Re: better apl array coding

[Dr . Jürgen Sauermann]

    Hi,
      
      the biggest problem with your old loop code is its quadratic
      runtime.
      
      The statement 
    
    rs←rs,ar[a[i]; b[i]]
    
takes more and more time as rs grows. GNU APL has occasionally been
blamed for being slow, but in most cases the above coding pattern was
the real reason. A faster way in your old loop code is to:

rs←(⍴A)⍴0

before the loop and then

rs[i]←ar[a[i]; b[i]]

inside the loop, which takes only linear time.

In general: never use , or ⍪ inside a (long) loop, even
though the code may look more intuitive or elegant.

Best Regards,
Jürgen

On 6/16/21 4:39 AM, enztec@gmx.com wrote:

> Hi
>
> thanks for this code - first time actually using ¨ and ⌷
>
> in my comparison test on an old hp dual core linux machine - with only mplayer running
> with ⍴a and ⍴b 29882 chars
>
> according to ⎕ts   my loop code takes ~39 seconds to put results into rs
> 2021 6 15 19 46 11 277
> 2021 6 15 19 46 50 354
>
> and the new code takes ~50 msec to put results into rs1    yes msec vs seconds   what is that 39000 msec vs 59 msec
> 2021 6 15 19 46 50 354
> 2021 6 15 19 46 50 404
>
> +/rs1=rs    and rs and rs1 don't exist before running them in fns
> 29882
>
> ----
>
> the only problem i have is in a fresh ws running the )copy as
>
> ⍎')copy mn9     mn9 is a file which has a string of 29882 chars assigned to mn9←'xxxxx' in it
> a←mn9             gives mn9 value error consistently - though when i check the )vars it is there and complete - running it in ram drive has no effect
>
> the unquoted ')copy'  doesn't actually finish coping the file into the ws as fast as the code returns and goes to the next line
>
> no real problem as i usually hard code the sequnces (cutnpaste) into the script - but with some in the 100K range i will have to do the )copy manually and run in workspace
>
> thank you for giving me this code - much appreciated
>
>
>
> On Tue, 15 Jun 2021 15:20:27 +0200
> Dr. Jürgen Sauermann <mail@xn--jrgen-sauermann-zvb.de> wrote:
>
> > Hi,
> >
> > how about:
> >
> >       (a,¨b)⌷¨⊂ar
> > 11 22 33 44 44 33 22 11
> >
> >
> > Best Regards,
> > Jürgen
> >
> >
> > On 6/15/21 2:45 AM, enztec@gmx.com wrote:
> > hello
> >
> > i've been using this looping for array work but there has to be a better apl way of coding the indexing of the ar array
> > any suggestions
> >
> >
> >
> > ∇rs←lp;a;b;ar;i
> > 'lp'
> > 'ar'
> > +ar←4 4⍴11 12 13 14 21 22 23 24 31 32 33 34 41 42 43 44
> > 'a '
> > +a←1 2 3 4 4 3 2 1
> > 'b '
> > +b←1 2 3 4 4 3 2 1
> > i←1
> > rs←⍴0
> > sp:
> > rs←rs,ar[a[i]; b[i]]
> > →((i←i+1)⍴a)/sp
> > ''
> > ∇
> >
> >
> >
> >       
>