Rotate a Matrix in R by 90 degrees clockwise

Question

I have a matrix in R like this:

|1|2|3| |1|2|3| |1|2|3| 

Is there an easy way to rotate the entire matrix by 90 degrees clockwise to get these results?

|1|1|1| |2|2|2| |3|3|3| 

and again rotating 90 degrees:

|3|2|1| |3|2|1| |3|2|1| 

?

Answer 1

t does not rotate the entries, it flips along the diagonal:

x <- matrix(1:9, 3) x ##      [,1] [,2] [,3] ## [1,]    1    4    7 ## [2,]    2    5    8 ## [3,]    3    6    9  t(x) ##      [,1] [,2] [,3] ## [1,]    1    2    3 ## [2,]    4    5    6 ## [3,]    7    8    9 

90 degree clockwise rotation of R matrix:

You need to also reverse the columns prior to the transpose:

rotate <- function(x) t(apply(x, 2, rev)) rotate(x) ##      [,1] [,2] [,3] ## [1,]    3    2    1 ## [2,]    6    5    4 ## [3,]    9    8    7  rotate(rotate(x)) ##      [,1] [,2] [,3] ## [1,]    9    6    3 ## [2,]    8    5    2 ## [3,]    7    4    1  rotate(rotate(rotate(x))) ##      [,1] [,2] [,3] ## [1,]    7    8    9 ## [2,]    4    5    6 ## [3,]    1    2    3  rotate(rotate(rotate(rotate(x)))) ##      [,1] [,2] [,3] ## [1,]    1    4    7 ## [2,]    2    5    8 ## [3,]    3    6    9 

90 degree counter clockwise rotation of R matrix:

Doing the transpose prior to the reverse is the same as rotate counter clockwise:

foo = matrix(1:9, 3) foo ## [,1] [,2] [,3] ## [1,]    1    4    7 ## [2,]    2    5    8 ## [3,]    3    6    9  foo <- apply(t(foo),2,rev) foo  ## [,1] [,2] [,3] ## [1,]    7    8    9 ## [2,]    4    5    6 ## [3,]    1    2    3 

Answer 2

m <- matrix(rep(1:3,each=3),3)       [,1] [,2] [,3] [1,]    1    2    3 [2,]    1    2    3 [3,]    1    2    3  t(m[nrow(m):1,])       [,1] [,2] [,3] [1,]    1    1    1 [2,]    2    2    2 [3,]    3    3    3  m[nrow(m):1,ncol(m):1]       [,1] [,2] [,3] [1,]    3    2    1 [2,]    3    2    1 [3,]    3    2    1  t(m)[ncol(m):1,]       [,1] [,2] [,3] [1,]    3    3    3 [2,]    2    2    2 [3,]    1    1    1