Quick look ups

//  is the format for adding comments in Rcpp

int n= b.size();    //finds the length of a vector b

int m=B.nrow();  //finds the number of rows in a matrix

double x=runif(1)(0);  //gets one random Uniform(0,1);  runif returns a vector so we just use element (0).

pow(), not ^, for exponentiation.

sqrt() and exp()  work

for a function that returns nothing use   void func_name(int a){  return; }