Integer64 class doesn't survive reshape2 melt function

Question

I don't know whether this is an integer64 (from bit64) problem, or a melt problem (from reshape2, but if I try to reshape a data.frame containing integer64 data then the class information is destroyed in the process and it reverts to the double representation:

library(bit64)
library(reshape2)

DF = data.frame(I =letters, Num1 = as.integer64(1:26), Num2 = as.integer64(1:26))
DFM = melt(DF, id.vars = "I")

sapply(DF, class)
sapply(DFM, class)

gives:

> sapply(DF, class)
          I        Num1        Num2 
   "factor" "integer64" "integer64" 
> sapply(DFM, class)
        I  variable     value 
 "factor"  "factor" "numeric" 

And because integer64 is double underneath, the data is "corrupted"

> DF
   I Num1 Num2
1  a    1    1
2  b    2    2
3  c    3    3
4  d    4    4
5  e    5    5
...
> DFM
   I variable         value
1  a     Num1 4.940656e-324
2  b     Num1 9.881313e-324
3  c     Num1 1.482197e-323
4  d     Num1 1.976263e-323
5  e     Num1 2.470328e-323
6  f     Num1 2.964394e-323

What is causing this? Is this a integer64 problem or a melt problem? When creating classes what can be done to avoid this sort of thing?

Answer 1

It seems to be a limitation of the package which is also mentioned in their documentation here on page 9. For example:

x <- data.frame(a=as.integer64(1:5), b=as.integer64(1:5))
> x
#   a b
# 1 1 1
# 2 2 2
# 3 3 3
# 4 4 4
# 5 5 5

> unlist(x)

#            a1            a2            a3            a4            a5            b1 
# 4.940656e-324 9.881313e-324 1.482197e-323 1.976263e-323 2.470328e-323 4.940656e-324 
#            b2            b3            b4            b5 
# 9.881313e-324 1.482197e-323 1.976263e-323 2.470328e-323 

> as.matrix(x)
#                  a             b
# [1,] 4.940656e-324 4.940656e-324
# [2,] 9.881313e-324 9.881313e-324
# [3,] 1.482197e-323 1.482197e-323
# [4,] 1.976263e-323 1.976263e-323
# [5,] 2.470328e-323 2.470328e-323

x <- as.integer64(1:5)

> is.vector(x)
# [1] FALSE

> as.vector(x)
# [1] 4.940656e-324 9.881313e-324 1.482197e-323 1.976263e-323 2.470328e-323