Extract numerics from matrix A with logical matrix B

Question

I have a matrix A containing numeric values and a matrix B containing 0/1s:

A <- matrix(1:25, 5, 5)
B <- matrix(c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0), 5, 5)
A B
1 6 11 16 21 0 0 1 0 0
2 7 12 17 22 0 0 0 0 1
3 8 13 18 23 0 0 0 0 1
4 9 14 19 24 0 0 1 0 0
5 10 15 20 25 0 0 1 0 0

I want to extract the elements of A where B==1 in row order. The desired result is:

11 22 23 14 15

I've come up with two possible ways to do this:

#1
rowSums(A*B)
#2
t(A)[as.logical(t(B))]

It seems like there should be a better (more elegant or faster) way to do this...

• Mast: Did you profile both?
• DanY: @Mast -- Not yet. In my application, my matrices are fairly small, but they occur in a loop that is called many times. I just figured that this would be a fairly common matrix operation, but I couldn't find "direct" way to implement it.
• DanY: @Amstel made a point (that is now deleted), but maybe the "solution" is to store information as t(A) and t(B) instead of as A and B so that A[B] will work.
• flodel: If how you store your information is up for changes/review, I'd recommend you replace the B matrix by a vector of indices, here b = c(3, 5, 5, 3, 3). (I am basing my assumption that there is only one 1 per row from your rowSums(A*B) suggestion). Storing the info in a vector is the most memory efficient. Then you could do A[cbind(seq_along(b), b)]. Otherwise I find t(A)[as.logical(t(B))] elegant enough and it is probably very fast.

Answer 1

While rowSums(A*B) looks pretty nice for me, you can also try

A[which(t(B) == 1, arr.ind = T)[,2:1]]

Answer 2

Although I don't have any substantial improvements, this is another way:

t(A)[t(B) == 1]