sub tobase { my $radix = int($_[0]); (my $num = int($_[1])) =~ y/-/_/;
+ chomp(my $str = `dc -e${radix}o${num}p`); $str } 
print tobase(13, 54), "\n"; # outputs 42
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Update: fixed the syntax errors, I've no idea how I got them there, sorry.

surely there's a way to avoid `dc`

-Paul

This one is longer than a one-liner. Let's call the stringification function from the gmp library.

First, make sure you have the gmp library and the headers for it installed, eg. if you have a debian linux system, install the libgmp3-dev package. Then make a directory called Math-Tobase-1.0 and enter the following code to a file called Tobase.xs in it:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include <gmp.h>

MODULE = Math::Tobase  PACKAGE = Math::Tobase

SV *
tobase(radix, num)
        int radix
        long num
    CODE:
        ST(0) = sv_newmortal();
        if (2 <= radix && radix <= 62 || -36 <= radix && radix <= -2) 
+{
            mpz_t big;
            mpz_init_set_si(big, num);
            if (mpz_sizeinbase(big, radix) <= 70) {
                char buf[72];
                mpz_get_str(buf, radix, big);
                sv_setpv(ST(0), buf);
            }
            mpz_clear(big);
        }
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Then enter the following to a file called lib/Math/Tobase.pm under the Math-Tobase-1.0 directory (create the subdirectories):

package Math::Tobase;

require Exporter;
require DynaLoader;
our $VERSION = "1.00";
our @ISA = (Exporter::, DynaLoader::);
our @EXPORT = "tobase";

bootstrap Math::Tobase::;

1;
__END__
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Then enter the following to the Makefile.PL file under the Math-Tobase-1.0 directory.

use ExtUtils::MakeMaker;

WriteMakefile(
    NAME => "Math::Tobase",
    VERSION_FROM => "lib/Math/Tobase.pm",
    LIBS => ["-lgmp"],
);
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Now compile the code with the command

perl Makefile.PL && make
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If all is successful, you can install the module now and get a Math::Tobase module with a tobase function that does what you want.

Try it out before installing using the command

perl -I./lib -Iblib/arch -we 'use Math::Tobase; print tobase(13, 54), 
+"\n";'
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The output shall be 42.

Update 2011-03-18: see also Re: Module for 128-bit integer math? for a list of bigint modules. See also Re: Convert big number from decimal to hexadecimal where I reuse this code.

Update 2012-10-16: note to self (as I reference this code frequently): you may want to use XSLoader.

Update 2013-11-19: for more XS examples, see Re: Perl XS.

That's a nice little demo of how to set up a simple perl extension - though there are already a number of modules in existence that will do the conversion.

<plug>
Math::GMPz already uses the very same gmp library function:

perl -MMath::GMPz=":mpz" -wle "print Rmpz_get_str(Math::GMPz->new(54),
+ 13)"
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</plug>

It's also available with Math::GMP, too (though the Math::GMP function we use is undocumented):

perl -MMath::GMP=":constant" -wle "print Math::GMP::get_str_gmp(54, 13
+)"
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There are other modules, too, that one could use, but these are the only modules I know of that use the gmp library for the task.

Cheers,
Rob

Wow, I didn't know there were interfaces to GMP other than Math::BigInt::GMP, and that one does not have an interface to this.

There's also Math::BaseCnv to print numbers in an arbitary base though.

sub ltostr {
    my ($num, $base) = @_;
    Carp::croak("Invalid base $base") if $base < 2 || $base > 36;
    my $neg = $num < 0;
    $num = -$num if $neg;
    my $str = '';
    do {
        $str = ( '0'..'9', 'a'..'z' )[ $num % $base ] . $str;
        $num = int( $num / $base );
    } while $num;
    $str = "-$str" if $neg;
    return $str;
}
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Actually, POSIX::strtol does not "parse a string representation of a number in any base into a number" but does a "String to (long) integer translation", which is quite different, but in Perl --IMO-- seldom useful.

A Perl scalar will pretend to be a number or a string as the circumstances require, so you do not need a "string to/from number" function.

You do if you want a non-base-2/8/10/16 string representation.

Actually, <c>POSIX::strtol<c> does not "parse a string representation of a number in any base into a number" but does a "String to (long) integer translation", which is quite different, but in Perl --IMO-- seldom useful.

In principle, yes, but if you know it will work with the range of numbers you have, strtol is possibly fine.
--
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