library(Rcpp)

cppFunction('
NumericMatrix imembrandc(int n, int k) {
  NumericMatrix u( n , k );
  IntegerVector sequ = seq(1,100);
  NumericVector sampled;
  for (int i=0; i < k; ++i) {
    sampled = sample(sequ, n);
    u(_,i) = sampled / sum(sampled);
  }

  if (is_true(any(rowSums(u) != 1))) {
    u(_,1) = u(_,1) + (1 - rowSums(u));
  }

  return(u);
}')

res = imembrandc(n = 10, k = 5)
rowSums(res)

rowSums(res) == 1