Taking Logarithms
Problem
You want to take a logarithm in various bases.
Solution
For logarithms to base e, use the built-in
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);
}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 defined as above
$answer = log_base(10, 10_000);
print "log10(10,000) = $answer\n";
log10(10,000) = 4The
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) = 10.000000even though no complex number is involved here. This is not very
efficient, but there are plans to rewrite
Math::Complex in C for speed.
See Also
The log function in perlfunc
(1) and Chapter 3 of Programming Perl; the
documentation for the standard POSIX module (also in Chapter 7 of
Programming Perl)