[q-lang-users] "listof" surprises

[<ma...@ma...>]

Hi,

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

   delta X X      =3D 1;
   delta _ _      =3D 0 otherwise;
   tabulate N M F =3D [[F I J : I in [1..N]] : J in [1..M]];
   eye N          =3D tabulate N N delta;

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

This is not the point anyway.  My question rather is: wouldn't be a
different behavior more appropriated to avoid this kind of traps?
E.g., I would like to propose something like:

   special comprehension E V ~R;
   comprehension E V R =3D map (lambda V E) R;

for which the definition like

   tabulate N M F =3D comprehension (comprehension (F I J) I [1..N]) J [1..M=
];

seems to show a more predictable behavior.

Just trying to learn Q ...
M.

--=20
Marco Maggesi
Universit=E0 degli Studi di Firenze
http://www.math.unifi.it/~maggesi/


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