Hi all !
I will try to explain this example.
$(i.5) (2 1$ =)"1 0 [i.2
2 2 1
$(i.5) (2 1$ =)"1 0 [i.1
1 2 1
$(i.5) (2 1$ =)"1 0 [i.0
0 2 1
$(i.1) (2 1$ =)"1 0 [i.0
0 2 1
$(i.0) (2 1$ =)"1 0 [i.0
0
f=: 4 : 'try. 2 1$x=y catch. 2 1$_1 end.'
$(i.0) f"1 0 [i.0
0 2 1
As we can see , in this expression:
$(i.0) (2 1$ =)"1 0 [i.0
the expression (2 1$ =) throws an error, which the interpreter hides and
replaces with i.0
As we can understand, taking a 0-cell from the right argument, i.0 , is
not possible, so what J does is to use a default element, 0, as the
0-cell taken. This is needed to determine the type and shape of the
result. However, this is highly questionable behaviour, since the
default element might not be a valid value and with a valid value the
result might well be of another type and shape. It is also questionable
because the calculation should not occur and doing it anyway might well
cause side effects like a patients death or a plane crash.
(i.0) (2 1$ =) [0
|length error
| (i.0) (2 1$=)[0
This resulting length error is hidden. The questionable result i.0
replaces it.
With knowledge about this strange internal behaviour of J you can create
a function like f and get the result you want in the particular case.
Without this knowledge you will probably not understand why you get i.0
as the result. You will have a fuzzy understanding of how rank works.
Rank, which is often used implicitly and which is fundamental for the
understanding of J.
The usage of the default element as 0-cell is something I think every J
programmer have to learn and remember. I think it should be described in
the dyadic rank dictionary page and in the dictionary pages of all verbs
which implicitly use dyadic rank, the non-scalar dydic verbs for which
one rank is 0, if any.
I think the length error should not be hidden.
Cheers,
Erling Hellenäs
Den 2017-12-13 kl. 17:12, skrev Erling Hellenäs:
Hi all !
It seems some related info did not make it into my blog and it is not
needed in my manual. My tweets about this are advertently cryptic:
-I found some interesting features of the real #J dyadic rank operator.
-Let's say the rank is zero and the argument is empty, how do you find
the resulting type and shape?
-Nothing can not be scalar, which can be executed to find the rank,
can it?
-If you execute the verb on a scalar default object, what do you do if
you get an error? Which rank and type will you use?
More clearly:
I have found that J determines the type and shape of the result when
both arguments of the dyadic array operation helper program are empty
by executing the verb against default objects of both arguments. If
the result is an error, J hides this error. The result then gets
another rank than it should. The rank is then not consistent with the
rank obtained when the arguments are not empty.
This means that if you have a theory of how rank works, the theory
will be falsified by some of the results you get, which means your
understanding will most probably be fuzzy. You have to test all cases
in the terminal to verify that it indeed works when one or the other
argument, or both, are empty.
According to my notes, this is such a case:
$(i.0) (2 1$ =)"1 0 [i.0
0
$(i.1) (2 1$ =)"1 0 [i.1
1 2 1
Cheers,
Erling Hellenäs
Den 2017-12-13 kl. 16:24, skrev Erling Hellenäs:
This blogpost also contains related information.
https://erlhelinfotech.wordpress.com/2016/08/17/jwithatwist-scalar-operations/
/Erling
Den 2017-12-13 kl. 15:46, skrev Erling Hellenäs:
It seems you didn't read the parts of my manual I referenced. /Erling
Den 2017-12-13 kl. 14:39, skrev Raul Miller:
On Wed, Dec 13, 2017 at 3:46 AM, Erling Hellenäs
<[email protected]> wrote:
I doubt you can find reasonably accurate descriptions of this
functionality
(these four helper programs) anywhere else.
This suggests to me that you have not read
http://www.jsoftware.com/help/dictionary/dictb.htm
nor
http://www.jsoftware.com/help/dictionary/dicta.htm
I am surprised how many people have not read the reference manual -
not just in the J community but among technical people in general.
I really don't know what to make of this.
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