How to convert a data frame to a 3d array in R

Question

I have a data frame that I want to convert to a three-dimensional array. One of the columns in the data frame should serve as the grouping variable for splitting the frame i

Answer 1

Maybe I'm reading the question wrong, but the MWE describes a 2x2x2 array (x, y, i (a.k.a. z)). The current answers appear to provide solutions that provide arrays of data.frames rather than arrays of 2D matrices (per OP). array() will convert a data.frame to an array of n-dimensional matrices:

dfa <- array(data = df$l, 
             dim=c(length(unique(df$x)), 
                   length(unique(df$y)), 
                   length(unique(df$i))), 
             dimnames=list(unique(df$x), unique(df$y), unique(df$i))
            )
dfa
> dfa
, , 1

           0          1
0 -0.5604756 1.55870831
1 -0.2301775 0.07050839

, , 2

          0          1
0 0.1292877  0.4609162
1 1.7150650 -1.2650612