Extendable piecewise function for any number of knots/breaks - r

Question

I have the following collection of slopes, breaks, and intercepts:
slopes <- c(4, 2, 8, 4)
breaks <- c(0.0150, 0.030, 0.035)
intercepts <- c(0.0299, 0.0599, -0.1201, 0.0199)
They define the following lines:
# y = slopes[1] * x + intercepts[1]
# y = slopes[2] * x + intercepts[2]
# y = slopes[3] * x + intercepts[3]
# y = slopes[4] * x + intercepts[4]
Graphing the lines yields:
tibble(x = seq(0.0025, 0.06, 0.0025), y = x) %>%
ggplot(aes(x, y)) +
geom_point(alpha = 0) +
geom_abline(intercept = intercepts[1], slope = slopes[1], color = "red") +
geom_abline(intercept = intercepts[2], slope = slopes[2], color = "orange") +
geom_abline(intercept = intercepts[3], slope = slopes[3], color = "yellow") +
geom_abline(intercept = intercepts[4], slope = slopes[4], color = "green2") +
scale_y_continuous(limits = c(0, 1))
I'd like to create a piecewise function based on the lines and breaks/knots, like so (follow: red -> orange -> yellow -> green):
I could wrap a function over a couple of if/else statements to get what I want. But I'd like for the solution to be extendable for any number of breaks/knots (instead of just 3, in this example).
How might I accomplish this?

Answer 1

This should be fairly extensible:
piecewise <- function(x, slopes, intercepts, breaks) {
i = 1 + findInterval(x, breaks)
y = slopes[i] * x + intercepts[i]
return(y)
}
Note that I put the breaks argument last, since that seemed most natural to me.
It automatically implements the piecewise defined function for any number of pieces.
Example:
slopes <- c(4, 2, 8, 4)
intercepts <- c(0.0299, 0.0599, -0.1201, 0.0199)
breaks <- c(0.0150, 0.030, 0.035)
df <- tibble(x = seq(0.0025, 0.06, 0.0025)) %>%
mutate(y = piecewise(x, slopes, intercepts, breaks))
df %>%
ggplot(aes(x, y)) +
geom_line()