How to select columns in a matrix of variable size?

I have a matrix that I am trying to split into two separate matrices. The original matrix is composed of an unknown number of columns but will always be divisible by 3. I am trying to create a new matrix composed of the 1st column of the original matrix and then every 3rd column after that. E.g. Columns 1,4,7,10.....

I am also trying to create a new matrix that is composed of all of the columns not counted in creating the other new matrix. E.g. Columns 2,3,5,6,8,9.....

How would I go about doing this?

Answer 1

[Azzi Abdelmalek]

m=size(A,2)
B=A(:,1:3:m)
C=A(:,setdiff(1:m,1:3:m))