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 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);
}```

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
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.

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)

 Chapter 1. Strings
 Chapter 3. Dates and Times
 Chapter 4. Arrays
 Chapter 5. Hashes
 Chapter 6. Pattern Matching
 Chapter 7. File Access
 Chapter 8. File Contents
 Chapter 9. Directories
 Chapter 10. Subroutines
 Chapter 11. References and Records
 Chapter 12. Packages, Libraries, and Modules
 Chapter 13. Classes, Objects, and Ties
 Chapter 14. Database Access
 Chapter 15. Interactivity
 Chapter 16. Process Management and Communication
 Chapter 17. Sockets
 Chapter 18. Internet Services
 Chapter 19. CGI Programming
 Chapter 20. Web Automation
 Chapter 21. mod_perl
 Chapter 22. XML