The reason J does not implement dictionaries (and seems to be designed
to discourage their use) has to do with how bad dictionaries can
mangle your architecture.
In my experience you usually want to find multiple different pieces of
information when looking things up. With a dictionary approach that
typically means you wind up with objects that hold all these different
things.
But when you are working with collections what this means is that for
each kind of information you want to extract you need to visit each
object in the collection. And, while that is fine for some things, it
is really bad for dealing with lots of things.
So, instead, J encourages you to think through the process.
Anyways, I typically wind up working with indices (which I get through
some lookup mechanism), and then on sequences extracted using indices.
This all seems simple and second nature to me, but apparently it
baffles some people (and I do remember having to be taught some tricks
involving grade-up and/or grade-down to deal with some of the
complications that can arise when dealing with things which are
ordered in different ways).
Put differently, I think people spend way too much time trying to add
abstraction to J, when J already provides way more abstraction than
they have had time to understand.
Put differently, it's usually best to learn J while focusing on
"non-computer-science" problems: Physics, chemistry, engineering,
biology, music, 3d printing, art, even [human] language drills all can
lead to quite instructive questions (and you should *not* be
embarrassed about asking them nor should you be hesitant about
clarifying your focus and needs when the initial exchange is not
satisfying).
Put differently, there's reasons people keep re-inventing abstractions
like dictionaries and one of those reasons is that the thousands (or
more) of existing implementations all have baggage that make them
inappropriate for some kinds of problem solving.
Or, at least, that's my opinion.
That said, here's basic J lookup:
inds=. keys i. names
vals=. inds { values
...
Or, if you really don't have multiple things you're after:
vals=. (keys i. names) { values
Or, if you must have shorter lines (never mind that we're already more
concise than a variety of implementations of dictionaries):
v=.V{~k i.nm
(or skip the assignment and plug the expression in where you need the values).
Put differently: how are you going to use this?
Thanks,
--
Raul
On Wed, Apr 12, 2017 at 9:28 AM, 'Jon Hough' via Programming
<[email protected]> wrote:
> I created one a while ago.
> https://github.com/jonghough/msgpack-j/blob/master/hashmap.ijs
> This is a crude implementation of a hashmap, using separate chaining.
> Admittedly, it isn't particularly great. E.g. hashing function implementation
> isnt very good. It uses symbols to represent inner hashmaps as a horrific
> workaround.
>
> As Pascal said, maybe it is better to use a functional / J-esque approach. J
> oop has some serious limitations. E.g. there is no typeof function (that i am
> aware of), and object references are represented as boxed literals. . . So
> what do you do if your dictionary contains another dictionary, and boxed
> literals? There is no way to distinguish them.
>
> On the same topic, I wonder if anyone has implemented a good noncrypto hash
> function in J? E.g. murmurhash? Would be useful for implementing a hashmap.
>
> Regards,
> Jon
>
> On Apr 12, 2017, 21:34, at 21:34, Herbert Weissenbaeck // Privat
> <[email protected]> wrote:
>>Next beginner's question: What would be an efficient/fast (or the
>>recommended) J way to implement a dictionary with strings as keys and
>>arbitrary contents (nouns, verbs, boxes) as values, which allows
>>inserting, removing, fetching and editing?
>>
>>Thank you.
>>
>>
>>
>>
>>Sent from my iPhone
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