I don't see catalog used frequently in natural J code. It does come up on these
Forums from time to time, mostly in contrived situations. In my own code, I
sometimes use it to generate J sentences in order to compare performance
statistics for semantically-equivalent phrases. But outside of that, I don't
see it too often. I suspect one reason for its rarity is, at least, that in any
real-world context, a complete MxN catalog would be prohibitively expensive.
So it's unsurprising that APL didn't have a primitive catalog function. Of
course, you could express it as {jot} {dot} {comma}, dot as being outer product
(and jot in this context a placeholder, not unlike [:) with the usual caveats
about the differences between J's rank and APL's multidimensionality (ie you
could expect catalog to work for 1D vectors in APL and generalized lists in J).
So, when he got his chance to "rationalize" APL and rebuild it from
the ground up as J, did Ken have a change of heart about catalog's centrality
to the discipline of computer science? Is that why he assigned to it that
scarce and desirable resource, a primitive spelling? A one-character primitive,
no less?
I suspect not. Recall the other valence of {, to wit: from. You might think
having this kind of partner is an even greater endorsement of catalog's virtues
... until you realize:
row_2n3n6_col_7n13 =: 2 3 6 ; 7 13
table =: i. 10 20
(< row_2n3n6_col_7n13) { table NB. "from"
47 53
67 73
127 133
({ row_2n3n6_col_7n13) NB. "catalog"
+---+----+
|2 7|2 13|
+---+----+
|3 7|3 13|
+---+----+
|6 7|6 13|
+---+----+
({ row_2n3n6_col_7n13) { table NB. "catalog from"
47 53
67 73
127 133
Ah, "from" *is* "catalog". Catalog is a primitive because from needed a
thematically related definition for monad { (and from, of course, got a
precious one-character primitive because it *is* central to array programming).
And, incidentally, this is why the results of catalog are boxed, even though
they don't need to be, and we end up writing >@:{ instead of plain { every time
we use it.
That is, by definition, the results of { will be homogenous in both type and
shape, and so we know for certain boxes aren't required ... except that dyad {
has a scalar left rank, and so to completely address an atom (or anything more
granular than an item, ie a Cartesian point in the frame specified as a vector
of indices, one per axis), each selection must be boxed.
True story.
-Dan
Sent from my iPhone
> On May 8, 2014, at 5:34 PM, "'Pascal Jasmin' via Programming"
> <[email protected]> wrote:
>
> Its the same. Just more descriptive name than catalogue perhaps. Google
> seems to only find J references to query "catalogue function permutations".
> I feel as though catalogue and my use of it is a generalizable pattern, and
> the J designers agreed, but I don't know if other languages did something
> similar, and just used a different name?
>
> Was catalogue in APL?
>
>
> ----- Original Message -----
> From: David Lambert <[email protected]>
> To: programming <[email protected]>
> Cc:
> Sent: Thursday, May 8, 2014 4:50:40 PM
> Subject: Re: [Jprogramming] a cool permutation verb
>
> Indeed cool, does it differ from ,@{ :[:
>> Date: Thu, 8 May 2014 10:53:53 -0700 (PDT)
>> From: "'Pascal Jasmin' via Programming"<[email protected]>
>> To: Programming forum<[email protected]>
>> Subject: [Jprogramming] a cool permutation verb
>> Message-ID:
>> <[email protected]>
>> Content-Type: text/plain; charset=utf-8
>>
>> cperm =: [: , [: < "1 [: > [: ,"0 1/ each/ (_2&}. , [: ,"0 0/ each/ _2&{.)
>>
>> this creates constrained permutations where each position in the list can
>> only be drawn from its own set:
>>
>> ? ?cperm 3 4 ; 1 2
>> ?????????????????
>> ?3 1?3 2?4 1?4 2?
>> ?????????????????
>>
>> ? ?cperm 0 ; 3 4 ; 1 2; 5
>> ?????????????????????????????????
>> ?0 3 1 5?0 3 2 5?0 4 1 5?0 4 2 5?
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