首先,让我复制中提出的线性插值的稳定解决方案我之前的回答 https://stackoverflow.com/a/52650890/4891738.
## given (x, y) data, find x where the linear interpolation crosses y = y0
## the default value y0 = 0 implies root finding
## since linear interpolation is just a linear spline interpolation
## the function is named RootSpline1
RootSpline1 <- function (x, y, y0 = 0, verbose = TRUE) {
if (is.unsorted(x)) {
ind <- order(x)
x <- x[ind]; y <- y[ind]
}
z <- y - y0
## which piecewise linear segment crosses zero?
k <- which(z[-1] * z[-length(z)] <= 0)
## analytical root finding
xr <- x[k] - z[k] * (x[k + 1] - x[k]) / (z[k + 1] - z[k])
## make a plot?
if (verbose) {
plot(x, y, "l"); abline(h = y0, lty = 2)
points(xr, rep.int(y0, length(xr)))
}
## return roots
xr
}
对于由返回的三次插值样条stats::splinefun
有方法"fmm"
, "natrual"
, "periodic"
and "hyman"
,以下函数提供稳定的数值解。
RootSpline3 <- function (f, y0 = 0, verbose = TRUE) {
## extract piecewise construction info
info <- environment(f)$z
n_pieces <- info$n - 1L
x <- info$x; y <- info$y
b <- info$b; c <- info$c; d <- info$d
## list of roots on each piece
xr <- vector("list", n_pieces)
## loop through pieces
i <- 1L
while (i <= n_pieces) {
## complex roots
croots <- polyroot(c(y[i] - y0, b[i], c[i], d[i]))
## real roots (be careful when testing 0 for floating point numbers)
rroots <- Re(croots)[round(Im(croots), 10) == 0]
## the parametrization is for (x - x[i]), so need to shift the roots
rroots <- rroots + x[i]
## real roots in (x[i], x[i + 1])
xr[[i]] <- rroots[(rroots >= x[i]) & (rroots <= x[i + 1])]
## next piece
i <- i + 1L
}
## collapse list to atomic vector
xr <- unlist(xr)
## make a plot?
if (verbose) {
curve(f, from = x[1], to = x[n_pieces + 1], xlab = "x", ylab = "f(x)")
abline(h = y0, lty = 2)
points(xr, rep.int(y0, length(xr)))
}
## return roots
xr
}
It uses polyroot
分段,首先找到所有根complex字段,然后只保留real分段区间上的那些。这是可行的,因为三次插值样条只是许多分段三次多项式。我的回答在如何在R中保存和加载样条插值函数? https://stackoverflow.com/q/51540182/4891738已经展示了如何获得分段多项式系数,因此使用polyroot
很简单。
使用问题中的示例数据,两者RootSpline1
and RootSpline3
正确识别所有根。
par(mfrow = c(1, 2))
RootSpline1(x, y, 2.85)
#[1] 3.495375 6.606465
RootSpline3(f3, 2.85)
#[1] 3.924512 6.435812 9.207171 9.886640