[R] sort rows of matrix by rows of another matrix

[Timothy W. Hilton]

Hello all,

I am trying to sort rows of one matrix by rows of another.  Given a1 and a2:

------
> a1
     [,1] [,2] [,3]
[1,]    7    6    8
[2,]    4    2    4
[3,]    4    7    2
[4,]    0    3    8

a1 <-
structure(c(7, 4, 4, 0, 6, 2, 7, 3, 8, 4, 2, 8), .Dim = c(4L, 3L))

> a2
     [,1] [,2] [,3]
[1,]  101  102  103
[2,]  101  102  103
[3,]  101  102  103
[4,]  101  102  103

a2 <-
structure(c(101L, 101L, 101L, 101L, 102L, 102L, 102L, 102L, 103L, 
103L, 103L, 103L), .Dim = c(4L, 3L))
------

I want to get a3:

> a3
     [,1] [,2] [,3]
[1,]  102  101  103
[2,]  102  101  103
[3,]  103  101  102
[4,]  101  102  103

where the rows of a3 are the rows of a2 sorted according to the rows of a1.

I can get the necessary sorting index:
> apply(a1, 1, order)
     [,1] [,2] [,3] [,4]
[1,]    2    2    3    1
[2,]    1    1    1    2
[3,]    3    3    2    3

and I can get the rows of a1 sorted according to the rows of a1:
> t(apply(a1, 1, function(x) x[order(x)]))
     [,1] [,2] [,3]
[1,]    6    7    8
[2,]    2    4    4
[3,]    2    4    7
[4,]    0    3    8

but I can't get the rows of a2 sorted according to the rows of a1...

Thanks,
Tim