My role was to create simple problems that students could understand fully,
so they would be ready for the graduate courses you describe. All the issues
like rank are useless when they do not yet have an appreciation for the
elegance of a vector.
Your comments could form the basis for an excellent syllabus for a course in
J at the college level! In addition, they should raise questions the you
programmers on this forum will need to understand as they are ready.
Maybe there are changes in j701 that are causing this not to work.
I'll leave this one for the real computer wizards.
table4 =: 2 2 $ '';];([:,.[);+/
a=:2 3 5
b=:0 1 2 3
a table b
|domain error
| (((#~LF-.@e.])5!:5<'u');,.y),.({.;}.)":x,y u/x
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Fraser Jackson
Sent: Sunday, November 20, 2011 3:19 PM
To: Programming forum
Subject: Re: [Jprogramming] table challenge in simple J
In this thread there has been a focus on the visual appearance
of the output. But many of the functions fail what I consider
a basic requirement in many programming situations.
We can get visual identity, but the displayed objects may
be of different shape or type.
If the objective is purely to produce a pretty display then
conversion to literal may be fine. If it is to create
a result which is to be used in further calculations it may not.
After my post Dan Lettow suggested off line that I should
incorporate ": in future solutions. I looked at it, drafted
a simple addendum to my post but did not post it.
Now that we have some more solutions it is clear that Linda's
problem specification is not really adequate in a J environment.
(1) We need to specify the ranks of the function, or some
very powerful defaults will be applied.
If we want the full power of insert then the default is _ _ _ .
I think there are strong grounds for wanting to preserve that.
To handle what Kip called extreme cases, it was suggested
that the arguments should be converted to vectors. That is
fine for functions defined over a pair of numeric scalars.
We might want to generalise that in many different ways.
Suppose we had two tables of points with three coordinates,
and wish to tabulate some function dependent on them -
perhaps the distance between them.
The function in table need not be of rank 0 0 though that
is the commonest form used for illustration.
Linda's problem was interpreted by most contributors as
writing a function for rank 1 arguments on left and right
and the table function as having rank 0.
(2)If we are to preserve fidelity then we need to
consider the type of the result. Again several contributors
have proposed solutions in which some of the boxed displays
are literal. That gives a pretty display, but it is not
true to the original argument. A sum of a numeric and
a literal vector does not make sense.
One thing we often find in J is that a structure which is
based on the algorithmic needs is different from the
best way of presenting the information.
There is a need for a table function which preserves
all features of the J objects, but also a need for a
listtable function which displays the results using forms
which assist visual interpretation.
They are different needs and we should recognize them. In
statistical programs having both table and listtable which
operates on the result a table call is a common way of
dealing with this.
(3) Sometimes it is easier to build up a set of results
of interest and then assemble them. Someone may have
suggested it, but if they have I did not notice it.
table4 adopts that approach.
table4 =: 2 2 $ '';];([:,.[);+/
It works when the left argument is of rank 0 or 1 but it is not
totally satisfactory visually and does not work at all
well otherwise.
Linda raised a very simple problem. Her intuition that it
would raise interesting and important issues has indeed
been true.
----- Original Message -----
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