Last active
April 12, 2024 07:35
-
-
Save danlewer/5a6759d341fb1f4422ffde28c4b1a3ad to your computer and use it in GitHub Desktop.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
# linear interpolation of lines between x and y coordinates, with option to find specific intercepts | |
# x must be positive monotonic, y does not need to be | |
interpolate <- function (x, y, xIncrements = 0.01, findx = NULL, findy = NULL) { | |
if (length(x) != length(y)) stop('length(x) does not equal length(y)') | |
if (is.unsorted(x)) stop('x is not positive monotonic') | |
dx <- diff(x) | |
dy <- diff(y) | |
xNotches <- round(dx / xIncrements, 0) | |
yIncrements <- dy / xNotches | |
yd <- rep(yIncrements, times = xNotches) | |
yfinal <- c(y[1], y[1] + cumsum(yd)) | |
xIncrements2 <- dx / xIncrements | |
xd <- unlist(lapply(xNotches, function (x) rep(xIncrements, times = x))) | |
xfinal <- c(x[1], x[1] + cumsum(xd)) | |
findxY <- if (!is.null(findx)) yfinal[which.min(abs(findx - xfinal))] | |
findyX <- if (!is.null(findy)) xfinal[which.min(abs(findy - yfinal))] | |
return (list( | |
x = xfinal, | |
y = yfinal, | |
xPoint = findyX, | |
yPoint = findxY | |
)) | |
} | |
# example with monotonic y, and find y where x = 10 | |
x <- c(0, 3, 5, 8, 11, 18, 20) | |
y <- c(0, 3, 7, 13, 16, 18, 20) | |
i <- interpolate(x = x, y = y, xIncrements = 0.001, findx = 10) | |
par(mfrow = c(1, 2)) | |
plot(x, y) | |
lines(i$x, i$y) | |
segments(x0 = 10, y0 = 0, y1 = i$yPoint, lty = 2) | |
segments(x0 = 0, y0 = i$yPoint, x1 = 10, lty = 2) | |
# example where y is not monotonic | |
y <- c(0, -1, -3, 2, 10, 12, 9) | |
i <- interpolate(x = x, y = y, xIncrements = 0.001, findx = 10) | |
plot(x, y) | |
abline(h = 0) | |
lines(i$x, i$y) | |
segments(x0 = 10, y0 = 0, y1 = i$yPoint, lty = 2) | |
segments(x0 = 0, y0 = i$yPoint, x1 = 10, lty = 2) |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment