More fun with C/C++

In order to write a program it's useful (but not necessary) to have an accurate mental model of how the language works, so you can mentally interpret your code before handing it off the compiler. So if you're a C/C++ programmer take a look at the following code and see if you can correctly identify the output:

#include <stdio.h>

int foo(int a, int b, int c, int d)
{
    if (a>b)
    {
        return c;
    }
    else
    {
        return d;
    }
}

int main(int argc, char * argv[])
{
    int x = 2;
    int y = 1;

    printf("Result: %d\n", foo(x+=y, y+=x, x+=y, y+=x));

    return 0;
}

In this case the output is non-deterministic. While C/C++ specify what order arguments will be pushed onto the stack it doesn't specify what order they'll be evaluated in. So there are several possibilities for the output. This flexibility allows compiler writers to order the evaluation of operations in the most efficient way. If you want to avoid confusing/broken code don't mutate variables in argument positions. The person that ends up maintaining your code will thank you for it.