data.frame with a column containing a matrix in R

Question

I'm trying to put some matrices in a dataframe in R, something like :

m <- matrix(c(1,2,3,4), nrow=2, ncol=2)
df <- data.frame(id=1, mat=m)

But when I do that, I get a dataframe with 2 rows and 3 columns instead of a dataframe with 1 row and 2 columns.

Reading the documentation, I have to escape my matrix using I().

df <- data.frame(id=1, mat=I(m))

str(df)
'data.frame':   2 obs. of  2 variables:
 $ id : num  1 1
 $ mat: AsIs [1:2, 1:2] 1 2 3 4

As I understand it, the dataframe contains one row for each row of the matrix, and the mat field is a list of matrix column values.

Thus, how can I obtain a dataframe containing matrices ?

Thanks !

Answer 1

I find data.frames containing matrices mind-bendingly weird, but: the only way I know to achieve this is hidden in stats:::simulate.lm

Try this, poke through and see what's happening:

d <- data.frame(y=1:5,n=5)
g0 <- glm(cbind(y,n-y)~1,data=d,family=binomial)
debug(stats:::simulate.lm)
s <- simulate(g0,n=5)

This is the weird, back-door solution. Create a list, change its class to data.frame, and then (this is required) set the names and row.names manually (if you don't do those final steps the data will still be in the object, but it will print out as though it had zero rows ...)

m1 <- matrix(1:10,ncol=2)
m2 <- matrix(5:14,ncol=2)
dd <- list(m1,m2)
class(dd) <- "data.frame"
names(dd) <- LETTERS[1:2]
row.names(dd) <- 1:5
dd

Answer 2

A much easier way to do this is to define the data frame with a placeholder for the matrix

m <- matrix(c(1, 2, 3, 4), nrow = 2, ncol = 2) 
df <- data.frame(id = 1, mat = rep(0, nrow(m)))

Then to assign the matrix. No need to play with the class of a list or to use an *apply() function.

df$mat <- m

Answer 3

I came across the same problem trying to understand the gasoline data in pls package. Used $ for the job. First, lets create a matrix, lets call it spectra_mat, then a vector called response_var1.

spectra_mat = matrix(1:45, 9, 5)
response_var1 = seq(1:9)

Now we put the vector response_var1 in a new data frame - lets call it df.

df = data.frame(response_var1)
df$spectra = spectra_mat

To check,

str(df)

'data.frame':   9 obs. of  2 variables:
 $ response_var1: int  1 2 3 4 5 6 7 8 9
 $ spectra      : int [1:9, 1:5] 1 2 3 4 5 6 7 8 9 10 ...