JacksonDunstan.com | Loop Safety

Loop Safety

July 10, 2009 Tags: loop

This is a curiosity I’ve had for far too long. Why didn’t I make these simple tests years ago when I was first learning AS3? I’m not sure, but judging by a lot of other people’s AS3 that I’ve read, many people don’t seem to understand it.

What does this print?

trace("before");
for (var i:* in null)
{
	trace("inside");
}
trace("after");
 
trace("before");
for each (var j:* in null)
{
	trace("inside");
}
trace("after");

Does the loop crash on the null when it tries to iterate over it? Would it necessitate the kinds of null checking I’ve seen so often in AS3 code? Nope! The for-in and for-each loops conveniently and efficiently skip the loop and we get:

before
after
before
after

On that happy note, enjoy your weekend!

Comments

#1 by golgobot on July 10th, 2009 · Reply

That got me to thinking.

What do you think the following code would do?

var dict:Dictionary = new Dictionary(); dict[null] = "null"; dict[undefined] = "undefined"; dict[NaN] = "NaN"; dict[int] = "int"; for (var i:* in dict) { trace("dict["+i+"] = " + dict[i]); } for each (var j:* in dict) { trace("j = " + j); }

Well apparently it’s all valid. Here’s the output.

dict[null] = null
dict[undefined] = undefined
dict[NaN] = NaN
dict[[class int]] = int
j = null
j = undefined
j = NaN
j = int
- #2 by jackson on July 10th, 2009 · Reply
  
  This is actually in my notes for an upcoming post. I will cover some details of dictionary keys. For example, what kinds of type conversion do you think it does? Consider this:
  
  var dict:Dictionary = new Dictionary(); dict[3] = "hello"; trace(dict["3"]); // what is printed?

#3 by golgobot on July 10th, 2009 · Reply

on that note what would this code trace out?

var dict:Dictionary = new Dictionary();
dict[NaN] = "NaN Real";
dict["NaN"] = "NaN String";
dict[null] = "null Real";
dict["null"] = "null String";
dict[Dictionary] = "Dictionary real";
dict["flash.utils.Dictionary"] = "Dictionary String";
trace(dict[NaN] + ", " + dict[null] + ", " + dict[Dictionary])

When you actually run this code, you’ll be surprised, but not taken aback.

#4 by jackson on July 18th, 2009 · Reply

Thanks for this. It’s the subject of post for Friday the 24th.