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perlman:Math::Trig

by root (Scribe)
on Dec 23, 1999 at 01:30 UTC ( #1286=perlfunc: print w/ replies, xml ) Need Help??

Math::Trig

See the current Perl documentation for Math::Trig.

Here is our local, out-dated (pre-5.6) version:


Math::Trig - trigonometric functions



        use Math::Trig;
        
        $x = tan(0.9);
        $y = acos(3.7);
        $z = asin(2.4);
        
        $halfpi = pi/2;

        $rad = deg2rad(120);



Math::Trig defines many trigonometric functions not defined by the core Perl which defines only the sin and cos. The constant pi is also defined as are a few convenience functions for angle conversions.


TRIGONOMETRIC FUNCTIONS

The tangent

tan

The cofunctions of the sine, cosine, and tangent (cosec/csc and cotan/cot are aliases)

csc, cosec, sec, sec, cot, cotan

The arcus (also known as the inverse) functions of the sine, cosine, and tangent

asin, acos, atan

The principal value of the arc tangent of y/x

atan2(y, x)

The arcus cofunctions of the sine, cosine, and tangent (acosec/acsc and acotan/acot are aliases)

acsc, acosec, asec, acot, acotan

The hyperbolic sine, cosine, and tangent

sinh, cosh, tanh

The cofunctions of the hyperbolic sine, cosine, and tangent (cosech/csch and cotanh/coth are aliases)

csch, cosech, sech, coth, cotanh

The arcus (also known as the inverse) functions of the hyperbolic sine, cosine, and tangent

asinh, acosh, atanh

The arcus cofunctions of the hyperbolic sine, cosine, and tangent (acsch/acosech and acoth/acotanh are aliases)

acsch, acosech, asech, acoth, acotanh

The trigonometric constant pi is also defined.

$pi2 = 2 * pi;


ERRORS DUE TO DIVISION BY ZERO

The following functions

        acoth
        acsc
        acsch
        asec
        asech
        atanh
        cot
        coth
        csc
        csch
        sec
        sech
        tan
        tanh

cannot be computed for all arguments because that would mean dividing by zero or taking logarithm of zero. These situations cause fatal runtime errors looking like this

        cot(0): Division by zero.
        (Because in the definition of cot(0), the divisor sin(0) is 0)
        Died at ...

or

        atanh(-1): Logarithm of zero.
        Died at...

For the csc, cot, asec, acsc, acot, csch, coth, asech, acsch, the argument cannot be 0 (zero). For the atanh, acoth, the argument cannot be 1 (one). For the atanh, acoth, the argument cannot be -1 (minus one). For the tan, sec, tanh, sech, the argument cannot be pi/2 + k * pi, where k is any integer.


SIMPLE (REAL) ARGUMENTS, COMPLEX RESULTS

Please note that some of the trigonometric functions can break out from the real axis into the complex plane. For example asin(2) has no definition for plain real numbers but it has definition for complex numbers.

In Perl terms this means that supplying the usual Perl numbers (also known as scalars, please see perldata) as input for the trigonometric functions might produce as output results that no more are simple real numbers: instead they are complex numbers.

The Math::Trig handles this by using the Math::Complex package which knows how to handle complex numbers, please see Complex for more information. In practice you need not to worry about getting complex numbers as results because the Math::Complex takes care of details like for example how to display complex numbers. For example:

        print asin(2), "\n";
    
should produce something like this (take or leave few last decimals):

        1.5707963267949-1.31695789692482i

That is, a complex number with the real part of approximately 1.571 and the imaginary part of approximately -1.317.


PLANE ANGLE CONVERSIONS

(Plane, 2-dimensional) angles may be converted with the following functions.

        $radians  = deg2rad($degrees);
        $radians  = grad2rad($gradians);
        
        $degrees  = rad2deg($radians);
        $degrees  = grad2deg($gradians);
        
        $gradians = deg2grad($degrees);
        $gradians = rad2grad($radians);

The full circle is 2 pi radians or 360 degrees or 400 gradians.


RADIAL COORDINATE CONVERSIONS

Radial coordinate systems are the spherical and the cylindrical systems, explained shortly in more detail.

