Vector Input, Scalar output:

sumR <- function(x) {
  total <- 0
  for (i in seq_along(x)) {
    total <- total + x[i]
  }
  total
}

x<- runif(100)
sumR(x)
#> [1] 51.28519

Translation:

cppFunction('double sumC(NumericVector x) {
  int n = x.size();
  double total = 0;
  for(int i = 0; i < n; ++i) {
    total += x[i];
  }
  return total;
}')

Some observations:

• vector indices start at 0
• The for statement has a different syntax: for(init; check; increment)
• Methods are called with .
• total += x[i] is equivalent to total = total + x[i].
• other in-place operators are -=, *=, and /=

To check for the fastest way we can use:

?bench::mark

x <- runif(1e3)
bench::mark(
  sum(x),
  sumC(x),
  sumR(x)
)
#> # A tibble: 3 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 sum(x)       2.04µs   2.07µs   472511.        0B        0
#> 2 sumC(x)      4.46µs   4.52µs   211768.        0B        0
#> 3 sumR(x)     20.27µs  20.59µs    48112.        0B        0