[Jprogramming] Request for code critique '&. Hello'

[Fred Bone]

On 9 Feb 2006 at 17:00, John Randall said:

[...]

> I am thinking of a case like
> 
> if.     c1 y. do. a1 y.
> elseif. c2 y. do. a2 y.
> elseif. c3 y. do. a3 y.
> elseif. 1     do. default y.
> end.
> 
> I can rewrite this as
> 
> ((default`a3 at .c3)`a2 at .c2)`a1 at .c1
> 
> but it is very easy to go wrong.

I'm very much a newcomer to J, but is it not possible to approach this 
using the I. function?
   c1 =. >&5  NB. some arbitrary condition functions
   c2 =. >&3
   c3 =. <&0
   a1 =. *:   NB. and some arbitrary action functions
   a2 =. >:
   a3 =. <:
   default =. ]
(so: if > 5, square;
     elseif > 3, increment;
     elseif < 0, decrement;
     else identity)

   {.I.(c1,c2,c3,c4,1:) 4    NB. which function?
1
   (a1`a2`a3`default @. 1) 4  NB. function 1 is increment
5
NB. Now combine the expressions
   (a1`a2`a3`default @. ({. at I.@(c1,c2,c3,1:) )) 4
5   
   (a1`a2`a3`default @. ({. at I.@(c1,c2,c3,1:) )) 6
36
   (a1`a2`a3`default @. ({. at I.@(c1,c2,c3,1:) )) 2
2
   (a1`a2`a3`default @. ({. at I.@(c1,c2,c3,1:) )) _3
_4

Of course it looks horrible ... especially if you want it to handle a 
vector argument:
   (a1`a2`a3`default @. ({. at I.@(c1,c2,c3,1:) ))"(0) 6 4 3 _3
36 5 3 _4


Or have I completely misunderstood what you're trying to do?