Game Maker Studio, script arguments not responding to variables

Question

To make my game more dynamic, I have created a script in which arguments are used in order to be substituted by pre defined variables.

Upon clicking the left mouse_button, there is the code:

script_execute(scrShoot, weapon1, ammo1);

Where weapon1 and ammo1 are local variables.

The simple script 'scrShoot' is as follows:

if argument0 = 1
{
    argument1 -= 0.05;
    instance_create(x,y,objBullet);
}

if argument0 = 2
{
    argument1 -= 0.05;
    repeat(4)
    {
        instance_create(x,y,objBullet2);
    }
}

argument0 works as expected, being successfully substituted by weapon1, however, the variable ammo1 never decreases.

When I manually write in ammo1 in place of argument1, the script works fine; decreasing by 0.05 with every click.

Other tests using scripts have lead me to believe that the problem lies with using variables to substitute arguments: strings and numbers work as you would expect.

I have encountered this problem in more than one scenario and I'm baffled that nobody else on the internet seems to have come across the same problem.

score 3 · Answer 1 · edited May 23 '17 at 12:08

When you pass a number as an argument to a script, you’re giving that script a copy of the number to work with. That is, the script doesn’t see that you’re passing it ammo1 – it only sees that you passed it, say, 50. The line argument1 -= 0.05 just modifies the copy the script receives, not ammo1 itself.

This is called passing an argument by value (giving the script a copy that it can modify), as opposed to by reference (pointing the script to a variable to modify).

See: What's the difference between passing by reference vs. passing by value?

GML itself doesn’t have syntax for passing arguments by-reference, so you’re out of luck. What you can do, I believe, is pass an instance ID (like self or other, or the result of a call to instance_create) to the script:

/// scrShoot()
var o = argument0;
if (o.weapon == 1) {
    o.ammo -= 0.05;
    instance_create(x, y, objBullet);
}

/// Your object
script_execute(scrShoot, self);

Game Maker Studio, script arguments not responding to variables

1 Answers1