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description: Messages from Usenet group comp.lang.idl-pvwave, compiled by Paulo Penteado
items: Re: Help with matrix operations

Re: Help with matrix operations
type: item
description: In article <1rk7gs$4m6@morrow.stanford.edu>, pln@egret0.Stanford.EDU (Patrick L. Nolan) writes: |> I have a little matrix problem that I'm trying to do without using |> loops. So far I'm not clever enough to figure out how to do it. |> Suppose we have |> A = fltarr(N,N) |> B = fltarr(N) |> C = fltarr(N,N,N) |> I want to have C(i,j,k) = A(i,j) + B(k) |> for all i,j,k < N. Is there a way to do this without writing |> ugly loops? I'm sure it's trivial, and I'll feel like a dope |> when the first person points it out. Fire away. |> |> -- |> * Patrick L. Nolan (415)723-0133 * |> * W. W. Hansen Experimental Physics Laboratory (HEPL) * |> * Stanford University * |> * Bitnet: PLN@SLACVM Internet: pln@egret0.stanford.edu * How about for k=1,n do c(*,*,k) = a(*,*) + b(k) That has a loop but its not to ugly. ************************************************** ** Dan Bergmann dbergmann@llnl.gov ** ************************************************** -- ************************************************** ** Dan Bergmann dbergmann@llnl.gov ** **************************************************
creator: dan
date: 1993-04-27T08:33:49-00:00

Re: Help with matrix operations
type: item
description: In article <1rk7gs$4m6@morrow.stanford.edu> pln@egret0.Stanford.EDU writes: > I have a little matrix problem that I'm trying to do without using > loops. So far I'm not clever enough to figure out how to do it. > Suppose we have > A = fltarr(N,N) > B = fltarr(N) > C = fltarr(N,N,N) > I want to have C(i,j,k) = A(i,j) + B(k) > for all i,j,k < N. Is there a way to do this without writing > ugly loops? I'm sure it's trivial, and I'll feel like a dope > when the first person points it out. Fire away. > > -- > * Patrick L. Nolan (415)723-0133 * > * W. W. Hansen Experimental Physics Laboratory (HEPL) * > * Stanford University * > * Bitnet: PLN@SLACVM Internet: pln@egret0.stanford.edu * Some one has suggested what may probably be the best (fastest) way to do this: for k=0,n-1 do a(0,0,k) = a(*,*) * b(k) . However there is a way to do this without loops. Let's take a more general situation. Let A = FLTARR(x,y) B = FLTARR(z) C = FLTARR(x,y,z) to get C(i,j,k) = A(i,j) * b(k) do the following ( make sure you are doing integer math here). ; Make the indicies ma = LINDGEN(x,y,z) mod (x*y) mb = LINDGEN(x,y,z) / (x*y) ; define the destination array c = fltarr(x,y,z) ; DO IT! c(*) = a(ma(*)) * b(mb(*)) That is all there is to it. I'll argue that you have done way too much math this way then you would have done with the signle loop idea. Some may agrue that the use of LINDGEN is an implicit loop. I won't argue with that either. -- Joseph M. Zawodny (KO4LW) NASA Langley Research Center Internet: zawodny@arbd0.larc.nasa.gov MS-475, Hampton VA, 23681-0001 Packet: ko4lw@wb0tax.va.usa
creator: zawodny
date: 1993-04-28T11:47:13-00:00

Re: Help with matrix operations
type: item
description: pln@egret0.Stanford.EDU (Patrick L. Nolan) writes: > I have a little matrix problem that I'm trying to do without using > loops. So far I'm not clever enough to figure out how to do it. > Suppose we have > A = fltarr(N,N) > B = fltarr(N) > C = fltarr(N,N,N) > I want to have C(i,j,k) = A(i,j) + B(k) > for all i,j,k < N. Is there a way to do this without writing > ugly loops? I'm sure it's trivial, and I'll feel like a dope > when the first person points it out. Fire away. I don't think it's trivial. Here's how I would solve it. ; First expand A and B out to NxNxN arrays. ; AA = A(*) # REPLICATE(1,N) ;AA is now (N*N, N) array AA = REFORM(AA,N,N,N) ;Make it (N, N, N) BB = REPLICATE(N*N) # B ;Do the same for B BB = REFORM(BB,N,N,N) C = AA + BB I don't know if this is any more or less "ugly" than doing it in a loop, but it should be much quicker. Of course you can combine all this into one command if you want. Bill Thompson
creator: thompson
date: 1993-04-29T14:48:33-00:00