R: reverse reshape2's melt() on n-dimensional arrays

Question

I've got a 3-dimensional array in R that I can use melt() on to produce a long data.frame:

library(abind)
library(reshape2)
a = matrix(1:4, nrow=2, ncol=2)
x = abind(a, a, along=3)
y = melt(x)
y

which yields:

  Var1 Var2 Var3 value
1    1    1    1     1
2    2    1    1     2
3    1    2    1     3
4    2    2    1     4
5    1    1    2     1
6    2    1    2     2
7    1    2    2     3
8    2    2    2     4

How can I transform the data back to the original array? It should work with acast, but I can't find the right arguments for it:

y['idx'] = rownames(y)
acast(y, idx~Var1+Var2+Var3, value.var='value') # does not work this way

A5C1D2H2I1M1N2O1R2T1 · Accepted Answer · 2014-05-08T17:09:06.660

2

For multiple dimensions, you need some more tildes (if I understood your question correctly):

acast(y, Var1 ~ Var2 ~ Var3)
# , , 1
# 
#   1 2
# 1 1 3
# 2 2 4
# 
# , , 2
# 
#   1 2
# 1 1 3
# 2 2 4

To generalize this behavior, you can try constructing a formula with paste:

Form <- as.formula(paste(grep("^Var", names(y), value=TRUE), 
                         collapse = " ~ "))
acast(y, Form)

edited May 08 '14 at 17:09

answered May 08 '14 at 16:14

A5C1D2H2I1M1N2O1R2T1

190,393
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...and it takes the one remaining column as the values. – Spacedman May 08 '14 at 16:15
One thing, actually: is it possible to generalize this for `Var1~Var2~...~VarN` without using `eval()`? – Michael Schubert May 08 '14 at 17:01

R: reverse reshape2's melt() on n-dimensional arrays

1 Answers1