How to get frequencies then add it as a variable in an array?

Question

Let\'s say I have an array of this format

X  Y  Z
A  1  0
A  2  1
B  1  1
B  2  1
B  1  0

I want to find the frequency of X and the frequen

Answer 1

Here's a data.table way:

require(data.table)
DT <- data.table(dat)
DT[,nx:=.N,by=X][,nxy:=.N,by=list(X,Y)]

That last step created the two columns:

DT
#    X Y Z nx nxy
# 1: A 1 0  2   1
# 2: A 2 1  2   1
# 3: B 1 1  3   2
# 4: B 2 1  3   1
# 5: B 1 0  3   2

And it could have been written in two lines instead of one:

DT[,nx:=.N,by=X]
DT[,nxy:=.N,by=list(X,Y)]