Why is my double integral is R not working

问题

I folloed this post here double integral in R and switched the function and limits to match below but it's not working.

InnerFunc = function(x) { x + (y^2) }
InnerIntegral = function(z) { sapply(y, 
        function(z) { integrate(InnerFunc, 0, 2)$value }) }
integrate(InnerIntegral, 0, 1)

I get this error:

Error in integrate(InnerFunc, 0, 2) : evaluation of function gave a result of wrong type

回答1:

Your variables are all out of wack. This should do what you want

InnerFunc <- function(x, y) { x + (y^2) }
InnerIntegral <- function(y) { sapply(y, 
  function(z) { integrate(InnerFunc, 0, 2, y=z)$value }) }
integrate(InnerIntegral, 0, 1)
# 2.666667 with absolute error < 3e-14

Notice how InnerFunc is now a function of both x and y. Also notice how we apply over the vector passed to InnerIntegral via the y parameter (rather than ignoring the z value passed in). We also pass in the current value of y to InnerFunc.

Although you typed

InnerFunc <- function(x, y) { x + (y^2) }

the math you drew looks should really result in

InnerFunc <- function(x, y) { x * (y^2) }

so I'm not sure what you were really after.

来源：https://stackoverflow.com/questions/45471499/why-is-my-double-integral-is-r-not-working