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periodicSpline: Create a Periodic Interpolation Spline

periodicSpline

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 = 4L)

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

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)
pispl
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" )