Re: Ask for recommended module for precise number

Hi Harry,

In fact those number are required to interact with a conjugate gradient
library in C++.
I tried to write a simple program in C and then use SWIG to generate
the PERL interface:

===============================================================
C:

double addOne(double x)
{
double y = x + 1.0;

return y;
}
===============================================================
PERL:

my $x = new Math::BigFloat(0.1);
$x = addOne($x);
printf "%.5f\n", $x;
===============================================================

It is amazing that it output "1.10000" to me !

After that I modify the program as:
===============================================================
PERL:

my $x = new Math::BigFloat(0.1);
foreach my $i (0..99) {
$x = $x * 0.1;
}
my $x = 10.0;
$x = Simplest::addOne($x);

printf "%.5f\n", $x;
===============================================================

And it can output "1.00000" to me!


Tom



harryfmudd [AT] comcast [DOT] net wrote:
> Tom wrote:
> > Sorry, I should call you "Tom"...........
> >
>
> No problem. Harry Mudd is a quasi-alias (quasi- because I always sign my
> real name), and a character from the original Star Trek. It was intended
> to be thrown away when it started collecting too much spam, but so far I
> haven't done that. Though I'm tempted daily.
>
> >
> > Tom wrote:
> >
> >>Hi Harry,
> >>
> >>Thanks for you help. It was really my misunderstanding before. I write
> >>another program which simulate my typical application: calculating
> >>posterior from prior and likelihood, which are very small number.
> >>
> >>================================================================
> >>use strict;
> >>
> >>use Math::BigFloat;
> >>use Carp;
> >>
> >>my $x = new Math::BigFloat(0.1);
> >>my $y = new Math::BigFloat(0.1);
> >>my $num = 100;
> >>
> >>foreach my $i (0..$num) {
> >> $x = $x * 0.1;
> >>}
> >>foreach my $j (0..$num-1) {
> >> $y = $y * 0.1;
> >>}
> >>
> >>my $z = $x/$y;
> >>
> >>printf "z: %.5f\n", $z;
> >>================================================================
> >>Now, the z is "0.1". It fits my usage as those small numbers are only
> >>stored internally. :)
> >>
> >>
> >>For the printf() issue, it seems to be inconsistency between the string
> >>and the printf() function. I have tried the following tiny program:
> >>
> >>================================================================
> >>use strict;
> >>use Carp;
> >>
> >>my $i = 0;
> >>while (1) {
> >> my $zeroString = join('', map { "0" } (1..$i));
> >> my $numString = sprintf("0.%s1", $zeroString);
> >> eval {
> >> printf "%d: %.5f\n", $i, $numString;
> >> };
> >> confess ($@) if ($@);
> >>
> >># confess (qq{Convert to "0" at $i-th iterations.\n}) if ($numString
> >>== 0);
> >>
> >> $i++;
> >>}
> >>================================================================
> >>With the "commented" statement, the program always output "0.00000"
> >>without error (the program was terminated at 9xxx-th iterations).
> >>
> >>With the "commented" statement, the program converts to "0" at around
> >>300 iterations. (it means the Math::BigFloat case is not reproductable
> >>at normal PERL operations?)
> >>
> >>
> >>Tom
> >>
> >>
>
> [snip! /]
>
> I think we can conclude that printf is simply not going to work right
> with Math::BigFloat numbers.
>
> The bottom line is that Math::BigFloat is _almost_ magic. I have not
> used it much, but I get the impression that as long as you're so-called
> "pure Perl" you're OK. That is, if you write a script using BigFloat,
> and don't invoke any functions that you didn't write in Perl, you're
> fine. If you invoke built-ins (for example, log (), sin (), and so
> forth) or libraries that do "C" call-outs, you don't get arbitrary
> precision. At least, that's my explanation for printf's behaviour.
>
> Even if Math::BigFloat were fully magic and worked for the built-ins,
> transcendental functions like log () and sin () would still be unusable,
> because the calculation would never terminate.
>
> If you read the Math::BigFloat docs, you will find methods to "take
> apart" BigFloat numbers, and I suppose you (or someone) could write
> arbitrary-precision functions to replace the built-ins. Precision of
> transcendental functions would have to be specified "up front" in some
> way, to prevent atan2 (1, 0) from trying to compute the last digit of pi.
>
> Whether this has been done I don't know. I personally would hesitate to
> do it because I don't need it, and therefore it would be inadequately
> tested.
>
> If I could change the subject briefly, I'd like to make a couple
> comments on Perl style:
>
> * You don't need to "use Carp" and then "confess" unless you're
> interested in a full call stack trace. The "die" command also terminates
> your program with a message. If the message does not end in "\n", Perl
> appends the line number of the "die".
>
> * For a string of $i zeros, you can say '0' x $i instead of using join
> (), and I think you'll find it is more efficient.
>
> * Similarly, for large numbers of iterations, "for (i = 0; i < $num;
> $i++)" would be preferred to "foreach $i (0 .. $num)", because the "0 ..
> $num" actually builds a list of all the numbers, which might be a bad
> thing if $num were something like 1e9.
>
> Tom Wyant

.

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