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Math::Trig(3pm)        Perl Programmers Reference Guide        Math::Trig(3pm)




NAME

       Math::Trig - trigonometric functions


SYNOPSIS

           use Math::Trig;

           $x = tan(0.9);
           $y = acos(3.7);
           $z = asin(2.4);

           $halfpi = pi/2;

           $rad = deg2rad(120);

           # Import constants pi2, pip2, pip4 (2*pi, pi/2, pi/4).
           use Math::Trig ':pi';

           # Import the conversions between cartesian/spherical/cylindrical.
           use Math::Trig ':radial';

               # Import the great circle formulas.
           use Math::Trig ':great_circle';


DESCRIPTION

       "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, and great circle formulas for spherical movement.


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).  Note that atan2(0, 0) is not well-defined.

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

       asinh, acosh, atanh

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

       acsch, acosech, asech, acoth, acotanh

       The trigonometric constant pi and some of handy multiples of it are
       also defined.

       pi, pi2, pi4, pip2, pip4

   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.

       Note that atan2(0, 0) is not well-defined.

   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 Math::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.

       deg2rad
               $radians  = deg2rad($degrees);

       grad2rad
               $radians  = grad2rad($gradians);

       rad2deg
               $degrees  = rad2deg($radians);

       grad2deg
               $degrees  = grad2deg($gradians);

       deg2grad
               $gradians = deg2grad($degrees);

       rad2grad
               $gradians = rad2grad($radians);

       The full circle is 2 pi radians or 360 degrees or 400 gradians.  The
       result is by default wrapped to be inside the [0, {2pi,360,400}[
       circle.  If you don't want this, supply a true second argument:

           $zillions_of_radians  = deg2rad($zillions_of_degrees, 1);
           $negative_degrees     = rad2deg($negative_radians, 1);

       You can also do the wrapping explicitly by rad2rad(), deg2deg(), and
       grad2grad().

       rad2rad
               $radians_wrapped_by_2pi = rad2rad($radians);

       deg2deg
               $degrees_wrapped_by_360 = deg2deg($degrees);

       grad2grad
               $gradians_wrapped_by_400 = grad2grad($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 Gulf of Guinea (think of the
       missing big chunk of Africa) 0, pi/2, rho.  In geographical terms phi
       is latitude (northward positive, southward negative) and theta is
       longitude (eastward positive, westward negative).

       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 AND DIRECTIONS

       A great circle is section of a circle that contains the circle
       diameter: the shortest distance between two (non-antipodal) points on
       the spherical surface goes along the great circle connecting those two
       points.

   great_circle_distance
       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, $phi1, [, $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.

       If you think geographically the theta are longitudes: zero at the
       Greenwhich meridian, eastward positive, westward negative -- and the
       phi are latitudes: zero at the North Pole, northward positive,
       southward negative.  NOTE: this formula thinks in mathematics, not
       geographically: the phi zero is at the North Pole, not at the Equator
       on the west coast of Africa (Bay of Guinea).  You need to subtract your
       geographical coordinates from pi/2 (also known as 90 degrees).

         $distance = great_circle_distance($lon0, pi/2 - $lat0,
                                           $lon1, pi/2 - $lat1, $rho);

   great_circle_direction
       The direction you must follow the great circle (also known as bearing)
       can be computed by the great_circle_direction() function:

         use Math::Trig 'great_circle_direction';

         $direction = great_circle_direction($theta0, $phi0, $theta1, $phi1);

   great_circle_bearing
       Alias 'great_circle_bearing' for 'great_circle_direction' is also
       available.

         use Math::Trig 'great_circle_bearing';

         $direction = great_circle_bearing($theta0, $phi0, $theta1, $phi1);

       The result of great_circle_direction is in radians, zero indicating
       straight north, pi or -pi straight south, pi/2 straight west, and -pi/2
       straight east.

   great_circle_destination
       You can inversely compute the destination if you know the starting
       point, direction, and distance:

         use Math::Trig 'great_circle_destination';

         # $diro is the original direction,
         # for example from great_circle_bearing().
         # $distance is the angular distance in radians,
         # for example from great_circle_distance().
         # $thetad and $phid are the destination coordinates,
         # $dird is the final direction at the destination.