You can import radial coordinate conversion functions by using the :radial tag:

    use Math::Trig ':radial';

    ($rho, $theta, $z)     = cartesian_to_cylindrical($x, $y, $z);
    ($rho, $theta, $phi)   = cartesian_to_spherical($x, $y, $z);
    ($x, $y, $z)           = cylindrical_to_cartesian($rho, $theta, $z);
    ($rho_s, $theta, $phi) = cylindrical_to_spherical($rho_c, $theta, $z);
    ($x, $y, $z)           = spherical_to_cartesian($rho, $theta, $phi);
    ($rho_c, $theta, $z)   = spherical_to_cylindrical($rho_s, $theta, $phi);

All angles are in radians.


COORDINATE SYSTEMS

Cartesian coordinates are the usual rectangular (x, y, z)-coordinates.

Spherical coordinates, (rho, theta, pi), are three-dimensional coordinates which define a point in three-dimensional space. They are based on a sphere surface. The radius of the sphere is rho, also known as the radial coordinate. The angle in the xy-plane (around the z-axis) is theta, also known as the azimuthal coordinate. The angle from the z-axis is phi, also known as the polar coordinate. The `North Pole' is therefore 0, 0, rho, and the `Bay of Guinea' (think of the missing big chunk of Africa) 0, pi/2, rho.

Beware: some texts define theta and phi the other way round, some texts define the phi to start from the horizontal plane, some texts use r in place of rho.

Cylindrical coordinates, (rho, theta, z), are three-dimensional coordinates which define a point in three-dimensional space. They are based on a cylinder surface. The radius of the cylinder is rho, also known as the radial coordinate. The angle in the xy-plane (around the z-axis) is theta, also known as the azimuthal coordinate. The third coordinate is the z, pointing up from the theta-plane.


3-D ANGLE CONVERSIONS

Conversions to and from spherical and cylindrical coordinates are available. Please notice that the conversions are not necessarily reversible because of the equalities like pi angles being equal to -pi angles.

cartesian_to_cylindrical

        ($rho, $theta, $z) = cartesian_to_cylindrical($x, $y, $z);
cartesian_to_spherical

        ($rho, $theta, $phi) = cartesian_to_spherical($x, $y, $z);
cylindrical_to_cartesian

        ($x, $y, $z) = cylindrical_to_cartesian($rho, $theta, $z);
cylindrical_to_spherical

        ($rho_s, $theta, $phi) = cylindrical_to_spherical($rho_c, $theta, $z);

Notice that when $z is not 0 $rho_s is not equal to $rho_c.

spherical_to_cartesian

        ($x, $y, $z) = spherical_to_cartesian($rho, $theta, $phi);
spherical_to_cylindrical

        ($rho_c, $theta, $z) = spherical_to_cylindrical($rho_s, $theta, $phi);

Notice that when $z is not 0 $rho_c is not equal to $rho_s.


GREAT CIRCLE DISTANCES

You can compute spherical distances, called great circle distances, by importing the great_circle_distance function:

        use Math::Trig 'great_circle_distance'

    $distance = great_circle_distance($theta0, $phi0, $theta1, $phi, [, $rho]);

The great circle distance is the shortest distance between two points on a sphere. The distance is in $rho units. The $rho is optional, it defaults to 1 (the unit sphere), therefore the distance defaults to radians.


EXAMPLES

To calculate the distance between London (51.3N 0.5W) and Tokyo (35.7N 139.8E) in kilometers:

        use Math::Trig qw(great_circle_distance deg2rad);

        # Notice the 90 - latitude: phi zero is at the North Pole.
        @L = (deg2rad(-0.5), deg2rad(90 - 51.3));
        @T = (deg2rad(139.8),deg2rad(90 - 35.7));

        $km = great_circle_distance(@L, @T, 6378);

The answer may be off by up to 0.3% because of the irregular (slightly aspherical) form of the Earth.


BUGS

Saying use Math::Trig; exports many mathematical routines in the caller environment and even overrides some (sin, cos). This is construed as a feature by the Authors, actually... ;-)

The code is not optimized for speed, especially because we use Math::Complex and thus go quite near complex numbers while doing the computations even when the arguments are not. This, however, cannot be completely avoided if we want things like asin(2) to give an answer instead of giving a fatal runtime error.


AUTHORS

Jarkko Hietaniemi <jhi@iki.fi> and Raphael Manfredi <Raphael_Manfredi@grenoble.hp.com>.


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