Re: fastest operations/fctns.

[Craig Markwardt <craigmnet(at)cow.physics.wisc.edu>]

Johnny Lin <donotreply@interbulletin.bogus> writes:
> hi folks,
> 
> i was wondering, has anyone compiled a ranked list of the relative
> speeds of IDL operations functions?  i've read postings re general
> rules of thumb, but was wondering if anything more specific and
> complete is available online anywhere.

The fastest operation is the one that takes the least amount of time
:-)

Actually I'm being quite serious.  If you really have a deep question
about the speed difference between two implementations, then go ahead
and try both and see which is fastest.

Things to bear in mind:

 * vectorize where you can; if you can vectorize the inner loop, then
   you are usually fine.

 * there really are not *that* many different ways to do things; You
   add with the "+" symbol, divide with the "/" symbol, etc.

 * the RSI web page does have some good tips about speeding up your
   code.  Things like "group scalars into parentheses;" or "take
   common expressions outside of FOR loops;" or "how to speed up array
   indexing assignments"

 * not all platforms are the same speed; some do certain operations
   faster than others, even with the same IDL code.

Some techniques discussed here are not really about speed.  For
example, I and others have mentioned "chunking," or dividing your huge
array up into smaller segments for processing.  While this may save
time, what it really does is save *memory*.  If you use too much
memory then your computer slows way down.

One legitimate question was over how to quickly expand an array (using
REBIN vs the "#" operator).  You can find the newsgroup discussion --
with timings -- here:

http://cow.physics.wisc.edu/~craigm/idl/archive/msg00327.html

There are some other fancy tips like using NOZERO keyword for array
creation, but this is getting to be a fairly "niche" optimization.

Search around the archive and you can find other examples.  

Craig

-- 
--------------------------------------------------------------------------
Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
--------------------------------------------------------------------------