Re: [q-lang-users] "listof" surprises

[Alexander N. <AN...@sp...>]

Hello maggesi,

Sunday, May 13, 2007, 3:01:10 AM, you wrote:

mmui> Hi,

mmui> I just got burned by this unexpected (for me, of course) behavior of
mmui> list comprehension.  Given the program

mmui>    delta X X      = 1;
mmui>    delta _ _      = 0 otherwise;
mmui>    tabulate N M F = [[F I J : I in [1..N]] : J in [1..M]];
mmui>    eye N          = tabulate N N delta;

mmui> then (eye 2) evaluates to [[0,0],[0,0]] instead of [[1,0],[0,1]] as I
mmui> would have imagined.  I vaguely understand that the first argument is
mmui> passed to delta 'too early' in tabulate, but I didn't try to read
mmui> cond.q and understand precisely why it works like this (yet).

Yes, it is evaluated 'too early', but there's a simple method to fix
it:

delta X:Int Y:Int     = 1 if Y = X;
                      = 0 otherwise;

thus delta I J will just remain not evaluated.

-- 
Best regards,
 Alexander                            mailto:AN...@sp...