Why is mean() so slow?

Question

Everything is in the question! I just tried to do a bit of optimization, and nailing down the bottle necks, out of curiosity, I tried that:

t1 <- rnorm(10         


        

Answer 1

mean is slower than computing "by hand" for several reasons:

1. S3 Method dispatch
2. NA handling
3. Error correction

Points 1 and 2 have already been covered. Point 3 is discussed in What algorithm is R using to calculate mean?. Basically, mean makes 2 passes over the vector in order to correct for floating point errors. sum only makes 1 pass over the vector.

Notice that identical(sum(t1)/length(t1), mean(t1)) may be FALSE, due to these precision issues.

> set.seed(21); t1 <- rnorm(1e7,,21)
> identical(sum(t1)/length(t1), mean(t1))
[1] FALSE
> sum(t1)/length(t1) - mean(t1)
[1] 2.539201e-16