Recipe 2.12 Taking Logarithms
2.12.1 Problem
You
want to take a logarithm in various bases.
2.12.2 Solution
For logarithms to base e, use the builtin
log
:
$log_e = log(VALUE);
For logarithms to base 10, use the POSIX module's
log10
function:
use POSIX qw(log10);
$log_10 = log10(VALUE);
For other bases, use the mathematical identity:
where x is the number whose logarithm you want,
n is the desired base, and
e is the natural logarithm base.
sub log_base {
my ($base, $value) = @_;
return log($value)/log($base);
}
2.12.3 Discussion
The
log_base function lets you take logarithms to any
base. If you know the base you'll want in advance, it's more
efficient to cache the log of the base instead of recalculating it
every time.
# log_base as defined earlier
$answer = log_base(10, 10_000);
print "log10(10,000) = $answer\n";
log10(10,000) = 4
The Math::Complex module does the caching for you via its
logn( ) routine, so you can
write:
use Math::Complex;
printf "log2(1024) = %lf\n", logn(1024, 2); # watch out for argument order!
log2(1024) = 10.000000
even though no complex number is involved here. This is not very
efficient, but there are plans to rewrite Math::Complex in C for
speed.
2.12.4 See Also
The log function in
perlfunc(1) and Chapter 29 of
Programming Perl; the documentation for the
standard POSIX and Math::Complex modules (also in Chapter 32 of
Programming Perl)
