Re: Turning a sequence of expr into expr

[Karim BELABAS <Karim.Belabas@math.u-psud.fr>]

[Ilya:]
> I got an impression that functions which take a chunk of code as an
> argument always take a statement, though the parser gives them a
> sequence of statements.  Otherwise I cannot explain what happens here:
> 
>  \p 9
>  ploth(t=100,110,z=zeta(0.5+I*t);[real(z),imag(z)])
>  ***   impossible assignment t_COMPLEX --> t_REAL.
> 
> As you see, the execution started (so the skipwhatever() was expecting
> a sequence), but apparently the second statement is ignored.
> 
> If we suppose that this is not a bug in ploth(), then how to wrap
> these statements into one?  I hate to use 
> 
>   ploth(t=100,110,[real(z=zeta(0.5+I*t)),imag(z)]) 

The usual trick to feed a "seq" to an "expr-reader" is to group the
statements into an "if (TRUE, seq)".

  ploth(t=100, 110, if (1, z=zeta(0.5+I*t); [real(z),imag(z)]) )

> and defining a function is a performance hit.

For that specific example, this is definitely exagerated.

The "side-effect" solution is indeed ugly, but as efficient as you can get:

  (20:08) gp > ploth(t=100,110,[real(z=zeta(0.5+I*t)),imag(z)]) 
  time = 17,542 ms.

The "performance impaired" version is not that bad:

  (20:08) gp > f(t)= local(z); z=zeta(0.5+I*t);[real(z),imag(z)]
  (20:09) gp > ploth(t=100,110,f(t))
  time = 17,547 ms.

[ the "if" solution is slightly faster ]


For really trivial examples, the function call overhead is about 40ms on my
(very slow) machine

(20:09) gp > f(t) = [0, 0]
(20:10) gp > ploth(t=100,110,f(t))
time = 76 ms.
%2 = [100.000000, 110.000000, 100.000000, 109.999969]
(20:10) gp > ploth(t=100,110,[0,0])
time = 33 ms.

  Karim.
__
Karim Belabas                    email: Karim.Belabas@math.u-psud.fr
Dep. de Mathematiques, Bat. 425
Universite Paris-Sud             Tel: (00 33) 1 69 15 57 48
F-91405 Orsay (France)           Fax: (00 33) 1 69 15 60 19
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