A question for Oleg - forgive me if I am incorrect in raising this as I do
not feel so experienced to query J code as yet ...
Oleg, I think your solution has introduced a small, subtle bug compared to
Henry or Raul's, as your test 1&< is based on (1-px)^2, instead of on px.
Ronan's data (v1) does not produce any px's in the test range _1<:px<:1, so
your solution agrees with Henry/Raul in this instance.
px =: (|. v1) p. (%~ i:) 30 NB. Ronan's px data from 60, v1
(<./ , >./) px NB. Range of px values
_190.837 _30.0942 NB. None in range _1<:px<:1
oleg =: 3 : '+/ (* 1&<) *:-. y'
henry =: [: +/ [: *: -. * _1&> + 1&<
raul =: [: +/ [: *: (1-]) * 1 < |
0j6":(oleg , henry , raul) px
676667.956501 676667.956501 676667.956501 NB. No problems for Ronan's data
However, if the px's were to contain values within _1<:px<:1 ...
px1 =: _2 _0.7 _0.3 0.3 0.7 2 NB. Make up test data
(oleg , henry , raul) px1
13.58 10 10
The difference being the test on px or (1-px)^2 for px, between _1 and 0...
> px1 ; ((* 1&<)*:-. px1) ; ([: *: -. * _1&> +. 1&<) px1
_2 _0.7 _0.3 0.3 0.7 2 NB. px1 made up test data
9 2.89 1.69 0 0 0 NB. Oleg's test based on 1<(1-px1)^2
9 0 0 0 0 1 NB. Henry/Raul's test based on _1<:px1<:1
Is this a correct analysis ?.../Rob Hodgkinson
PS: Written in Courier, but my emails seem to be transformed upon sending,
apologies if this does not display nicely.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Oleg Kobchenko
Sent: Wednesday, 28 June 2006 4:49 PM
To: Programming forum
Subject: Re: [Jprogramming] Precision query
0j6": +/ (* 1&<) *:-. (|. v1) p. (%~ i:) 30
676667.956501
----- Original Message ----
From: Henry Rich <[EMAIL PROTECTED]>
To: Programming forum <[email protected]>
Sent: Tuesday, June 27, 2006 9:56:31 PM
Subject: RE: [Jprogramming] Precision query
You need to do the 1-px thing like so:
calcCost =: [: +/ [: *: -. * _1&> + 1&<
and then, if you want to see the result in full precision,
you have to ask for it:
15j6 ": calcCost (|. v1) p. dxVect
676667.956501
The default display doesn't show the lower digits.
It doesn't seem right to me to look for values outside
of [0,1] and then square the distance from 1. Shouldn't
you square the distance from 1 if it's >1, and square
the distance from _1 if it's <_1? That is,
-.@:| instead of -. above
Henry Rich
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Ronan Reilly
> Sent: Tuesday, June 27, 2006 8:17 PM
> To: Programming forum
> Subject: [Jprogramming] Precision query
>
> Hi,
>
> In light of the recent discussion on precision and rounding
> error, I have a
> concrete example of a problem I'm having with rounding error (at least
> that's where I think the problem lies).
>
> The goal is to write a cost function for a polynomial
> evaluation so that any
> result outside [-1,1] is penalised. The work is part of an
> implementation
> of a genetic algorithm.
>
> Here is a stripped down C function which I'm using as the
> basis for my J
> implementation (plus a sample vector for evaluation):
> ====
> #include "stdio.h"
>
> double evaluate (int, double[]);
>
> int main() {
>
> double v1[]={54.735, 31.069, -79.616, -22.279, -76.028,
> -42.021, -34.925,
> -44.066, -53.706};
> printf ("evaluate: %f\n", evaluate(9, v1));
>
> return;
> }
>
> double evaluate(int D, double tmp[]) {
> int const M=60;
> int i, j;
> double px, x, dx, result=0.0;
>
> x = -1.0;
> dx = 2.0/(double)M;
> for (i=0; i<=M; i++) {
> px = tmp[0];
> for (j=1; j<D; j++) {
> px = x*px + tmp[j];
> }
> if (px<-1.0 || px>1.0) result+=(1.0-px)*(1.0-px);
> x+=dx;
> }
> return result;
> }
> ===
>
> Here is the equivalent J code:
>
> v1 =: 54.735 31.069 _79.616 _22.279 _76.028 _42.021 _34.925
> _44.066 _53.706
> dxVect =: +/\ _1, 60 $ 2 % 60
> calcCost =: [: +/ [: *: ] * _1&> + 1&<
>
> When I run the J code (J601 o beta) I get:
>
> calcCost (|. v1) p. dxVect
> 665922
>
> whereas running the C program gives me:
>
> evaluate: 676667.956501
>
> The problem seems to be with calcCost, and I was wondering if
> there was a
> different way of formulating that function to reduce the
> discrepancy in the
> two results.
>
> Many thanks,
>
> Ronan
>
>
> --
> Ronan Reilly
> NUI Maynooth
>
> t: +353-1-7083847
> e: [EMAIL PROTECTED]
> w: http://www.cs.nuim.ie; http://cortex.cs.nuim.ie
>
>
>
> ----------------------------------------------------------------------
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> http://www.jsoftware.com/forums.htm
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