Scripting Question

[P.M.B]

So I'm following a beginners scripting lesson for AE and my code sent AE into an infinite loop.

When I changed the line myValue = myValue++;   to  myValue = myValue+1;   it fixed the issue and the script ran correctly.

As I learned this operator using MEL which I believe is derived from C+ I thought maybe it's just unique to C+ but

a quick search of JavaScript operators shows me that ++ is a valid operator for increment.

Why did this fail but work when I changed the last line to myValue = myValue+1; ?

//defines selected active composition in active project in active app

var myComp = app.project.activeItem;

//defines layer 1 of "myComp"

var myLayer = myComp.layer(1);

//defines value for conditional loop

var myCounter = 1;

//conditional loop

while(myCounter <= 10)

    {

      //duplicates active layer and defines it as

    var dupLayer = myLayer.duplicate();

    //rotates dupLayer additive 45 degrees

    dupLayer.rotation.setValue(45*myCounter);

    //increases reference value by one

    myCounter = myCounter++;

    }

Thanks,

Paul

[Mike_Abbott]

I'm a scripting newby myself - so, don't bet the farm on this, but...

I think this is a case of the increment operator being available for use as a ''prefix' or 'postfix' operation.

Your original code: myValue = myValue++ is a postfix operation. MyValue is set to the value of myValue - then the value is incremented.

See the entry on this page:

Arithmetic operators - JavaScript | MDN

I don't think this is an issue when using the plain addition operator - hence the infinite loop of the original code versus the success of the +1 version.

[P.M.B]

Thanks Mike,

I saw the example you're talking about but I can't say i fully understand.  Meaning I can't think of real world scenario where I would do what that example did.  I simply want to increment the current value of variable(x) by one.  All of those examples define a second variable that I don't understand.

// Postfix
var x = 3;
y = x++; // y = 3, x = 4

// Prefix
var a = 2;
b = ++a; // a = 3, b = 3

I'm sure it's perfectly logical but I cannot see how it applies to what i am trying to do.

I just want to increment the var x by 1.

Would it be   x = ++x;  ​?

Or maybe it's just  x++;  ?

I suppose I'll just stick with  x=x+1;

[Mike_Abbott · Accepted solution]

I think the example you show above is just using a second variable to try and explain what is going on - but is confusing matters further. Here's Google's explanation, which I think is helpful:

Increment (++) The increment operator increments (adds one to) its operand and returns a value. If used postfix, with operator after operand (for example, x++), then it returns the value before incrementing. If used prefix with operator before operand (for example, ++x), then it returns the value afterincrementing.

In place of your code:

myCounter = myCounter++;

I think you could just use:

++myCounter

[P.M.B]

Thanks so much for taking the time to explain it, Mike.

If I may ask one more question...

If postfix returns the value before incrementation than why, in the first example, does x return a value of 4?

/ Postfix

var x = 3;

y = x++; // y = 3, x = 4

In fact in both examples the original variable gets incremented by one. 

[Mike_Abbott]

This thread needs an expert... : )

[P.M.B]

Mike, for the record your solution was correct.

It's just I just don't understand the example & that bugs me.

The answer will come eventually.

Thanks again for your help.

-Paul

[Dan Ebberts]

This is the order of operations in your two examples:

// post fix

x = 3;

y = x;

x = x+1;

// pre fix

a = 2;

a = a+1;

b = a;

I hope that helps.

Dan

[P.M.B]

Thanks Dan.

Unfortunately that does not help.

Hopefully you'll indulge me once more.

I've been playing around in Toolkit trying to understand but it makes no sense to me.

////////////////////////////

Example 1:

1. var myNum = 3;

2. myNum;

3. //result 3

4. myNum;

5. //result 3

6. myNum++;

7. //result 3

8. myNum;

9. //result 4???

So on lines 2 & 4 I call "myNum" and it outputs "3".  (this makes sense to me)

On line 6 I use the operator postfix & it outputs "3" (this makes sense to me)

On line 8, when I call my variable again it outputs "4" (I do not understand why)

/////////////////////////////

Thanks for your time,

Paul

Wait....I think it just hit me.

When I use prefix it applies the increment "pre" execution of the statement.

When I use postfix it applies the increment "post" execution of the statement.

So the statement returns the original value of the variable & THEN applies the increment.

So the next time I call the variable the increment has been applied.

Is that right?