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description: Messages from Usenet group comp.lang.idl-pvwave, compiled by Paulo Penteado
items: Re: Matrix indexing question

Re: Matrix indexing question
type: item
description: Dear Matt what do you think on this solution a=[3,4] b=[1,0] x = [[1,2,3,4,5,6],[7,8,9,10,11,12]] print,x[[a],[b]] 10 5 My explanation: The problem here is that there are the same signs used to create an array and the same signs are used to assign an index array. May be David has a better idea. I hink what happens is by using an operation you did by {{{ print, x[[3,4],[1,0]] }}} the outer [] are used to adress the values because then they are not interpreted to create a new array. If you do create a temporary array like this example shows. It is not necessary to create the {{{y}}} variable. You will get the results you see in your example too. print, (x)([[3,4],[1,0]]) 4 5 2 1 Both could be wanted and both could give terrible results if they are not clear described or coded. Thanks for the advice! Reimar Matt Feinstein wrote: > If I set > > x = [[1,2,3,4,5,6],[7,8,9,10,11,12]] > > then (case A) > > print, x[[3,4],[1,0]] > > gives > > 10 5 > > which is slick, and is the kind of indexing I want. However, if (Case > B) I set > > y = [[3,4],[1,0]] > > then > > print, x[y] > > gives > > 4 5 > 2 1 > > which, I guess, is also slick-- but is not what I want. Is there any > way to set a variable 'y' that will give me the kind of indexing in > Case A? > > And, yes, I know that I can set > > y = [9, 4] > > and get the 'right' answer. Is this the only way? > > Matt Feinstein > > -- > There is no virtue in believing something that can be proved to be true. -- Forschungszentrum Juelich email: R.Bauer@fz-juelich.de http://www.fz-juelich.de/icg/icg-i/ ============================================================ ====== a IDL library at ForschungsZentrum Juelich http://www.fz-juelich.de/icg/icg-i/idl_icglib/idl_lib_intro. html
creator: R.Bauer
date: 2004-04-03T00:47:15-00:00

Re: Matrix indexing question
type: item
description: In article <gm1r60p1cdu66r7h8b1bornsh99m1aa05d@4ax.com>, "Matt Feinstein" <nospam@here.com> wrote: > If I set > > x = [[1,2,3,4,5,6],[7,8,9,10,11,12]] > then (case A) > print, x[[3,4],[1,0]] > gives > 10 5 > which is slick, and is the kind of indexing I want. However, if (Case B) > I set > y = [[3,4],[1,0]] > then > print, x[y] > gives > > 4 5 > 2 1 > which, I guess, is also slick-- but is not what I want. Is there any way > to set a variable 'y' that will give me the kind of indexing in Case A? > And, yes, I know that I can set > y = [9, 4] > and get the 'right' answer. Is this the only way? Matt Feinstein > -- > There is no virtue in believing something that can be proved to be true. Hi Matt, I have two answers, neither is particularly clean. ;1 print, x[y[*,0], y[*,1]] 10 5 ;2 , the generic,Index the elements yourself, method. x=[[1,2,3,4,5,6],[7,8,9,10,11]] y=[[3,4],[1,0]] ;;;;; dx=size(x, /dimensions) dy=size(y,/dimensions) q=replicate(1,n_elements(dx)) for i=1, n_elements(dx)-1 do for j=0, i-1 do q[i]=q[i]*dx[j] ;;q holds the number of elements per dimension. indices=y#q ; 2 dimensions in y only print, x[indices] 10 5 Also, some bounds checking might be needed. Chris.
creator: Chris Lee
date: 2004-04-03T10:26:59-00:00