问题
I'm attempting to calculate the area between two plots using R's integrate function. I have two forecast curves that I'm able to place on the same plot and also shade in the area between the two curves, for visualisation:
x1 = seq(1,200,1)
y1 = # 200 numbers from 0.02 - 1.000
y2 = # 200 numbers from 0.00 - 0.95
plot(x1,y1,type="l",bty="L",xlab="Forecast Days",ylab="Curve Actuals")
points(x1,y2,type="l",col="red")
polygon(c(x1,rev(x1)),c(y2,rev(y1)),col="skyblue")
Following the example here https://stat.ethz.ch/pipermail/r-help/2010-September/251756.html I've attempted to execute the same code for my data to calculate the are between the two curves. As stated in the example "the area between two curves is the same as the integral of the difference between these two curves (resp. its absolute values)":
f1 = approxfun(x1, y1-y2) # piecewise linear function
f2 = function(x) abs(f1(x)) # take the positive value
integrate(f2, 1, 200)
However I get the following error:
Error in integrate(f2, 1, 200) : maximum number of subdivisions reached
Appreciate any clarity that can be shed on this. Thanks!
回答1:
Increasing the number of subdivisions, as suggested in an earlier comment by @Roland, executes correctly. It does produce an absolute error, but very minute.
f1 = approxfun(x1, y1-y2) # piecewise linear function
f2 = function(x) abs(f1(x)) # take the positive value
area1 = integrate(f2, 1, 200, subdivisions = 300)
> area1
9.327692 with absolute error < 8.1e-05
回答2:
Is approxfun() returning a function? You need to have the difference of f1 and f2 saved as a function, not a set a point differences. For example
f1 <- function(x) { 2 * x - 1}
f2 <- function(x) { x^2 - 3 * x}
abs_dif <- function(x) { abs( f1(x) - f2(x) ) }
integrate(abs_dif, -1, 1)
Try expressing the f1 and f2 as functions then try the second two lines of code.
来源：https://stackoverflow.com/questions/40083837/error-calculating-the-area-between-two-lines-using-integrate