When something is slow, it's usually because you
are operating with too low a rank.
Here, you are interpreting up82 6400 times,
which is about 2/3 of the problem.
Also, it's better to use &.> to do the boxing
and unboxing, since that's optimized.
The following runs fast (I'm on J6):
,. {&(,M0 , (< 0 0))&.> ,&.> ,m
I had to ,&.> the m to get the same results as the
original.
I suspect ,M0 , (< 0 0) is not creating what you
think.
Henry Rich
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of p j
> Sent: Thursday, February 23, 2006 5:41 PM
> To: [email protected]
> Subject: [Jprogramming] Strange performance problem
>
> At the bottom of this message is timing code for what
> I naively think should be the same function call. But
> one is over 1000 times faster.
>
> The code (x up82 y) is called with x as a column of
> boxes, where each box contains 1 to 3 indexes to look
> up in y.
> x up82"1 y returns the result without filling (what I
> want), but is slow. IE if a box in x has only one
> index, the function returns only one box in the result
> instead of filling the result with 0{y.
> x up82 y fills result with 0{y
>
> 1. I don't really understand the difference between
> the 2 function calls, so thank you for any patience in
> explaining the difference.
> 2. I'm curious why one is 1400x times slower
>
> -------SETUP CODE
>
>
> M =: i.80 80
> m=: (1+i.(80*80)),(80+i.(80*80)),(_80+i.(80*80))
> m=: 80 80 $(<"1 (>"1 (|: (3 6400 $ m))))
> m=: (<"1 (}:"1 (>0{m))) 0}m
> m=: (<"1 (0 2 {"1 (>_1{m))) _1}m
> m=: (<"0 (1+ 80*i.80) ) (< a: ; 0) } m
> m=: (<"0 (80$ 6400)) (< a: ; _1) } m NB. add trailing
> 0 in 6401st position of input matrix
> M0 =. 80 80 $ <"1 (,. , +/\."1 M),"1 (,.,M)
>
> up82 =: 4 : 0
> <"1((> x.) { y.)
> )
>
> ------------TIMING CODE
> ts '(,. ,m) up82 ,M0 , (< 0 0)'
> 0.0216966 994048
> ts '(,. ,m) up82"1 ,M0 , (< 0 0)'
> 27.8915 1.27974e6
>
>
>
>
>
>
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