         ($thetad, $phid, $dird) =
           great_circle_destination($theta, $phi, $diro, $distance);

       or the midpoint if you know the end points:

   great_circle_midpoint
         use Math::Trig 'great_circle_midpoint';

         ($thetam, $phim) =
           great_circle_midpoint($theta0, $phi0, $theta1, $phi1);

       The great_circle_midpoint() is just a special case of

   great_circle_waypoint
         use Math::Trig 'great_circle_waypoint';

         ($thetai, $phii) =
           great_circle_waypoint($theta0, $phi0, $theta1, $phi1, $way);

       Where the $way is a value from zero ($theta0, $phi0) to one ($theta1,
       $phi1).  Note that antipodal points (where their distance is pi
       radians) do not have waypoints between them (they would have an an
       "equator" between them), and therefore "undef" is returned for
       antipodal points.  If the points are the same and the distance
       therefore zero and all waypoints therefore identical, the first point
       (either point) is returned.

       The thetas, phis, direction, and distance in the above are all in
       radians.

       You can import all the great circle formulas by

         use Math::Trig ':great_circle';

       Notice that the resulting directions might be somewhat surprising if
       you are looking at a flat worldmap: in such map projections the great
       circles quite often do not look like the shortest routes --  but for
       example the shortest possible routes from Europe or North America to
       Asia do often cross the polar regions.  (The common Mercator projection
       does not show great circles as straight lines: straight lines in the
       Mercator projection are lines of constant bearing.)


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.
           sub NESW { deg2rad($_[0]), deg2rad(90 - $_[1]) }
           my @L = NESW( -0.5, 51.3);
           my @T = NESW(139.8, 35.7);
           my $km = great_circle_distance(@L, @T, 6378); # About 9600 km.

       The direction you would have to go from London to Tokyo (in radians,
       straight north being zero, straight east being pi/2).

           use Math::Trig qw(great_circle_direction);

           my $rad = great_circle_direction(@L, @T); # About 0.547 or 0.174 pi.

       The midpoint between London and Tokyo being

           use Math::Trig qw(great_circle_midpoint);

           my @M = great_circle_midpoint(@L, @T);

       or about 69 N 89 E, in the frozen wastes of Siberia.

       NOTE: you cannot get from A to B like this:

          Dist = great_circle_distance(A, B)
          Dir  = great_circle_direction(A, B)
          C    = great_circle_destination(A, Dist, Dir)

       and expect C to be B, because the bearing constantly changes when going
       from A to B (except in some special case like the meridians or the
       circles of latitudes) and in great_circle_destination() one gives a
       constant bearing to follow.

   CAVEAT FOR GREAT CIRCLE FORMULAS
       The answers may be off by few percentages because of the irregular
       (slightly aspherical) form of the Earth.  The errors are at worst about
       0.55%, but generally below 0.3%.

   Real-valued asin and acos
       For small inputs asin() and acos() may return complex numbers even when
       real numbers would be enough and correct, this happens because of
       floating-point inaccuracies.  You can see these inaccuracies for
       example by trying theses:

         print cos(1e-6)**2+sin(1e-6)**2 - 1,"\n";
         printf "%.20f", cos(1e-6)**2+sin(1e-6)**2,"\n";

       which will print something like this

         -1.11022302462516e-16
         0.99999999999999988898

       even though the expected results are of course exactly zero and one.
       The formulas used to compute asin() and acos() are quite sensitive to
       this, and therefore they might accidentally slip into the complex plane
       even when they should not.  To counter this there are two interfaces
       that are guaranteed to return a real-valued output.

       asin_real
               use Math::Trig qw(asin_real);

               $real_angle = asin_real($input_sin);

           Return a real-valued arcus sine if the input is between [-1, 1],
           inclusive the endpoints.  For inputs greater than one, pi/2 is
           returned.  For inputs less than minus one, -pi/2 is returned.

       acos_real
               use Math::Trig qw(acos_real);

               $real_angle = acos_real($input_cos);

           Return a real-valued arcus cosine if the input is between [-1, 1],
           inclusive the endpoints.  For inputs greater than one, zero is
           returned.  For inputs less than minus one, pi is returned.


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.

       Do not attempt navigation using these formulas.

       Math::Complex(3)


AUTHORS

       Jarkko Hietaniemi <jhi!at!iki.fi>, Raphael Manfredi
       <Raphael_Manfredi!at!pobox.com>, Zefram <zefram@fysh.org>


LICENSE

       This library is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.



perl v5.24.0                      2016-02-05                   Math::Trig(3pm)

perl 5.24 - Generated Wed Nov 16 19:44:36 CST 2016
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