On Sun, May 26, 2013 at 2:01 PM, Robert Herman <[email protected]> wrote:
> I have worked through the solution arrived at by a few of you to better
> learn J, namely this one:
>
> +/,(57=x+/y+/z)*x</y*y</z
>
> In order to fully understand the way the matrix operations are carried out,
> I minimized the answer space by making P = 13, and worked through the
> output (not shown for brevity's sake) of the following input lines I placed
> in a project file (learning the IDE with this problem too):
>
> P =: 13
> x =: >:i.4
> y =: >:x
> z =: >:y
>
> 3 4 $ x,y,z
>
> (13=x+/y+/z)*x</y*y</z
> +/,(13=x+/y+/z)*x</y*y</z
>
> +/,((13=[+/+/)*[</]*</)~i.7
Since this is a rank 3 bit array, we can ask J to display it more compactly.
<"2 '.*'{~((13=[+/+/)*[</]*</)~i.7
We can also use this prefix to inspect some of the intermediate results:
<"2 '.*'{~([</]*</)~i.7
<"2 '.*'{~(</)~i.7
That said, some interemdiate results are not bit arrays and are
perhaps best inspected directly:
(]*</)~i.7
> Anyway, I was tempted to try and make the J program look like the Perl
> oneby using
> for. but I realized I was lapsing into old habits. My minimal programming
> experience is all with imperative languages or some Lisp too. My next
> question is how do you get the J program to print the solutions as in the
> Perl program?
That's an open-ended question, since "as in" has a contextual and case
specific meaning.
Personally, I usually try and understand intermediate results (usually
there is only one and I can just extract an expression which computes
it and see what it does with sample data.
> In my 3 x 4 matrices of the reduced problem set, I could see
> how the 1's in the solution selection matrix set related to the original 3
> x 4 matrix of x, y and z rows. Without re-writing it, is there a way to
> index the solution?
I am not sure what you mean by this.
I mean, of course you can index the solution, but the solution has
only a single number. You can also rewrite the expression which
creates that solution and index from those results. But I feel like I
am not understanding your question.
> One question that is on my mind is the tacit vs. explicit style. I am in
> love with the way you all distill the solution down to a tacit one-liner,
> just as a maths formula is simplified, however, I am starting to think that
> if and when I revisit this problem in the future, it will take a bit of
> jarring to read it, translate it if you will. I am guessing for. loops are
> very expensive cycle-wise and it would be just as easy to make words for
> phrases if I wish to preserve future readability. I am getting addicted to
> the fast way of working in the J environment, and I am grateful to all on this
> list for sharing their wisdom.
I think that any time we delve into some area of computation that
leaving and coming back after an extended absence can leave us in a
jarring situation where we need to reacquaint ourselves with the
original topic. I do not know of any general cure for this, finding
good names for things, and good examples, can be fruitful.
Thanks,
--
Raul
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