So we access it by saying STD, colon, colon, accumulate. The accumulate function is in the standard library. Here we will declare the resulting variable and define it at the same time. And we call fast sum two, that takes a numeric vector.

We create another function and we export it. C plus plus also offers an accumulate function where it can automatically iterate over a vector or an array and add up the numbers. Again, we source with control, shift, S or on a Mac, command, shift, S. We can now source this and see what happens. When the loop is done, return the result. We just have to keep adding in each element of X.

We aren't iterating of a result because result is just a number that accumulates everything. We do that by saying result, plus gets X sub I. Now we've looped through X and add each value to result. For, int, I equals zero, keep going as long as I is less than X dot size and at each iteration, add one to I. 'Cause what we will do is take this value of zero and successively add in each element of the vector X. And we will initialize it with a value of zero. This will be a double and we will call it result. The first thing we have to do is declare a variable to hold the result. We enclose the body of the function in curly braces just like in R. The one argument for this function will be a numeric vector named x. In reality, this probably won't be any faster than the base R sum function because that is written in either C or fortran and is already completely optimized. The name of the function will be fast sum since it should be fast since it's in C plus plus. In C plus plus we do that by returning a double. So we need to be able to handle a decimal result. When adding up a vector of numbers there could be integers there could be decimals.

We're going to create a new function and to ensure it's available in R, we need to export it, so slash, slash and then double square brackets, Rcpp, colon, colon, export. So we are going to take a look at some of those ways to add up a vector of numbers. With C plus plus and even more so with Rcpp, there is more than one way to do things.

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