Consider the following code:
set.seed(1)
M = matrix(rnorm(9), ncol = 3)
dimnames(M) = list(LETTERS[1:3], LETTERS[1:3])
print(M)
A B C
A -0.6264538 1.5952808 0.4874291
B 0.1836433 0.3295078 0.7383247
C -0.8356286 -0.8204684 0.5757814
melt(M)
Var1 Var2 value
1 A A -0.6264538
2 B A 0.1836433
3 C A -0.8356286
4 A B 1.5952808
5 B B 0.3295078
6 C B -0.8204684
7 A C 0.4874291
8 B C 0.7383247
9 C C 0.5757814
If i call melt
using a data.frame
, i get a different result:
DF = data.frame(M)
melt(DF)
variable value
1 A -0.6264538
2 A 0.1836433
3 A -0.8356286
4 B 1.5952808
5 B 0.3295078
6 B -0.8204684
7 C 0.4874291
8 C 0.7383247
9 C 0.5757814
I found the docs a little bit confusing on this, so anyone can help me understand this behavior? Can i get the first result using a data.frame?
The basic reason is that there are different methods
for melt
, which you can see by running methods("melt")
. Most of these can be accessed by, say, reshape2:::melt.matrix
or reshape2:::melt.data.frame
, which can send you on your hunt for figuring out exactly why the results are different.
But, to summarize what you will find, basically, melt.matrix
will end up doing something like:
cbind(expand.grid(dimnames(M)), value = as.vector(M))
# Var1 Var2 value
# 1 A A -0.6264538
# 2 B A 0.1836433
# 3 C A -0.8356286
# 4 A B 1.5952808
# 5 B B 0.3295078
# 6 C B -0.8204684
# 7 A C 0.4874291
# 8 B C 0.7383247
# 9 C C 0.5757814
... while melt.data.frame
will end up doing something like this:
N <- data.frame(M)
data.frame(var1 = rep(names(N), each = nrow(N)), value = unlist(unname(N)))
# var1 value
# 1 A -0.6264538
# 2 A 0.1836433
# 3 A -0.8356286
# 4 B 1.5952808
# 5 B 0.3295078
# 6 B -0.8204684
# 7 C 0.4874291
# 8 C 0.7383247
# 9 C 0.5757814
Of course, the actual functions do a lot more error checking and are designed to let you conveniently specify which columns should be melted and so on.
Note that the data.frame
method doesn't make use of the rownames
, so as mentioned in the comments, to get the same result with the data.frame
method, you'll have to add them in to the melt
command.