## check if a given integer is prime
isPrime = function(n) {
  if (n <= 3) {
    return (TRUE)
  }
  if (any((n %% 2:floor(sqrt(n))) == 0)) {
    return (FALSE)
  }
  return (TRUE)
}

## estimate mean only using observation with prime indices
estMeanPrimes = function (x) {
  n = length(x)
  ind = sapply(1:n, isPrime)
  return (mean(x[ind]))
}

print(estMeanPrimes(rnorm(100000)))