Re: Fast matrix filling in IDL

[davidf(at)dfanning.com (David Fanning)]

Timm Weitkamp (weitkamp@my-dejanews.com) writes:

> What is the fastest way of filling a matrix with identical
> column vectors in IDL?
> 
> More precisely, if A is a float matrix (m x n) and V is
> a vector with m elements, is there a faster way than
> 
>   FOR i=0,n-1 DO A[*,i]=V ?

I don't know about "fastest", but 

   vector = Replicate(1, n)
   array = V ## vector

is about 5 times faster for a 1000 by 500 array.
Here is my example program:

*************************************************************
 PRO TEST
 
 array = Fltarr(1000, 500)
 v = RandomU(seed, 1000)

 time = systime(1)
 FOR i=0,500-1 DO array[*,i] = v
 Print, 'Time for Loop: ', systime(1) - time
 
 time = systime(1)
 vector = Replicate(1, 500)
 array = v ## vector
 Print, 'Time for Matrix Operations: ', systime(1) - time
 END
 *************************************************************

And the results:

  IDL> test
  Time for Loop:                   0.10000002
  Time for Matrix Operations:      0.019999981

Cheers,

David

-- 
David Fanning, Ph.D.
Fanning Software Consulting
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