#include <iostream>

// The following program compiles, but there are
// inputs for which it prints the wrong result.
//
// First, try several inputs and determine which
// inputs cause the program to produce an incorrect
// result. Then identify and fix the bug.
//
// Try to find the bug before reading the explanation
// below.

// Function declarations
int main();

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

    std::cout << "Enter an integer: ";
    std::cin  >> n;

    int result{ n };

    if ( n < 0 ) {
        int result{ -n };
    }

    std::cout << "The absolute value of "
              << n << " is " << result << std::endl;

    return 0;
}



























// Explanation:
//
// The program works correctly when n >= 0, but it
// produces the wrong result whenever n < 0.
//
// The problem is that 'result' is declared twice.
//
// First, it is declared in the body of main():
//
//     int result{ n };
//
// A second local variable with the same name is then
// declared inside the block of the if statement:
//
//     if ( n < 0 ) {
//         int result{ -n };
//     }
//
// These are two different local variables.
//
// Inside the block, the second 'result' shadows the
// first 'result'. Thus, while executing that block,
// the identifier 'result' refers to the variable
// declared inside the block.
//
// That variable is correctly initialized to -n, but
// it ceases to exist when execution reaches the end
// of the block.
//
// The original 'result' was never changed. Therefore,
// after the block, it still contains its original
// value of n.
//
// The solution is not to declare another variable
// named 'result'. Instead, assign -n to the variable
// that was already declared.



























// Here is the corrected program:
/*
int main() {
    int n{};

    std::cout << "Enter an integer: ";
    std::cin  >> n;

    int result{ n };

    if ( n < 0 ) {
        result = -n;
    }

    std::cout << "The absolute value of "
              << n << " is " << result << std::endl;

    return 0;
}
*/

























// If this is still confusing, consider how the
// compiler treats the two declarations.
//
// Despite containing exactly the same characters,
// the two occurrences of the identifier 'result'
// refer to different local variables while inside
// the block.
//
// We could rename the variable declared inside the
// block without changing what the program does:
//
//     int main() {
//         int n{};
//
//         std::cout << "Enter an integer: ";
//         std::cin  >> n;
//
//         int result{ n };
//
//         if ( n < 0 ) {
//             int douglas_adams{ -n };
//         }
//
//         std::cout << "The absolute value of "
//                   << n << " is "
//                   << result << std::endl;
//
//         return 0;
//     }
//
// It should now be clear that 'result' is never
// changed when n < 0. The variable 'douglas_adams'
// is initialized and then immediately ceases to
// exist.
//
// Declaring a local variable with the same name as
// another local variable that is still in scope is
// called shadowing. Although C++ permits shadowing,
// it can make programs more difficult to understand
// and, as this example demonstrates, can easily
// introduce bugs.