Rotate a Matrix in R by 90 degrees clockwise
An easy way to rotate a matrix by 180° is this:
m <- matrix(1:8,ncol=4)
# [,1] [,2] [,3] [,4]
# [1,] 1 3 5 7
# [2,] 2 4 6 8
rot <- function(x) "[<-"(x, , rev(x))
rot(m)
# [,1] [,2] [,3] [,4]
# [1,] 8 6 4 2
# [2,] 7 5 3 1
rot(rot(m))
# [,1] [,2] [,3] [,4]
# [1,] 1 3 5 7
# [2,] 2 4 6 8
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
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