Order a matrix by multiple column in r

Question

I have a matrix

df<-matrix(data=c(3,7,5,0,1,0,0,0,0,8,0,9), ncol=2) rownames(df)<-c("a","b","c","d","e","f")  [,1] [,2] a    3    0 b    7    0 c    5    0 d    0    8 e    1    0 f    0    9 

and I would like to order the matrix in descending order first by column 1 and then by column two resulting in the matrix

df.ordered<-matrix(data=c(7,5,3,1,0,0,0,0,0,0,9,8),ncol=2) rownames(df.ordered)<-c("b","c","a","e","f","d")     [,1] [,2]  b    7    0  c    5    0  a    3    0  e    1    0  f    0    9  d    0    8 

Any suggestions on how I could achieve this? Thanks.

Answer 1

The order function should do it.

df[order(df[,1],df[,2],decreasing=TRUE),] 

Answer 2

To complete the main answer, here is a way to do it programmatically, without having to specify the columns by hand:

set.seed(2013) # preparing my example mat <- matrix(sample.int(10,size = 30, replace = T), ncol = 3) mat       [,1] [,2] [,3]  [1,]    5    1    6  [2,]   10    3    1  [3,]    8    8    1  [4,]    8    9    9  [5,]    3    7    3  [6,]    8    8    5  [7,]   10   10    2  [8,]    8   10    7  [9,]   10    1    9 [10,]    9    4    5 

As a simple example, let say I want to use all the columns in their order of appearance to sort the rows of the matrix: (One could easily give a vector of indexes to the matrix)

mat[do.call(order, as.data.frame(mat)),]   #could be ..as.data.frame(mat[,index_vec])..       [,1] [,2] [,3]  [1,]    3    7    3  [2,]    5    1    6  [3,]    8    8    1  [4,]    8    8    5  [5,]    8    9    9  [6,]    8   10    7  [7,]    9    4    5  [8,]   10    1    9  [9,]   10    3    1 [10,]   10   10    2