Note: You probably want to use accuracy() instead. With the accuracy manpage you set the number of digits each result should have, with precision you set the place where to round!

Autocreating constants

After use Math::BigFloat ':constant' all the floating point constants in the given scope are converted to Math::BigFloat. This conversion happens at compile time.

In particular

  perl -MMath::BigFloat=:constant -e 'print 2E-100,"\n"'

prints the value of 2E-100. Note that without conversion of constants the expression 2E-100 will be calculated as normal floating point number.

perldoc2tree.cgi: /usr/lib/perl5/5.8.8/Math/BigFloat.pm: cannot resolve L in paragraph 429.

Please note that ':constant' does not affect integer constants, nor binary nor hexadecimal constants. Use bignum or the Math::BigInt manpage to get this to work.

Math library

Math with the numbers is done (by default) by a module called Math::BigInt::Calc. This is equivalent to saying:

        use Math::BigFloat lib => 'Calc';

You can change this by using:

        use Math::BigFloat lib => 'BitVect';

The following would first try to find Math::BigInt::Foo, then Math::BigInt::Bar, and when this also fails, revert to Math::BigInt::Calc:

        use Math::BigFloat lib => 'Foo,Math::BigInt::Bar';

Calc.pm uses as internal format an array of elements of some decimal base (usually 1e7, but this might be differen for some systems) with the least significant digit first, while BitVect.pm uses a bit vector of base 2, most significant bit first. Other modules might use even different means of representing the numbers. See the respective module documentation for further details.

Please note that Math::BigFloat does not use the denoted library itself, but it merely passes the lib argument to Math::BigInt. So, instead of the need to do:

        use Math::BigInt lib => 'GMP';
        use Math::BigFloat;

you can roll it all into one line:

        use Math::BigFloat lib => 'GMP';

It is also possible to just require Math::BigFloat:

        require Math::BigFloat;

This will load the neccessary things (like BigInt) when they are needed, and automatically.

Use the lib, Luke! And see Using Math::BigInt::Lite for more details than you ever wanted to know about loading a different library.

Using Math::BigInt::Lite

It is possible to use the Math::BigInt::Lite manpage with Math::BigFloat:

        # 1
        use Math::BigFloat with => 'Math::BigInt::Lite';

There is no need to ``use Math::BigInt'' or ``use Math::BigInt::Lite'', but you can combine these if you want. For instance, you may want to use Math::BigInt objects in your main script, too.

        # 2
        use Math::BigInt;
        use Math::BigFloat with => 'Math::BigInt::Lite';

Of course, you can combine this with the lib parameter.

        # 3
        use Math::BigFloat with => 'Math::BigInt::Lite', lib => 'GMP,Pari';

There is no need for a ``use Math::BigInt;'' statement, even if you want to use Math::BigInt's, since Math::BigFloat will needs Math::BigInt and thus always loads it. But if you add it, add it before:

        # 4
        use Math::BigInt;
        use Math::BigFloat with => 'Math::BigInt::Lite', lib => 'GMP,Pari';

Notice that the module with the last lib will ``win'' and thus it's lib will be used if the lib is available:

        # 5
        use Math::BigInt lib => 'Bar,Baz';
        use Math::BigFloat with => 'Math::BigInt::Lite', lib => 'Foo';

That would try to load Foo, Bar, Baz and Calc (in that order). Or in other words, Math::BigFloat will try to retain previously loaded libs when you don't specify it onem but if you specify one, it will try to load them.

Actually, the lib loading order would be ``Bar,Baz,Calc'', and then ``Foo,Bar,Baz,Calc'', but independend of which lib exists, the result is the same as trying the latter load alone, except for the fact that one of Bar or Baz might be loaded needlessly in an intermidiate step (and thus hang around and waste memory). If neither Bar nor Baz exist (or don't work/compile), they will still be tried to be loaded, but this is not as time/memory consuming as actually loading one of them. Still, this type of usage is not recommended due to these issues.

The old way (loading the lib only in BigInt) still works though:

        # 6
        use Math::BigInt lib => 'Bar,Baz';
        use Math::BigFloat;

You can even load Math::BigInt afterwards:

        # 7
        use Math::BigFloat;
        use Math::BigInt lib => 'Bar,Baz';

But this has the same problems like #5, it will first load Calc (Math::BigFloat needs Math::BigInt and thus loads it) and then later Bar or Baz, depending on which of them works and is usable/loadable. Since this loads Calc unnecc., it is not recommended.

