For loop over a function to get function output in R

Question

I want to compute the integral of a function associated with each value in a vector. Using an example, I precisely describe my aim as follows.

Suppose x and y are independent random variables, and I is an indicator variables such that I=1 if y > 0.25x or zero otherwise. I want to make n draws of y from an exponential distribution, and for each draw I wish to compute the integral of g = xf(x)I over the distribution of x, f(x). I want this distribution to be exponential with parameter value of 1. I tried the following code in r.

f = function(n){
  h = list()
  g = numeric()
  y = rexp(n)
  for(i in 1:n){
    h[i] = function(x){(x * dexp(x) * (y[i] > 0.5*x))}
    g[i] = integrate(h[i], lower=-Inf, upper=Inf)
  }
  return(summary(g))
}

But then when I run f(3) I get an error message "*Error in h[i] = function(x) { : cannot coerce type 'closure' to vector of type 'list'"*. But when I run the following codes

y = rexp(1)
h = function(x){x * dexp(x) * (y > 0.5*x)}
integrate(h, lower=-Inf, upper=Inf)
f(3)

I get results. Does somebody has suggestion on how to improve this code so that h function is evaluated at each values of y with the output being a function, which then will be passed to the g function?

Answer 1

I would suggest to use sapply instead of the loop:

f = function(n){
  y = rexp(n)
  g = sapply(1:n, function(i) 
                  integrate(function(x) (x * dexp(x) * (y[i] > 0.5*x)), lower=-Inf, upper=Inf))
  return(summary(g))
}