comp.lang.idl-pvwave archive - RDF feed
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description: Messages from Usenet group comp.lang.idl-pvwave, compiled by Paulo Penteado
items: Re: function of two variables

Re: function of two variables
type: item
description: In article <CMtu54.5o@ireq.hydro.qc.ca>, brooker@toka.ireq-ccfm.hydro.qc.ca writes: > Hello from Canada!! > > I have the following IDL question: > > Assume I have the functions Y=2.*X + 3. and I want to plot Y vs X over the > range [0,10] > > I could code this as > > X=findgen(101)/100.*10. > Y=2.*X + 3. > plot,x,y > > Very nice!! Only array operations. NO LOOPING!!!!!! Very FAST!!!!! > > But what if now I had Z=2.*X + 3.*Y and I want to plot Z as X and Y both > range from 0 to 10. A way to code this is > > X=findgen(101)/100.*10. > Y=X > num_y=n_elements(Y) > num_x=n_elements(X) > z=fltarr(num_x,num_y) > for j=0,num_y-1 do begin > z(*,j)=2.*X + 3.*Y(j) > endfor > surface,Z,X,Y > > Very inefficient because of the loop!! Very slow!! > > Is there anyway to do this more efficiently? > > Thanks, > Peter Brooker > brooker@toka.ireq-ccfm.hydro.qc.ca Peter, I would recommend trying... z = fltarr(num_x,num_y)+replicate(1,num_y)#(3.*y)+(2.*X)#replica te(1,num_x) I didn't find your example slow on a SUN Sparc 2, but hopefully this suggestion will be faster on your system. Andy -- ,__o Andrew F. Loughe (Code 971) -\_<, NASA Goddard Space Flight Center phone: (301) 286-5899 (*)/'(*) Greenbelt, MD 20771 email: andy@toto.gsfc.nasa.gov
creator: andy
date: 1994-03-18T18:48:05-00:00

Re: function of two variables
type: item
description: In article <CMtu54.5o@ireq.hydro.qc.ca>, <brooker@toka.ireq-ccfm.hydro.qc.ca> wrote: [Stuff deleted] > But what if now I had Z=2.*X + 3.*Y and I want to plot Z as X and Y both > range from 0 to 10. A way to code this is > > X=findgen(101)/100.*10. > Y=X > num_y=n_elements(Y) > num_x=n_elements(X) > z=fltarr(num_x,num_y) > for j=0,numy-1 do begin > z(*,j)=2.*X + 3.*Y(j) > endfor > surface,Z,X,Y > > Very inefficient because of the loop!! Very slow!! > > Is there anyway to do this more efficiently? > > Thanks, > Peter Brooker > brooker@toka.ireq-ccfm.hydro.qc.ca What you need are two two dimensional arrays that contain X and Y values. The size of number of X co-ords by number of Y co-ords. These arrays essentaially stor the X & Y values at each point on the grid. So the value in the elements in the X array change in one direction say along the rows, but stays consatnt in the other direction. The Y array does the opposite. Given that the Y=X in your code you only need to come up with one array, since Y is the transpose of X. So the next trick is to come up with the X array. This is simply done by taking your existing X array and using the matrix multiply in the following way 1) create a 1d array that contains 1 of the size required - the number of Y indices call this UNITY 2) do X#UNITY. This does what you want. John Black.
creator: black
date: 1994-03-21T14:01:57-00:00