Selecting an item on a specific axis can also be performed using compress (/ in
APL, # in
J).
dim <- (shape array)[Axis] NB. dim =. Axis{$array
sel <- (-item-[]IO) rotate dim take 1 NB. sel =. (-item) |. dim{.1
+/[Axis] sel/[Axis] array NB. sel #"((#dim)-Axis) array
This only works for a single item tho.
--- Mike Day <[EMAIL PROTECTED]> wrote:
> Not J-programming but...
>
> Here's a simple "non-nested array" method, easily
> worked up into an APL function, which might do what
> you want. I think it generalises to a rank higher
> (or lower) than 3, but you'd best check. I suggest
> you look at the APL*PLUS III definition of transpose,
> which should allow you to do more along the same
> lines.
>
> (NB. I'm adding extra line-feeds to beat T/bird's
> line-wrapping)
>
> Using Jim Weigang's transliteration of APL primitives.
>
> Adding J expressions as this is the J prog group!
> J's transpose is a bit different from APL's....
>
> NB. using APL index origin 1
>
> A <- 2 3 4 {rho} {iota} ! 4
>
> NB. J: A =. i. 2 3 4
>
> r <- (1-n) {rot} {iota} {rho} {rho} A
>
> NB. J: r =. n |. i. $ $ A NB. J origin 0
>
> rtA <- r {transpose} A
>
> NB. J: rtA =. r |: A
>
> tAn <- (1{drop}{rho rtA) {rho} , rtA
>
> NB. J: tAn =. (, $~ ([EMAIL PROTECTED])) rtA
>
> NB. tAn is the required sub-array, but might need transposing...
>
> An <- ({gradeup} (-1) {drop} r) {transpose} tAn
>
> NB. J: An =. (/: }. r) |: tAn
>
> Mike
>
>
> Will Slade wrote:
> > Hello,
> >
> > I'm crossposting this in the Usenet APL group.
> >
> > I'm learning APL*PLUS III for work, and I have a question for you
> > experienced APLers.
> >
> > Lets say I have a matrix, A, with dimensions 2 3 4.
> >
> > A2 3 4 ½ ¼!4
> >
> > I can return the first dimension of A like so: A[1;;]
> >
> > Is there a way to return the nth dimension of A without knowing what the
> > dimensions of A are? I'd like the flexibility to arbitrarily return
> > the nth
> > dimension, but I don't know how to do this in APL, and the error messages
> > are a little cryptic for someone with my inexperience.
> >
> > Thanks,
> > Will
> > ----------------------------------------------------------------------
> > For information about J forums see http://www.jsoftware.com/forums.htm
> >
> >
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>
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