| Bizarre slowness from sort() [message #30456] |
Tue, 23 April 2002 07:59 |
Jonathan Joseph
Messages: 69
Registered: September 1998
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Member |
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Hello,
My colleague complained of an incredible slowness when trying to sort
an array of long integers (on the order of 400,000 of them). I said
"you're nuts. Must be a bug in your code" and proceeded to generate
a random array of 400,000 long integers and sort them very quickly.
"See, it works fine."
So, he showed me his code, and it all looked perfectly normal, and
the sort took minutes! The data looked fine (no bizarre values)
so we created a save file, opened up a new IDL session tried to sort
the data and saw the same slowness!
I've found that the problem occurs on SUN and Windows 2K running IDL
5.5, but not on HP-UX running IDL 5.3. Also, we have found
a workaround for the integer case. Adding a small (less than 1) random
offset to each element of the array before sorting will make it work
quickly and yield the correct result. But this will not work properly
unless the array to be sorted is an integer type array, otherwise you
could be changing the sort order by adding the random offset. Just
converting the array to float or adding a constant offset to each
element does NOT fix the problem.
This behavior seems very strange - possibly a bug in IDL. Anyone
have any thoughts on this? Can you reproduce this bug on your
system?
The save file is located at
http://baritone.astro.cornell.edu/~jj/idl2/
and is called 'sort.bin' (about 1.4 megabytes)
IDL> restore, 'sort.bin'
IDL> help, sortme
SORTME LONG = Array[376467]
IDL> a = sort(sortme)
Works, but takes minutes to return.
If I add a random number between 0 and 0.1 to each element
and then sort, it works very rapidly (and produces the correct
result since it is not changing the sorting order)
IDL> b = sort(sortme + randomu(seed, n_elements(sortme)) * 0.1)
Works very fast as expected
Anyone know what's going on?
Thanks.
-Jonathan
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