R: calculate variance for data$V1 for each different value in data$V2

Question

I have data frame looking like this

V1   V2
..   1
..   2
..   1
..   3

etc.

For each distinct V2 value i would like to calculate v

Answer 1

There are a few ways to do this, I prefer:

dat <- data.frame(V1 = rnorm(50), V2=rep(1:5,10))
dat

aggregate (V1~V2, data=dat, var) # The first argument tells it to group V1 based on the values in V2, the last argument simply tells it the function to apply.

> aggregate (V1~V2, data=dat, var)
  V2        V1
1  1 0.9139360
2  2 1.6222236
3  3 1.2429743
4  4 1.1889356
5  5 0.7000294

Also look into ddply, daply etc in the plyr package.