| Re: function of two variables [message #1885] |
Mon, 21 March 1994 06:01 |
black
Messages: 39
Registered: August 1992
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Member |
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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.
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| Re: function of two variables [message #1888 is a reply to message #1885] |
Fri, 18 March 1994 10:48  |
andy
Messages: 31
Registered: November 1993
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Member |
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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
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