comp.lang.idl-pvwave archive: archive » Re: Using two different arrays in tha same calculation

Home » Public Forums » archive » Re: Using two different arrays in tha same calculation

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Using two different arrays in tha same calculation [message #2232]

Thu, 16 June 1994 06:52

landers
Messages: 45
Registered: May 1993

Member

In article <Cr326y.Etw@rsinc.com>, dan@rsinc.com (Dan Carr) writes:
[snip]
|> You can use the # operator (matrix multiply) to make the small (8)
|> array into a (300,8) array. Try this :
|>
|> IDL> arr1 = Findgen(8)
|> IDL> arr2 = Findgen(300, 8)
|> IDL> newarr = (Replicate(1.0, 300) # Cos(arr1)) * Sin(arr2)
|>
|> ------------------------------------------------------------ -------------------
|> Dan Carr
|> Research Systems
|> Boulder, Colorado
|> dan@rsinc.com
|> ------------------------------------------------------------ -------------------

OK - I'll bite on this one. Now that RSI has decided to post to the group, I
want to know - from the 'experts'....

Is this matrix multiply method any faster or more efficient than generating an
array of indices? Which is 'better':

replicate( 1.0, 300 ) # arr1

or

arr1( Lindgen(300,8) / 300 )

Seems to me the second way should be more efficient - it involves one integer
divide and some memory dereferencing, while the matrix method involves
floating point multiplies and addition.

The second method has the added advantage that it doesn't 'mess' with your
data. If you're not careful, you could convert ints to floats, etc.
Matrix multiply of any two integer types (byte, int, long) results in a long.
You also can't matrix multiply strings.

I'll grant you that the first way (#) is the easiest to remember - that's what
I generally use from the command line, and it's what I recommend to people
around here who need a 'quick fix' or a one-shot answer. But I am starting
to use this second way in my programs, for generality and efficiency. If I'm
wrong about this, let me know....

;;;; for the FAQ?

to convert an array1(M) to array2(n,M) :
array2 = array1( Lindgen(n,M) / n )
or array2 = replicate(1,n) # array1

to convert an array1(M) to array2(M,n) :
array2 = array1( Lindgen(M,n) / MOD M )
or array2 = array1 # replicate(1,n)

---
;Dave

Report message to a moderator

[Message index]

		Re: Using two different arrays in tha same calculation By: landers on Thu, 16 June 1994 06:52
		Re: Using two different arrays in tha same calculation By: dan on Wed, 08 June 1994 07:42
		Re: Using two different arrays in tha same calculation By: landers on Thu, 09 June 1994 09:09
		Re: Using two different arrays in tha same calculation By: steinhh on Fri, 10 June 1994 00:44
		Re: Using two different arrays in tha same calculation By: thompson on Thu, 09 June 1994 06:20

Previous Topic:	Does PVWAVE V4.2 work with Motif V1.2 under VMS?
Next Topic:	Re: Problems with Idl and Irix 5.2-alpha

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Fri Oct 10 07:23:12 PDT 2025

Total time taken to generate the page: 1.76030 seconds