matrixの１行目と２行目の入れ替え

> b
[,1] [,2]
[1,] 12 17
[2,] 33 10

> b[2,]
[1] 33 10

> matrix(c(b[2,],b[1,]),ncol=2,byrow=T)
[,1] [,2]
[1,] 33 10
[2,] 12 17