| Re: Modifying Arrays and Structures in HASH's (hint: you can't) [message #85361 is a reply to message #85360] |
Mon, 29 July 2013 02:44   |
Lajos Foldy
Messages: 176
Registered: December 2011
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Senior Member |
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On Monday, July 29, 2013 10:46:36 AM UTC+2, mschellens wrote:
> As of IDL 8.0, this is not correct. An IDL LIST is really a sinlge linked list made up of (PTR) heap variable nodes (IDL_CONTAINER_NODE). The IDL_CONTAINER::GET function creates then the array.
>
> But your method works, as the (copied) pointers access the same heap variables. This is also the core of the mechanism I suggested for _OVERLOADBRACKETSLEFTSIDE.
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> Also note, that at least with HEAP the IDL_CONTAINER::GET functionality cannot work anymore (as you cannot pick the right element).
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> And it is of course as well not efficient, to convert the complete container to a pointer array in order to left-access one element.
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> And the call to GET is almost as ugly as copying out one element, left-accessing it and copying it back.
>
With huge list elements, copying out and back is very unefficient, creating a pointer array is much faster.
I do not understand the idea behind list. If it is a linked list, then accessing elements through subscripting is O(n) vs. O(1) in arrays. This makes lists practically unusable.
Try this test program to access the last element in a list:
pro list_test
l=list(1,2,3)
t=systime(1)
for j=1,10l^6 do x=l[-1]
print, " 3 elements: ", systime(1)-t
for j=4,10l^3 do l.add, j
t=systime(1)
for j=1,10l^6 do x=l[-1]
print, " 10^6 elements: ", systime(1)-t
end
IDL 8.2.3:
3 elements: 7.6518829
10^6 elements: 47.781787
GDL CVS:
3 elements: 0.5020251
10^6 elements: 44.200854
FL 0.79.25:
3 elements: 0.044948101
10^6 elements: 0.042613983
My pointer array based LIST implementation is about 10-150x faster for the small list, and more than 1000x faster for the large list.
regards,
Lajos
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