Dan
I think a single "item" is what he asked for;
your suggestion is preferable to mine as it's
more general, especially if he needs hyperplanes
other than the first. It also avoids transposes
which could be expensive in a real application.
A problem might arise - does APL*PLUS III allow
the rank-reduction in the final step, +/[axis] ,
for a non-nested character or general nested
array? I expect it does.
Dick Bowman observes that modern APLs offer
forms of selection other than eg [i;j;]. Dyalog
APL now allows
item {squid} [axis] A
APL*PLUS III may well have a similar mechanism.
Perhaps Will would comment on our various
contributions before the powers that be shut
down this non-J thread. In particular, does
he need the J-language solutions?
Mike
Dan Baronet wrote:
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.
A„2 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
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