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Re: idl speed question [message #65624]

Sat, 14 March 2009 15:28

Michael Galloy
Messages: 1114
Registered: April 2006

Senior Member

oxfordenergyservices@googlemail.com wrote:
> I have the following strange result
>
> a=double(1.0)
> for i=0,10000 do begin
> for j=0,10000 do begin
> a=a+1.0
> endfor
> endfor
>
> takes 13 seconds whereas
>
> a=double(1.0)
> b=double(1.0)
> c=double(1.0)
> d=double(1.0)
> e=double(1.0)
> for i=0,10000 do begin
> for j=0,10000 do begin
> a=a+1.0
> b=b+1.0
> c=c+1.0
> d=d+1.0
> e=e+1.0
> endfor
> endfor
>
> takes 60 seconds? I thought the overhead with IDL was in the loops
> rather than the computing?

The real killer for speed is number of statements (each one has to be be
interpreted). Loops are bad only because they could execute a statement
a possibly large number of times. So in your example, the first case has
10001 * 10001 statements while the second has 5 * 10001 * 10001
statements. So if the statements are doing the same amount of work, one
would expect the second to take about 5 times more time.

The conclusion: try to do more work per statement.

Mike
--
www.michaelgalloy.com
Associate Research Scientist
Tech-X Corporation

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		Re: idl speed question By: Michael Galloy on Sat, 14 March 2009 15:28
		Re: idl speed question By: oxfordenergyservices on Mon, 16 March 2009 03:39

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