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

Question

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 1

The as.table and as.data.frame functions together will do this:

> m <- matrix( sample(1:12), nrow=4 )
> dimnames(m) <- list( One=letters[1:4], Two=LETTERS[1:3] )
> as.data.frame( as.table(m) )
   One Two Freq
1    a   A    7
2    b   A    2
3    c   A    1
4    d   A    5
5    a   B    9
6    b   B    6
7    c   B    8
8    d   B   10
9    a   C   11
10   b   C   12
11   c   C    3
12   d   C    4

Answer 2

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
...

Answer 3

Assuming 'm' is your matrix...

data.frame(col = rep(colnames(m), each = nrow(m)), 
           row = rep(rownames(m), ncol(m)), 
           value = as.vector(m))

This executes extremely fast on a large matrix and also shows you a bit about how a matrix is made, how to access things in it, and how to construct your own vectors.

Answer 4

You can use the function data.table() with the parameter keep.rownames to convert the matrix to the data.table class:

# Example data
x

  a b  c
1 1 5  9
2 2 6 10
3 3 7 11
4 4 8 12

# Conversion
data.table(x, keep.rownames = TRUE) # inserts rownames into column "rn"

   rn a b  c
1:  1 1 5  9
2:  2 2 6 10
3:  3 3 7 11
4:  4 4 8 12