Check how many times smooth spline intersects with x-axis

Question

I want to check how many times my smoothed spline intersects with the x-axis. Is there an elegant way to do this?

Example: (1 intersection in this case)

]1)

Answer 1

Check the number of times y values go from positive to negative

set.seed(1571933401)
x = 1:100
y = rnorm(100)
sp = smooth.spline(x, y)

with(sp, sum((sign(c(0, y)) * sign(c(y, 0))) == -1))
#6

graphics.off()
plot(sp, type = "l")
abline(h = 0, lty = 2)