Question

I have the following matrix

2    4    1
6    32   1
4    2    1
5    3    2
4    2    2

I want to make the following two matrices based on 3rd column

first

2    4
6    32
4    2

second

5    3
4    2

Best I can come up with, but I get an error

x <- cbind(mat[,1], mat[,2]) if mat[,3]=1

y <- cbind(mat[,1], mat[,2]) if mat[,3]=2

Answer 1

If you have a matrix A, this will get the first two columns when the third column is 1:

A[A[,3] == 1,c(1,2)]

You can use this to obtain matrices for any value in the third column.

Explanation: A[,3] == 1 returns a vector of booleans, where the i-th position is TRUE if A[i,3] is 1. This vector of booleans can be used to index into a matrix to extract the rows we want.

Disclaimer: I have very little experience with R, this is the MATLAB-ish way to do it.