Question

I am currently using this implementation:

getVal = function(i, x, margin) {
    rst = ifelse(margin==1, x[i, ], x[, i])
}

i.e. return the i-th row OR column of x, depending on the value of margin.

=== update ===

Just realized my usage of ifelse(x,y,z) statement here is wrong, as it returns a value the same length as its first argument. My implementation of getVal should have:

...
rst = if (margin == 1) x[i, ] else x[, i]
...

Answer 1

abind::asub() does something very much like what you’re trying to do (and also generalizes nicely to higher-dimensional arrays). Its idx and dims arguments correspond, respectively, to your i and margin arguments.

library(abind)

(m <- matrix(1:6, ncol=2))
#      [,1] [,2]
# [1,]    1    4
# [2,]    2    5
# [3,]    3    6

asub(x = m, idx = 2, dims = 1)   # Extract 2nd row
# [1] 2 5

asub(x = m, idx = 2, dims = 2)   # Extract 2nd column
# [1] 4 5 6