r - Elegant indexing up to end of vector/matrix

Question

Is it possible in R to say - I want all indices from position i to the end of vector/matrix? Say I want a submatrix from 3rd column onwards. I currently only know this way:

A = matrix(rep(1:8, each = 5), nrow = 5) # just generate some example matrix...

A[,3:ncol(A)] # get submatrix from 3rd column onwards

But do I really need to write ncol(A)? Isn't there any elegant way how to say "from the 3rd column onwards"? Something like A[,3:]? (or A[,3:...])?

Answer 1

Sometimes it's easier to tell R what you don't want. In other words, exclude columns from the matrix using negative indexing:

Here are two alternative ways that both produce the same results:

A[, -(1:2)]
A[, -seq_len(2)]

Results:

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    4    5    6    7    8
[2,]    3    4    5    6    7    8
[3,]    3    4    5    6    7    8
[4,]    3    4    5    6    7    8
[5,]    3    4    5    6    7    8

---

But to answer your question as asked: Use ncol to find the number of columns. (Similarly there is nrow to find the number of rows.)

A[, 3:ncol(A)]

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    4    5    6    7    8
[2,]    3    4    5    6    7    8
[3,]    3    4    5    6    7    8
[4,]    3    4    5    6    7    8
[5,]    3    4    5    6    7    8