Indexing a matrix by a vector [duplicate]

Question

m = matrix(1:10, nrow = 5, ncol = 2)
y = c(1,2,2,1,1)

I want a vector v whose ith element is m[i,y[i]].

I thought m[,y] would do it, but that's clearly wrong.

Answer 1

You can use cbind() to create a matrix to be used for indexing.

m[cbind(seq_along(y), y)]
# [1] 1 7 8 4 5