For-loop variable scope confusion

Question

I have noticed a weird behavior of the variables in for loops. It's not really a problem, but it disturbs me a lot.
Actually I've created two loops this way:

for (var i:uint; i<19; i++) SomeFunction (i);
for (var i:uint; i<26; i++) SomeOtherFunction (i);

What I received was a compilation warning:
Warning: Duplicate variable definition.

This warning really surprised me. Nothing like that ever happened to me in other languages.
It seems that the i variable gets into the scope that is higher in the hierarchy and becomes available out of the loop's block. I've also tried to embrace the loop block in a curly brace, but it didn't change anything.
Why does it happen? Is it normal? Is it possible to avoid it? For now I've just set different names for both of the variables, but that's not a real solution I think. I'd really like to use the i-named variable in most of my for-loops.

Ryan Guill · Accepted Answer · 2013-02-25T13:29:52.043

yes, the loop increment variable is in the scope of the loops parent, not inside the loop itself. This is intentional, for examples like this:

public function getPositionOfValue ( value:String ) : int
{
    for ( var i:int = 0; i < someArray; i++ )
    {
        if (someArray[i] == value )
        {
            break;
        }
    }

    return i;
}

this allows you to access the value of i once the loop is over. There are lots of cases where this is very useful.

What you should do in the cases where you have multiple loops inside the same scope is var the i outside of the loops:

public function getPositionOfValue ( value:String ) : int
{
    var i:int;

    for ( i = 0; i < 15; i++ )
    {
        //do something
    }

    for ( i = 0; i < 29; i++ )
    {
        //do something else
    }

    return i;
}

then you get rid of your warning. The other thing to consider is to name your loop increment variables something more descriptive.

Update: Two other things to consider:

~~1) you shouldn't use uints except for things like colors and places where Flex expects a uint. They are slower than int's to use. Source]1~~ Update: it looks like this may no longer be the case in newer versions of the flash player: source

2) when you var a loop increment variable inside of a loop declaration, you want to make sure you set it to the proper initialization value, usually 0. You can get some hard to track down bugs if you dont.

Thank you for the meaningful answer. :) BTW: 1) I didn't know that `uint` is slower. Thanks, it may help me in the future. 2) I know, but I've read that if an `int` or `uint` is uninitialized, its value is always 0. — rhino, Dec 18 '10 at 20:23
no problem, glad to help. Re: your response to 2), thats right, but if you var your i outside of the loops, your second loop is going to have the value of the previous loop now. so be careful. In my experience, its always better to be explicit. — Ryan Guill, Dec 18 '10 at 20:26

paxdiablo · Answer 2 · 2010-12-18T20:20:49.057

As mentioned here, as3 has global and local scope and that's about it.

It does not do block-level scoping (or for-level either). With hoisting, you can even write to variables before you define them. That's the bit that would do my head in :-)

Early versions of Visual C had this bug, leading to all sorts of wonderful funky macro workarounds but this is not a bug in as3, it's working as designed. You can either restrict your code to having the declaration in the first for only or move the declaration outside all the for statements.

Either way, it's a matter of accepting that the language works one way, even though you may think that's a bad way :-)

score 2 · Answer 3 · answered Dec 18 '10 at 20:10

2

Declare the variable i outside the loops to avoid this. As long as you reset it (i=0) you can still use it in all loops.

var i : uint;
for (i=0; i<19; i++) SomeFunction(i);
for (i=0; i<26; i++) SomeOtherFunction(i);

answered Dec 18 '10 at 20:10

Mattias

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For-loop variable scope confusion

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