Yeah, I found it, but it wouldn't be applicable here because it
implemented a fixed-size hashtable and kept only the most recent entries
(it was used as a lookaside buffer to speed negascout searches).
A simple hashtable wouldn't be hard to implement, methinks.
Henry Rich
On 12/3/2017 8:18 PM, Andrew Dabrowski wrote:
Can you locate your code?
On 12/03/2017 06:52 PM, Henry Rich wrote:
Yes; I've done that before. It might be worth seeing how well we
could make such an addon perform before making changes to the JE.
Henry Rich
On 12/3/2017 6:47 PM, bill lam wrote:
Can this be implemented using j script addon?
On Dec 4, 2017 5:26 AM, "Henry Rich" <[email protected]> wrote:
Perhaps we shouldn't try to make a full datatype for associative
arrays,
but just a set of functions (foreigns, or (m h.) ) that do what's
needed.
We would need to decide what's needed.
hashtable =: keys create values
indexes =: hashtable lookup keys (like i.)
values =: hashtable fetch indexes (like {)
hashtable =: hashtable add keys;values
hashtable =: hashtable delete indexes
That would not be the hardest thing to implement; perhaps we should
start
a strawman in Interpreter/Requests.
Henry Rich
On 12/3/2017 4:06 PM, Marshall Lochbaum wrote:
The associative lists that Andrew is talking about (also called
maps or
dictionaries) are essentially the same as JSON objects (more
precisely,
JSON objects are maps whose keys are strings). There's no need for
them
to be used in a particularly low-level way, although the
primitives used
to manipulate them would have to be different from J's array
primitives.
Examples might be combining two maps but giving preference to one of
them when they conflict, or inverting a map by swapping keys and
values.
Lisp stands for "list processing", and its fundamental datatype is
the
linked list, composed of value-reference pairs. These aren't
related to
associative arrays, at least not any more than they are related to
any
other datatype.
I feel pretty strongly that an associative array type is missing
in J,
but it's understandable given how hard such a type would be to
specify
and implement. That doesn't make it any easier to live without them,
though.
Marshall
On Sun, Dec 03, 2017 at 01:50:43PM -0500, Raul Miller wrote:
<<< means "box three times"
<2
┌─┐
│2│
└─┘
<<2
┌───┐
│┌─┐│
││2││
│└─┘│
└───┘
<<<2
┌─────┐
│┌───┐│
││┌─┐││
│││2│││
││└─┘││
│└───┘│
└─────┘
See also the documentation on the index operation (Henry gave you a
reference for that).
Also, as I understand it, JSON has only a loose relationship with
a-lists. My understanding of them is that they are the fundamental
datatype provided by the lisp / scheme / .. family of languages.
Basically, you're working with pointer chasing. In J, you can use
indices or names as references, if you want to build a workalike
- but
usually it's just not worth the bother.
And that "encode" method is building a name based on an arbitrary
string. You should probably think of it as a hash function. But
there's a typo - there should be a space after the v in inv. But
this
suggests also that rosettacode is having bitrot issues with its
storage system, and that has all sorts of unpleasant implications.
Basically, you are not going to be able to use rosettacode
implementations - in the general case - unless you are prepared to
debug them (which kind of defeats the purpose of having the site in
the first place.) ... wait, no.. when I go to that rosettacode
page, I
see a space after the 'inv' keyword. Well.. it could still be
bitrot,
but perhaps at the cloudfront level rather than on the site
itself...
Thanks,
--
Raul
On Sun, Dec 3, 2017 at 1:24 PM, Andrew Dabrowski
<[email protected]>
wrote:
1. Yes, I should been more specific: I wanted to know the
idiomatic way
to
delete the nth item of a list, so I think <<< is what I was
looking for.
What is <<< called and where is it documented?
2. Not sure what you mean by "Their focus is on micromanaging the
sequence
of operations." A-lists are built into just about every other
modern
language because they're so useful at managing information.
Does J not
support JSON for web interoperability?
One example is using an a-list as a structured object, without
having
to get
into full-blown OO. I could represent the state of a model
using an
a-list
and then update the values over time.
I saw that using classes is one alternative
(https://rosettacode.org/wiki/Associative_array/Creation#J).
coclass'assocArray'
encode=:'z',(a.{~;48 65 97(+ i.)&.>10 26 26) {~62x
#.inv256x #.
a.&i.
get=: ".@encode
has=:0 <: nc@<@encode
set=:4 :'(encode x)=:y'
I have absolutely no idea what is going on in the encode
function. I
hope
there are other options.
On 12/03/2017 12:59 PM, Raul Miller wrote:
Why do you want to delete an item from a list? This is *not* a
rhetorical question - we have a variety of ways of removing
items from
lists and to pick the correct mechanism we need to understand
what you
are trying to accomplish.
To illustrate, here are a few examples:
list=: 2 3 4 5 7 8 9 10
NB. delete specific elements
list -. 3 5 7
2 4 8 9 10
NB. delete based on a computation
(0=1&p: list)#list
4 8 9 10
NB. delete based on a range of indices
(3&{.,6&}.) list
2 3 4 9 10
NB. delete values at certain indices
(<<<3 5 6){ list
2 3 4 7 10
And then there's operations (like outfix) which offer regular
abstractions:
3 ]\. list
5 7 8 9 10
2 7 8 9 10
2 3 8 9 10
2 3 4 9 10
2 3 4 5 10
2 3 4 5 7
Note that "delete from a list" is a dual of "select from a list" -
maybe a shift in perspective can help?
As for the lack of "a-lists" - again, why do you want them?
Depending
on what you are trying to do, there's a lot of options. Their
focus is
on micromanaging the sequence of operations, though, so you
should not
expect the language to make most of your decisions for you, there
(unless of course you are working with a language which was
specifically designed with an "everything should be an a-list"
mentality - but that's not J).
Thanks,
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