Generate random numbers with fixed mean and sd

Question

When generating random numbers in R using rnorm (or runif etc.), they seldom have the exact mean and SD as the distribution they are sampled from.

Answer 1

The mvrnorm() function in the MASS package can do this.

library(MASS)
#empirical=T forces mean and sd to be exact
x <- mvrnorm(n=20, mu=5, Sigma=10^2, empirical=T)
mean(x)
sd(x)
#empirical=F does not impose this constraint
x <- mvrnorm(n=20, mu=5, Sigma=10^2, empirical=F
mean(x)
sd(x)