The logic is more or less correct but the "official" terminology would be
different; the official terminology reflects the actual implementation with
high fidelity.
The basic data structure in the interpreter has type A. It is used for
arrays as well as for verbs, adverbs, etc. (even parens!). Every type A
object has a header and a value. The header has the type, element count,
rank, shape, reference count, and other stuff. The value is interpreted
differently ("cast") depending on the type of object.
An A object of an indirect type (boxed, extended integer, rational, etc.)
can have type A objects in its value, and those objects can themselves have
reference counts, etc.
The reference counting in the J implementation can be improved.
Regarding your last example. It is highly unlikely that the space
accounting is incorrect, because the same simple mechanism is used for
every kind of object and expression. I believe that for c =. bb the
interpreter traversed the whole of bb updating all the reference counts.
It takes time to do that, but little space. It looks like c =. ] bb made a
copy of bb, but in theory it shouldn't have to.
The uncertainty I expressed is due to that I haven't looked at it for a
long time. The mechanism was devised in the early days (1989) and I
haven't had many reasons to revisit it.
Come to the J conference and I will give you a more polished story.
On Thu, Nov 24, 2011 at 10:18 AM, Henry Rich <[email protected]> wrote:
> As examples of what Roger is saying:
>
> abb =. (1e6 $ 5) ; (1e6 $ 5)
> ts '''a b'' =. abb'
> 8.36834e_6 1600 NB. no data-copying here
> ab =. (1e6 $ 5) ,: (1e6 $ 5)
> ts '''a b'' =. ab' NB. data was copied
> 0.0227605 1.25844e7
> ts 'c =. a;b'
> 7.53628e_6 960 NB. no copying
>
> My understanding has been that an unboxed noun contains a header which
> includes a pointer to a data block which comprises all the values of the
> noun. This data block carries a reference-count, with no
> reference-count for any subarrays. An boxed noun is represented as an
> array (or atom) of such headers, one for each atomic box.
>
> So, in your example
> 'a b c' =. abc =. i.3 4 5
>
> there is no header for each row of abc, and the data for a, b, and c
> will be copied.
>
> In
> 'n m' =. nm =. 12; 'monkeys'
>
> there is a header for each atom of nm, so no copy is required.
>
> In
>
> 'p q r' =. y
>
> it depends on y. If y was created from (a;b;<c) the case will be like
> the monkeys example, but if by (a,b,c) a copy will be required.
>
> I would be obliged if Roger would indicate what he wasn't sure about,
> because it would show where my understanding is deficient. I know there
> is more to boxed arrays than my model accounts for, in particular when
> and whether the array of headers gets copied when a verb is evaluated.
> A puzzling example is:
>
> bb =. <"0 i. 10 11 12 13 100
> ts 'c =. bb'
> 0.343118 704
> ts 'c =. ] bb'
> 0.543127 7.02944e6
>
> In c =. bb, the interpreter reports that it filled 704 bytes, but it
> took 1/3 of a second to do it. Could that be an error in space
> accounting? But the suggests that the array of headers is rebuilt when
> the noun is copied.
>
> In c =. ] bb, apparently it used an array that was 4 bytes per atom of
> bb, which suggests that the array of headers was rebuilt for the
> execution of ] .
>
> Henry Rich
>
>
> On 11/24/2011 1:44 AM, Roger Hui wrote:
> > This does not apply for the first example, nor for the general case of
> the
> > third example. Not sure about the second example. In any case you can
> get
> > answers to these questions yourself by doing some experiments and
> checking
> > the space consumed.
> >
> >
> >
> > On Wed, Nov 23, 2011 at 10:35 PM, Ben Gorte - LR<[email protected]
> >wrote:
> >
> >> Thanks for the explanation. Would this also apply to
> >>
> >> 'a b c' =. abc =. i.3 4 5
> >>
> >> or to
> >>
> >> 'n m' =. nm =. 12; 'monkeys' ?
> >>
> >> In that case it is perhaps not so expensive to pass more than two things
> >> to a verb and separate these inside it wilh
> >>
> >> 'p q r' =. y (?)
> >>
> >> Ben
> >> ________________________________________
> >> From: [email protected] [
> [email protected]]
> >> on behalf of Roger Hui [[email protected]]
> >> Sent: Wednesday, November 23, 2011 3:36 PM
> >> To: Programming forum
> >> Subject: Re: [Jprogramming] One noun with two names
> >>
> >> Names are reference-counted so that if you say a=:b=: 123 the reference
> >> counts for a and b are each one more than if you'd said a=:123 and
> b=:123.
> >> Before the interpreter modifies an array it checks the reference count
> and
> >> if that count is not the minimum it will make a copy of the array before
> >> modifying it.
> >>
> >>> (but what would be the prupose?)
> >>
> >> Shorter code. a=:b=:c=: blah is a common construct, in many
> programming
> >> languages.
> >>
> >>
> >>
> >> On Wed, Nov 23, 2011 at 6:16 AM, Ben Gorte - LR<[email protected]
> >>> wrote:
> >>
> >>>
> >>> Dear J forum,
> >>>
> >>> To my surprise I found out that
> >>>
> >>> a =. b =. 480 640$0
> >>>
> >>> is not the same as
> >>>
> >>> a =. 480 640$0
> >>> b =. 480 640$0
> >>>
> >>> as the first gives you two names pointing to the same data. I guess
> that
> >>> should be ok, and it saves space (but what would be the prupose?)
> >>>
> >>> I passed both as pointers to a DLL (a .so in linux), which was supposed
> >> to
> >>> put two different images in a and b, and then the first is not ok. It
> >> only
> >>> works with the second variant :-)
> >>>
> >>> Greetings from Cape Town,
> >>> Ben
> >>>
> >>> ----------------------------------------------------------------------
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> >>>
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> >>
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> >
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