sense of if - how is it possible that works

Question

I'm watching Mojam (live games programming, on HumbleBundle) and I saw this piece of code there (here is screenshot of code) I'm wondering how come the condition expression in last if statement (which is !wasmoving && ismoving) ever evaluates to true (it does, programmer compiled it and animation was running). I've tried this in my compiler

#include <iostream>
#include <cstdlib>
using namespace std;
int main(){
    bool ismoving=5>0;
    bool wasmoving=ismoving;
    cout << "ismoving"<< ismoving << " oraz wasmoving " << wasmoving << endl;
    if (!wasmoving && ismoving) cout << "1st = true 2nd = true" << endl;
    system("PAUSE");
    return 0;
}

and of course if the last if takes false path, nothing happens. Could anyone explain how it's posssible that code from screenshot worked?

Answer 1

Note that in the screenshot, the ismoving variable is updated after it is copied to wasmoving, so it is perfectly possible that the value of ismoving is different from the value of wasmoving by the time the condition is evaluated.

Answer 2

Your transformation of the code from the screenshot is inaccurate. You have changed the order of the operations.

C++ and Java, like most mainstream programming languages, are "imperative" languages. This means that programs consist of sequences of instructions that are performed one after another. When that code does:

boolean wasJumping = isJumping;
isJumping = ...someCalculation...;

That first sets the value of wasJumping to be the current value of isJumping, then changes the value of isJumping to something else. wasJumping doesn't change - it still has the old value that was taken from isJumping.

If you come from a mathematical background this can be confusing. I find it helps to read "wasJumping = isJumping" as "wasJumping becomes equal to isJumping", not "wasJumping equals isJumping". The former reinforces that it's describes a change in a variable, rather than describing a permanent relationship between two values.