R matrix to rownames colnames values

Question

I have a matrix A which I want to convert into a data.frame of the form:

rownames    colnames    values

Using

unlist(A)       

helps but does not give me the rownames

Thank you for your help.

Answer 1

You could use the reshape2-package:

# load package
> require(reshape2)
# create an example matrix
> mdat <- matrix(c(1,2,3, 11,12,13), nrow = 2, ncol=3, byrow=TRUE,
+                dimnames = list(c("row1", "row2"),
+                                c("C.1", "C.2", "C.3")))
> mdat
     C.1 C.2 C.3
row1   1   2   3
row2  11  12  13
# bring matrix to long format using melt()
> melt(mdat)
  Var1 Var2 value
1 row1  C.1     1
2 row2  C.1    11
3 row1  C.2     2
4 row2  C.2    12
5 row1  C.3     3
6 row2  C.3    13

Answer 2

Solution in base R:

Here's a matrix:

test <- matrix(1:9,nrow=3)
rownames(test) <- letters[1:3]
colnames(test) <- letters[4:6]

> test
  d e f
a 1 4 7
b 2 5 8
c 3 6 9

Convert to a data.frame as required:

data.frame(
           rownames=as.vector(row(test,as.factor=TRUE)),
           colnames=as.vector(col(test,as.factor=TRUE)),
           values=as.vector(test)
          )

Or even shorter, using expand.grid and making sure to name the inputs:

data.frame(
        expand.grid(rownames=rownames(test),colnames=colnames(test)),
        values=as.vector(test)
          )

Or even shorter again, using as.data.frame.table (which will give the majority of the result), and setting the names using setNames

setNames(as.data.frame.table(test),c("rownames","colnames","values"))

Result:

  rownames colnames values
1        a        d      1
2        b        d      2
3        c        d      3
4        a        e      4
5        b        e      5
6        c        e      6
7        a        f      7
8        b        f      8
9        c        f      9