| Re: LIST performance [message #73542 is a reply to message #73447] |
Fri, 12 November 2010 09:46   |
chris_torrence@NOSPAM
Messages: 528
Registered: March 2007
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Senior Member |
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Hi JD,
Just to put in my 2 cents, LIST (like much of IDL) is a general
purpose set of functionality. It was never designed to replace the use
of arrays, where the data is laid out in memory in the most efficient
manner. The IDL LIST is implemented as a doubly-linked list, which is
very efficient for adding & removing elements from arbitrary
positions, especially elements of different or complicated types. If
you're only using integers or floats, you probably don't want to use a
list.
Here's an example for adding & removing random elements from an array
of structures:
print,' N Array(s)' + $
' List(s) Ratio'
for nn=2,4 do begin
n = 10L^nn
iter = 10L^(6 - nn)
a = REPLICATE(!map, n)
t = systime(1)
for i=0,iter-1 do begin
index = RANDOMU(seed,1)*(n-2) + 1
a = [a[0:index-1], !map, a[index:*]]
a = [a[0:index-1], a[index+1:*]]
endfor
timearray = systime(1)-t
a = LIST(REPLICATE(!map, n), /EXTRACT)
t = systime(1)
for i=0,iter-1 do begin
index = RANDOMU(seed,1)*(n-2) + 1
a.Add, !map, index
a.Remove, index
endfor
timelist = systime(1)-t
print, n, timearray, timelist, timearray/timelist
endfor
end
When I run this on my Win32 XP laptop, I get the following results:
N Array(s) List(s) Ratio
100 3.3750000 0.38999987 8.6538491
1000 5.5160000 0.10899997 50.605520
10000 7.2500000 0.078000069 92.948636
Part of the reason the LIST is slower in your example is the overhead
with both error checking and the operator overloading. The List::Add
and List::overloadForeach are both method calls, so there is some
additional overhead for making a call instead of just doing it in-
place like in your pointer example.
Now, all that being said, we'll continue to make performance
improvements in future versions. For example, I just rewrote the
List::ToArray to be about 10 times faster. It's still "brutally slow"
for your particular example, but for other scenarios it's much faster.
Finally, I like your idea about making the ::ToArray method work
properly for list elements that are arrays. I'll see what I can do.
Keep the feedback coming.
Cheers,
Chris
ITTVIS
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