Since it also possible to just require Math::BigFloat, this poses the question about what libary this will use:

        require Math::BigFloat;
        my $x = Math::BigFloat->new(123); $x += 123;

It will use Calc. Please note that the call to import() is still done, but only when you use for the first time some Math::BigFloat math (it is triggered via any constructor, so the first time you create a Math::BigFloat, the load will happen in the background). This means:

        require Math::BigFloat;
        Math::BigFloat->import ( lib => 'Foo,Bar' );

would be the same as:

        use Math::BigFloat lib => 'Foo, Bar';

But don't try to be clever to insert some operations in between:

        require Math::BigFloat;
        my $x = Math::BigFloat->bone() + 4;             # load BigInt and Calc
        Math::BigFloat->import( lib => 'Pari' );        # load Pari, too
        $x = Math::BigFloat->bone()+4;                  # now use Pari

While this works, it loads Calc needlessly. But maybe you just wanted that?

BUGS

Please see the file BUGS in the CPAN distribution Math::BigInt for known bugs.

CAVEATS

stringify, bstr()

Both stringify and bstr() now drop the leading '+'. The old code would return '+1.23', the new returns '1.23'. See the documentation in the Math::BigInt manpage for reasoning and details.

bdiv

The following will probably not do what you expect:

        print $c->bdiv(123.456),"\n";

It prints both quotient and reminder since print works in list context. Also, bdiv() will modify $c, so be carefull. You probably want to use

        print $c / 123.456,"\n";
        print scalar $c->bdiv(123.456),"\n";  # or if you want to modify $c

instead.

Modifying and =

Beware of:

        $x = Math::BigFloat->new(5);
        $y = $x;

It will not do what you think, e.g. making a copy of $x. Instead it just makes a second reference to the same object and stores it in $y. Thus anything that modifies $x will modify $y (except overloaded math operators), and vice versa. See the Math::BigInt manpage for details and how to avoid that.

bpow

bpow() now modifies the first argument, unlike the old code which left it alone and only returned the result. This is to be consistent with badd() etc. The first will modify $x, the second one won't:

        print bpow($x,$i),"\n";         # modify $x
        print $x->bpow($i),"\n";        # ditto
        print $x ** $i,"\n";            # leave $x alone

precision() vs. accuracy()

A common pitfall is to use precision() when you want to round a result to a certain number of digits:

        use Math::BigFloat;

        Math::BigFloat->precision(4);           # does not do what you think it does
        my $x = Math::BigFloat->new(12345);     # rounds $x to "12000"!
        print "$x\n";                           # print "12000"
        my $y = Math::BigFloat->new(3);         # rounds $y to "0"!
        print "$y\n";                           # print "0"
        $z = $x / $y;                           # 12000 / 0 => NaN!
        print "$z\n";
        print $z->precision(),"\n";             # 4

Replacing precision with the accuracy manpage is probably not what you want, either:

        use Math::BigFloat;

        Math::BigFloat->accuracy(4);            # enables global rounding:
        my $x = Math::BigFloat->new(123456);    # rounded immidiately to "12350"
        print "$x\n";                           # print "123500"
        my $y = Math::BigFloat->new(3);         # rounded to "3
        print "$y\n";                           # print "3"
        print $z = $x->copy()->bdiv($y),"\n";   # 41170
        print $z->accuracy(),"\n";              # 4

What you want to use instead is:

        use Math::BigFloat;

        my $x = Math::BigFloat->new(123456);    # no rounding
        print "$x\n";                           # print "123456"
        my $y = Math::BigFloat->new(3);         # no rounding
        print "$y\n";                           # print "3"
        print $z = $x->copy()->bdiv($y,4),"\n"; # 41150
        print $z->accuracy(),"\n";              # undef

In addition to computing what you expected, the last example also does not ``taint'' the result with an accuracy or precision setting, which would influence any further operation.

SEE ALSO

the Math::BigInt manpage, the Math::BigRat manpage and the Math::Big manpage as well as the Math::BigInt::BitVect manpage, the Math::BigInt::Pari manpage and the Math::BigInt::GMP manpage.

perldoc2tree.cgi: /usr/lib/perl5/5.8.8/Math/BigFloat.pm: cannot resolve L in paragraph 507. perldoc2tree.cgi: /usr/lib/perl5/5.8.8/Math/BigFloat.pm: cannot resolve L in paragraph 507. perldoc2tree.cgi: /usr/lib/perl5/5.8.8/Math/BigFloat.pm: cannot resolve L in paragraph 507.

The pragmas bignum, bigint and bigrat might also be of interest because they solve the autoupgrading/downgrading issue, at least partly.

The package at http://search.cpan.org/search contains more documentation including a full version history, testcases, empty subclass files and benchmarks.

LICENSE

This program is free software; you may redistribute it and/or modify it under the same terms as Perl itself.

AUTHORS

Mark Biggar, overloaded interface by Ilya Zakharevich. Completely rewritten by Tels http://bloodgate.com in 2001 - 2004, and still at it in 2005.