periodicSpline            package:splines            R Documentation

Create a Periodic Interpolation Spline

Description:

     Create a periodic interpolation spline, either from 'x' and 'y'
     vectors, or from a formula/data.frame combination.

Usage:

     periodicSpline(obj1, obj2, knots, period = 2*pi, ord = 4)

Arguments:

    obj1: either a numeric vector of 'x' values or a formula.

    obj2: if 'obj1' is numeric this should be a numeric vector of the
          same length.  If 'obj1' is a formula this can be an optional
          data frame in which to evaluate the names in the formula.

   knots: optional numeric vector of knot positions.

  period: positive numeric value giving the period for the periodic
          spline.  Defaults to '2 * pi'.

     ord: integer giving the order of the spline, at least 2.  Defaults
          to 4.  See 'splineOrder' for a definition of the order of a
          spline.

Value:

     An object that inherits from class 'spline'.  The object can be in
     the B-spline representation, in which case it will be a 'pbSpline'
     object, or in the piecewise polynomial representation (a
     'ppolySpline' object).

Author(s):

     Douglas Bates and Bill Venables

See Also:

     'splineKnots', 'interpSpline'

Examples:

     require(graphics); require(stats)
     xx <- seq( -pi, pi, length.out = 16 )[-1]
     yy <- sin( xx )
     frm <- data.frame( xx, yy )
     ( pispl <- periodicSpline( xx, yy, period = 2 * pi ) )
     pispl2 <- periodicSpline( yy ~ xx, frm, period = 2 * pi )
     stopifnot(all.equal(pispl, pispl2))# pispl and pispl2 are the same

     plot( pispl )          # displays over one period
     points( yy ~ xx, col = "brown")
     plot( predict( pispl, seq(-3*pi, 3*pi, length.out = 101) ), type = "l" )

