Question

The following code is part of an easing equation. I’m just interested in the syntax.

sqrtf(1 - (t = t / d - 1) * t);

I haven’t seen the ‘=’ operator used like this before. What does it do in this context?

Edit: the code is from a well-known Robert Penner easing function, written in ActionScript:

static function easeOut (t:Number, b:Number, c:Number, d:Number):Number {
    return c * Math.sqrt(1 - (t=t/d-1)*t) + b;
}

Answer 1

The = operator here does what it always does. In this case it modifies t (setting it to t / d - 1). The value returned from that part of the equation will be the new value of t.

This becomes confusing because t is used later in the equation, and I wouldn’t be sure that every compiler would treat it the same. ie is the third t going to use the old value or the new value?

I would avoid writing equations in this way. It’s lazy and I don’t see any good reason to do it.