Vector input and output

pdistR <- function(x, ys) {
  sqrt((x - ys) ^ 2)
}

cppFunction('NumericVector pdistC(double x, NumericVector ys) {
  int n = ys.size();
  NumericVector out(n);

  for(int i = 0; i < n; ++i) {
    out[i] = sqrt(pow(ys[i] - x, 2.0));
  }
  return out;
}')

Note: uses pow(), not ^, for exponentiation

y <- runif(1e6)
bench::mark(
  pdistR(0.5, y),
  pdistC(0.5, y)
)[1:6]
#> # A tibble: 2 × 6
#>   expression          min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>     <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 pdistR(0.5, y)   4.18ms   5.11ms      197.    7.63MB     54.9
#> 2 pdistC(0.5, y)   4.08ms   4.15ms      239.    7.63MB     61.4