How to get a vector by indexing a matrix with pair of vectors?

Question

Given two vectors that are the components of a list of indices, how do I index a matrix by them pairwise? If I just use them as is, I get the cartesian product (as is consistent with fancy indexing in other languages):

> x
           [,1]      [,2]        [,3]
[1,]  0.4636395 0.7937225  0.81670694
[2,] -1.0591770 0.7515119  1.31597748
[3,] -0.5215888 1.2026196 -0.03518676
> x[c(1,2), c(1, 1)]
           [,1]       [,2]
[1,]  0.4636395  0.4636395
[2,] -1.0591770 -1.0591770

while I really want

> c(x[1, 1], x[2, 1])
[1]  0.4636395 -1.0591770

In Julia, I can write rather concisely:

julia> x[CartesianIndex.([1, 2], [1, 1])] # not the same x here, but still 3x3
2-element Array{Float64,1}:
 0.5815007881748451
 0.6563715338911154

In Matlab, I have been told that the equivalent of this is to convert manually to a linear index:

x(sub2ind(A, [1, 2], [1, 1]))

How do I do that in R?

(This is actually a different question for this answer, but I really could not find the solution easily by searching. Especially no duplicate, even though I imagine it must be a common question. That "R" is not a well-googleable keyword didn't help.)

Answer 1

"Indexing" is usually called "subsetting" in R. And you have to subset by a matrix in this case. Which can be constructed most easily by cbinding the vectors together:

> cbind(c(1,2), c(1, 1))
     [,1] [,2]
[1,]    1    1
[2,]    2    1
> x[cbind(c(1,2), c(1, 1))]
[1]  0.4636395 -1.0591770