AS3 Array“for each ... in”性能与for循环，并按顺序保证？

Question

This question is really twofold:

1. How does for each looping over an Array compare performance-wise with a simple for loop through its elements?

2. Does a loop guarantee in order traversal? The following code says yes:

 var sample_array:Array = []; for (var i:uint = 0; i < 10000; i++) sample_array.push(i); i = 0; for each(var value:uint in sample_array) { sample_array[i++] = value; } trace('in order was:', check_in_order(sample_array)); function check_in_order(array:Array):Boolean { for (var i:uint = 0, l:uint = array.length; i < l; ++i) { if (array[i] != i) return false; } return true; }

but I have heard other (senior-level) engineers swear up and down that the traversal does not always proceed in ascending order! Is this true?

Answer 1

As demonstrated by Jackson Dunstan , who compared for , for each and for in with different collections (arrays, vectors, etc), the performance ranking is (from fastest to slowest):

1. for
2. for each
3. for in (extremely slow)

In all cases for is the fastest of all. In particular when used with arrays or vectors, for has an overwhelming performance. Those are the numbers for all collections (smaller means faster):

Answer 2

here is fixed post from Adrian

var size:Number = 10000000;
var arr:Array = [];
var time:Number, o:Object, i:int;
// fill container
for (i = 0; i < size; i++) arr[i] = i;

time = getTimer();
for (i=0; i<arr.length; i++){ arr[i]; }
trace("for test: "+(getTimer()-time)+"ms");

// for()
time = getTimer();
for (i=arr.length; i>0; i--) { arr[i]; }
trace("for reversed test: "+(getTimer()-time)+"ms");

// for..in
time = getTimer();
for each(o in arr) { o; }
trace("for each test: "+(getTimer()-time)+"ms");

// Result
for test: 212ms
for reversed test: 26ms
for each test: 103ms

Answer 3

Please check my post Flex Array Performance: For vs. ForEach

Some code

var size:Number = 10000000;
var arr:Array = [];
for (var i:int=0; i
var time:Number, o:Object;

// for()
time = getTimer();
for (i=0; i=0; i--) { arr[i]; }
trace("for reversed test: "+(getTimer()-time)+"ms");

// for..in
time = getTimer();
for each(o in arr) { o; }
trace("for each test: "+(getTimer()-time)+"ms");

Results

for test: 124ms
for reversed test: 110ms
for each test: 261ms