r - Converting a numeric matrix into a data.table (or data.frame)

Hoping there's a simple answer here but I can't find it anywhere.

I have a numeric matrix with labelled rows and columns:

     1    2    3    4
a    6    7    8    9

b    8    7    5    7
c    8    5    4    1
d    1    6    3    2

I would like a data.table (or a data.frame I can then convert) of the form:

    col     row    value
    1       a      6
    1       b      8

    1       c      8
    1       d      1
    2       a      7
    2       b      7
    2       c      5
    2       d      6
    ...

Any tips appreciated.

Answer

Use melt from reshape2:

library(reshape2)
#Fake data
x <- matrix(1:12, ncol = 3)
colnames(x) <- letters[1:3]
rownames(x) <- 1:4
x.m <- melt(x)
x.m


   Var1 Var2 value
1     1    a     1
2     2    a     2
3     3    a     3
4     4    a     4
...