pchip | R Documentation |
Piecewise Cubic Hermitean Interpolation Polynomials.
pchip(xi, yi, x)
pchipfun(xi, yi)
xi , yi |
x- and y-coordinates of supporting nodes. |
x |
x-coordinates of interpolation points. |
pchip
is a ‘shape-preserving’ piecewise cubic Hermite polynomial
approach that apptempts to determine slopes such that function values do
not overshoot data values.
pchipfun
is a wrapper around pchip
and returns a function.
Both pchip
and the function returned by pchipfun
are vectorized.
xi
and yi
must be vectors of the same length greater or equal 3
(for cubic interpolation to be possible), and xi
must be sorted.
pchip
can be applied to points outside [min(xi), max(xi)]
, but
the result does not make much sense outside this interval.
Values of interpolated data at points x
.
Copyright of the Matlab version from Cleve Moler in his book “Numerical Computing with Matlab”, Chapter 3 on Interpolation. R Version by Hans W. Borchers, 2011.
Moler, C. (2004). Numerical Computing with Matlab. Revised Reprint, SIAM.
interp1
x <- c(1, 2, 3, 4, 5, 6)
y <- c(16, 18, 21, 17, 15, 12)
pchip(x, y, seq(1, 6, by = 0.5))
fp <- pchipfun(x, y)
fp(seq(1, 6, by = 0.5))
## Not run:
plot(x, y, col="red", xlim=c(0,7), ylim=c(10,22),
main = "Spline and 'pchip' Interpolation")
grid()
xs <- seq(1, 6, len=51)
ys <- interp1(x, y, xs, "spline")
lines(xs, ys, col="cyan")
yp <- pchip(x, y, xs)
lines(xs, yp, col = "magenta")
## End(Not run)
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