R - fastest way to select the rows of a matrix that satisfy multiple conditions

Question

This is an extension to the question on returning the rows of a matrix that meet a condition in R. Say I have the matrix:

       one two three four
 [1,]   1   6    11   16
 [2,]   2   7    12   17
 [3,]   3   8    11   18
 [4,]   4   9    11   19
 [5,]   5  10    15   20
 [6,]   1   6    15   20
 [7,]   5   7    12   20

I want to return all rows, where matrix$two == 7 AND matrix$three == 12 as fast as possible. This is the way I know to do it:

 out <- mat[mat$two == 7,]
 final_out <- out[out$three == 12, ]

There should obviously be a method to get the contents of final_out in a one-liner, something like: final_out <- which(mat$two == 7 && mat$three == 12) that is faster and more succinct than the two line of codes above.

What is the fastest R code to return this multiple condition matrix query?

Answer 1

Just use [ subsetting with logical comparison...

#  Reproducible data
set.seed(1)
m <- matrix( sample(12,28,repl=T) , 7 , 4 )
     [,1] [,2] [,3] [,4]
[1,]    4    8   10    3
[2,]    5    8    6    8
[3,]    7    1    9    2
[4,]   11    3   12    4
[5,]    3    3    5    5
[6,]   11    9   10    1
[7,]   12    5   12    5


#  Subset according to condition
m[ m[,2] == 3 & m[,3] == 12 , ]
[1] 11  3 12  4