[Jprogramming] delete item; ass. lists

[Henry Rich]

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 <dabrowsa at indiana.edu> 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=: ". at 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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