
//
//    Remember that there is already a variable named
//    'k' declared before the for loop.



























// Explanation:
//
// If you write
//
//     for ( int k{ 0 }; k < 20; ++k ) {
//
// the program still compiles.
//
// The declaration creates a new variable named 'k'
// whose scope is the for loop. This new variable
// shadows the 'k' declared at the beginning of
// main().
//
// Thus, inside the loop, 'k' refers to the variable
// declared by the for loop. After the loop ends,
// that variable no longer exists, and 'k' once again
// refers to the variable declared at the beginning
// of main().
//
// Because that outer variable was initialized to 0
// and was never changed, the final statement prints
//
//     The final value of 'k' is 0.
//
// This is one reason to pay careful attention to
// where variables are declared and to the scope of
// each variable.