Converting a data frame to a matrix with plyr daply

Question

I\'m trying to use the daply function in the plyr package but I cannot get it to output properly. Even though the variable that makes up the matrix is

Answer 1

If we take the OP at their word(s) in the title, then they may be looking for data.matrix() which is a standard function in the base package that is always available in R.

data.matrix() works by converting any factors to their numeric coding before converting the data frame to a matrix. Consider the following data frame:

dat <- data.frame(A = 1:10, B = factor(sample(c("X","Y"), 10, replace = TRUE)))

If we convert via as.matrix() we get a character matrix:

> head(as.matrix(dat))
     A    B  
[1,] " 1" "X"
[2,] " 2" "X"
[3,] " 3" "Y"
[4,] " 4" "Y"
[5,] " 5" "Y"
[6,] " 6" "Y"

or if via matrix() one gets a list with dimensions (a list array - as mentioned in the Value section of ?daply by the way)

> head(matrix(dat))
     [,1]      
[1,] Integer,10
[2,] factor,10 
> str(matrix(dat))
List of 2
 $ : int [1:10] 1 2 3 4 5 6 7 8 9 10
 $ : Factor w/ 2 levels "X","Y": 1 1 2 2 2 2 1 2 2 1
 - attr(*, "dim")= int [1:2] 2 1

data.matrix(), however, does the intended thing:

> mat <- data.matrix(dat)
> head(mat)
     A B
[1,] 1 1
[2,] 2 1
[3,] 3 2
[4,] 4 2
[5,] 5 2
[6,] 6 2
> str(mat)
 int [1:10, 1:2] 1 2 3 4 5 6 7 8 9 10 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:2] "A" "B"