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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