Programming Perl, 4th Edition

atan2

atan2 Y, X

This function returns the principal value of the arc tangent of Y/X in the range -π to +π. A quick way to get an approximate value of π is to say:

$pi = atan2(1,1) * 4;

For the tangent operation, you may use the tan function from the Math::Trig or the POSIX modules, or just use the familiar relation:

sub tan { sin($_[0]) / cos($_[0]) }

If either or both arguments are 0, the return value is implementation defined; see your atan2(3) manpage for more information.