[Jprogramming] Implementing a tacit "filter" function

[Alex Rufon]

When I started using J, I realized that learning J is a paradigm shift.

The simplest way for me to explain this is this way. 

For example, you have a table of information and you need find all rows which matches a certain condition. Before I learned to use J, what I would expect is something similar to this SQL DML command:
SELECT * FROM table1 WHERE column1 < 10

In J, I may code it this way:
   [data=. (10 $ 3) ? 15
13  0  8
 3  7  9
 1  8  2
 1  4  6
13  8  2
12  0  1
 8 14 10
 8  7  5
 3 11  9
 9 10  0
   10 > {. "1 data
0 1 1 1 0 0 1 1 1 1
   data #~ 10 > {. "1 data
3  7  9
1  8  2
1  4  6
8 14 10
8  7  5
3 11  9
9 10  0

One of the paradigm shifts for me is in knowing that you can get the masking vector (10 > {. "1 data) and you can manipulate it.

You just have to keep using it and asking question and then after a while it would suddenly click into place. 

Good luck on your journey. ;)

r/Alex

P.S.
Both my son and daughter are named Alex too. :D 

-----Original Message-----
From: programming-bounces at jsoftware.com [mailto:programming-bounces at jsoftware.com] On Behalf Of Alex Gian
Sent: Wednesday, August 25, 2010 5:10 PM
To: Programming forum
Subject: Re: [Jprogramming] Implementing a tacit "filter" function

On Wed, 2010-08-25 at 07:14 +0800, bill lam wrote:
> I guess you think J is a functional language.  What if you are told J 
> is not a functional language?

Hi Bill!

Thanks, but I want to learn J, not to get into a philosophical dilemma
about "functional languages"...   :)

Having said that, my view is that if you've got: 1) nameless functions,
2) can pass functions as arguments and 3) lazy evaluation (which I'm sure is manageable in J - though not often necessary) then you've got a functional language.

The main difference I can now see (with my newbie vision) between J and other FLs is that J does not rely as heavily on recursion, because of its marvellous handling of arrays.  This has got to impact on programming _style_ a bit, but even though I am somewhat used to the elegant recursive style, the incredible speed gain you get with J is more than worth it for me.


> There were lengthy discussions on this subject in J forums.  Please 
> search forum achieve for detail.

I am sure I'll read those discussions at some stage, but first I have to finish Rich's book, then Roger's, the the Dictionary... so loads of reading ahead.







----------------------------------------------------------------------
For information about J forums see http://www.jsoftware.com/forums.htm