Why is iterating through an array backwards faster than forwards

Question

Given this code:

var arr = [];

for (var i = 0; i < 10000; ++i)
    arr.push(1);

Forwards

for (var i = 0; i < arr.length; ++i) {}

Backwards

for (var i = arr.length - 1; i >= 0; --i) {}

Hard-coded Forward

for (var i = 0; i < 10000; ++i) {}

Why is backwards so much faster?

Here is the test: http://jsperf.com/array-iteration-direction

Solution 1

Because your forwards-condition has to receive the length property of your array each time, whilst the other condition only has to check for "greater then zero", a very fast task.

When your array length doesn't change during the loop, and you really look at ns-perfomance, you can use

for (var i=0, l=arr.length; i<l; i++)

BTW: Instead of for (var i = arr.length; i > 0; --i) you might use for (var i = arr.length; i-- > 0; ) which really runs through your array from n-1 to 0, not from n to 1.

Solution 2

Because in the first form you are accessing the property length of the array arr once for every iteration, whereas in the second you only do it once.

Solution 3

If you want to have them at same pace, you can do that for forward iteration;

for(var i=0, c=arr.length; i<c; i++){
}

So, your script won't need to take length of array on everystep.

Solution 4

I am not entirely sure about this, but here is my guess:

For the following code:

for (var i = 0; i < arr.length; ++i) {;
}

During runtime, there is an arr.length calculation after each loop pass. This may be a trivial operation when it stands alone, but may have an impact when it comes to multiple/huge arrays. Can you try the following:

 var numItems = arr.length;
    for(var i=0; i< numItems; ++i)
    {
    }

In the above code, we compute the array length just once, and operate with that computed number, rather than performing the length computation over and over again.

Again, just putting my thoughts out here. Interesting observation indeed!

Solution 5

i > 0 is faster than i < arr.length and is occurring on each iteration of the loop.

You can mitigate the difference with this:

for (var i = 0, len = arr.length; i < len; ++i) {;
}

This is still not as fast as the backwards item, but faster than your forward option.

View more solutions

Comments

[samccone]

look at the hard coded test jsperf.com/array-iteration-direction ... using your logic it should be faster no?

[samccone]

Ahhh... interesting your method now takes the crown for fastest iteration jsperf.com/array-iteration-direction

[Mike Dunlavey]

The backward case isn't right. What I do is for(i=len; --i>=0;){

[Mike Dunlavey]

It should be while(--L>=0) arr[L] = L;

[jfriend00]

@samccone - what result are you seeing (which test in which browser)? I'm seeing that caching the length is always at least as fast and usually faster.

[Jeffrey Sweeney]

Yep, this is how I do it for optimized functions. Just note that if the array is mutable (and it usually isn't), this may skip a resource or two. For example, this won't work if you're looping through elements that the loop itself is adding or deleting.

[samccone]

alright you are correct... it looks like it is really a mixed bag of results tho but caching does help it... thanks XD

[ajax333221]

you can even do for(... ; i-- ;)

[Breno Inojosa]

so basically it means that if I don't change the array and I keep array.length in a variable, then backwards speed = forward speed? Example: var length = array.length; for(var i = 0 ; i<length ; i++);

[Bergi]

afaik, i<l is a different computation than i>0, but yes they're rather equal (faster than uncached length): jsperf.com/array-iteration-direction/3 - yet some engines do recognize and optimize some of these cases

[bryc]

The condition of a loop is evaluated on each iteration. Think of it like var i = 0; var a = i < 10; while(a) { i++; a = i < 10; }.

[Bergi]

@bryc: So what? Of course it needs to be. Did I say something else?

[Max Yari]

Isnt this loop for (var i = arr.length; i-- > 0; ) going to miss 0 index as 1-- !>0? maybe this one needs to be for (var i = arr.length; i-- >= 0; )

[Bergi]

@MaxYari: No. The i in the loop body (after being decremented) is always one smaller than the i seen in the condition (before being decremented). What you would need is for (var i=arr.length-1; i>=0; --i)

[Max Yari]

tested, yes 0 is here, i honestly did not understand yet why, isnt it in the first place decrements i, then compares to 0, then decides to run iteration or not. Like this, it seems that 1 must be decremented to 0, therefore not starting iteration as it's not > 0

[Bergi]

@MaxYari: Read on prefix/postfix decrement operators then

[Max Yari]

Oh snap!! you opened my eyes, what a shame, thank you)

[Gaurang Patel]

Fast forward to 2018. it looks like the modern versions of browsers don't make loop slower even when accessing arr.length in each iteration (your forwards case)

[Vincent]

for (let i = arr.length; i-- > 0;) {