loop perfomance: for vs. for-in?

[petermichaux]

Hi,

Is there a general consenses which is faster?

a) a for loop to iterate through n objects stored in an array
b) a for-in loop to iterate through n objects stored as properties of
an object

I'm deciding which storage technique I should use for targets in a
dragdrop application. When I move the dragging object I must loop
through the possible targets to determine which targets the dragging
object is over. Since this must happen with every mouse move I'd like
to make this efficient.

Thanks,
Peter

[RobG]

pe**********@gm ail.com wrote:

Hi,

Is there a general consenses which is faster?

a) a for loop to iterate through n objects stored in an array
b) a for-in loop to iterate through n objects stored as properties of
an object
Test it and see - there are also do..while and while loops. You may
chose to use an array of objects, where each object has a reference to
an element and its bounding coordinates. You can use for..in with an
Array too (it's a JavaScript Object too) provided you deal with any
other enumerable properties you may have added.

However, be careful of premature optimisation - you may be searching
for fractions of a millisecond with loops but missing much greater
optimisations elsewhere.

I'm deciding which storage technique I should use for targets in a
dragdrop application. When I move the dragging object I must loop
through the possible targets to determine which targets the dragging
object is over. Since this must happen with every mouse move I'd like
to make this efficient.

It probably doesn't matter. If your drop zones don't overlap, have you
considered using a mouseover on the drop zones so they report "hey,
you're over me"?

--
Rob

[petermichaux]

Hi Rob,

Thanks for the info

RobG wrote:

pe**********@gm ail.com wrote:
It probably doesn't matter. If your drop zones don't overlap, have you
considered using a mouseover on the drop zones so they report "hey,
you're over me"?

My drop zones may overlap. However if they did not I haven't found a
way to do the mouseover listing because the draggable item is always
directly under the cursor. So the target never registers a mouse over.
Is there a new trick for this? Most libraries have to loop through the
targets for each mousemove to examine the region of each target and the
cursor position. It is slow.

Peter

[lofty00]

pe**********@gm ail.com wrote:

Hi Rob,

Thanks for the info

RobG wrote:

pe**********@gm ail.com wrote:
It probably doesn't matter. If your drop zones don't overlap, have you
considered using a mouseover on the drop zones so they report "hey,
you're over me"?

My drop zones may overlap. However if they did not I haven't found a
way to do the mouseover listing because the draggable item is always
directly under the cursor. So the target never registers a mouse over.
Is there a new trick for this? Most libraries have to loop through the
targets for each mousemove to examine the region of each target and the
cursor position. It is slow.

Peter

I saw a trick in the book 'DHTML Utopia' which you might be able to
use. The author suggests having an extra parallel layer of invisible
(transparent) elements above the layer with the draggable object in.
Each invisible object is above one of the real drag targets, so you can
use an onmouseover event on it.