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Re: Removing if then else loop for efficiency [message #69280 is a reply to message #69275]

Tue, 12 January 2010 05:09

Tom Ashbee
Messages: 3
Registered: January 2010

Junior Member

On Jan 10, 6:25 pm, pp <pp.pente...@gmail.com> wrote:
> I think this does the same as your code, but it does not use any
> loops, and it should be much faster and easier to read:
>
> function velocities,t,xyvec
> compile_opt idl2
> ;Constants
> N=50
> R=5d0
> ;Unpack the input
> xvec=xyvec[0:N-1]
> yvec=xyvec[N:(N*2)-1]
> ;Temporary arrays for x[j], x[i], y[j], y[i]
> xj=rebin(xvec,N,N)
> xi=transpose(xj)
> yj=rebin(yvec,N,N)
> yi=transpose(yj)
> ;Repeated terms in the expressions
> tmp1=(xi-xj)^2+(yi-yj)^2
> tmp2=R^2/(xj^2+yj^2)
> tmp3=(xi-xj*tmp2)^2+(yi-yj*tmp2)^2
> ;Terms of dxdt,dydt present everywhere
> dxdt=-(yi-yj*tmp2)/tmp3
> dydt=(xi-xj*tmp2)/tmp3
> ;Terms present only out of the diagonal
> tmp4=1d0-identity(N) ;this is 0 in the diagonal, 1 out of it
> dxdt+=((yj-yi)/tmp1)*tmp4
> dydt-=((xi-xj)/tmp1)*tmp4
> ;Put the gamma factor
> gamm=rebin(gamma(N,2.0d,10.0d)/(2d0*!dpi),N,N) ;this does not seem to
> be IDL's gamma function
> dxdt*=gamm
> dydt*=gamm
> ;Sum over the rows
> dxvecdt=total(dxdt,1)
> dyvecdt=total(dydt,1)
> ;Pack the results
> z=[dxvecdt,dyvecdt]
> return,z
> end
>
> You should check that I did not misidentify anything, which would not
> have been difficult in such convoluted expressions.
>
> Other points to note:
>
> 1) Do not use () for array indexes. Use [] instead. That makes it
> unambiguous that it is an array index, and not a function call.
>
> 2) When using doubles, as you did, use !dpi instead of !pi.
>
> 3) Your function has an argument t that is not used anywhere in it. I
> left it there, so that the argument order does not change.

Hi,

thanks a lot for this; it was very insightful and helpful.
Unfortunately it's just giving NaNs for z at the moment but I'm
working on debugging it.

Thanks again

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[Message index]

		Re: Removing if then else loop for efficiency By: penteado on Tue, 12 January 2010 08:04
		Re: Removing if then else loop for efficiency By: Tom Ashbee on Tue, 12 January 2010 05:09
		Re: Removing if then else loop for efficiency By: penteado on Sun, 10 January 2010 10:25
		Re: Removing if then else loop for efficiency By: Tom Ashbee on Sun, 10 January 2010 06:46
		Re: Removing if then else loop for efficiency By: penteado on Sun, 10 January 2010 06:42

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