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

Here is what I'd probably do:

library(abind)
abind(split(df, df$i), along=3)
# , , 1
# 
#   i x y           l
# 5 1 0 0 -0.56047565
# 6 1 0 1 -0.23017749
# 7 1 1 0  1.55870831
# 8 1 1 1  0.07050839
# 
# , , 2
# 
#   i x y          l
# 5 2 0 0  0.1292877
# 6 2 0 1  1.7150650
# 7 2 1 0  0.4609162
# 8 2 1 1 -1.2650612

Answer 2

It sounds like you are looking for split:

> split(df, df$i)
$`1`
  i x y           l
1 1 0 0 -0.56047565
2 1 0 1 -0.23017749
3 1 1 0  1.55870831
4 1 1 1  0.07050839

$`2`
  i x y          l
5 2 0 0  0.1292877
6 2 0 1  1.7150650
7 2 1 0  0.4609162
8 2 1 1 -1.2650612

This results in a list of two data.frames separated by your "i" column.

---

To get an array, you've got Josh's answer, or you can use simplify2array from base R:

> simplify2array(by(df, df$i, as.matrix))
, , 1

  i x y           l
1 1 0 0 -0.56047565
2 1 0 1 -0.23017749
3 1 1 0  1.55870831
4 1 1 1  0.07050839

, , 2

  i x y          l
1 2 0 0  0.1292877
2 2 0 1  1.7150650
3 2 1 0  0.4609162
4 2 1 1 -1.2650612

Answer 3

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