can the value.var in dcast be a list or have multiple value variables?

Question

In the help files for dcast.data.table, there is a note stating that a new feature has been implemented: \"dcast.data.table allows value.var column to be of typ

Answer 1

Using sample data frame mydf from A5C1D2H2I1M1N2O1R2T1's answer.

Edit December 2016 using tidyr

Reshape2 has been replaced with the tidyr package.

library(tidyr)
mydf  %>% 
    gather(variable, value, -x1, -x2)  %>% 
    unite(x2_variable, x2, variable)  %>% 
    spread(x2_variable, value)

#   x1 1_salt 1_sugar 2_salt 2_sugar 3_salt 3_sugar
# 1  1      3       1      4       2      6       2
# 2  2     10       5      3       3      9       6
# 3  3     10       4      7       6      7       7

Original answer based on reshape2

@AlexR added to his question:

Sure, you can 'melt' the 2 value variables into a single column,

For those who come here looking for an answer based on reshape2, here is how to melt the data and then use dcast based on the "variable". .

dt2 <- melt(mydf, id = c("x1", "x2")) 

The variable column will now contain 'var1','var2','var3'. You can achieve the desired effect with

dt3 <- dcast(dt2, x1 ~ x2 + variable, value.var="value")
dt3
#   x1 1_salt 1_sugar 2_salt 2_sugar 3_salt 3_sugar
# 1  1      3       1      4       2      6       2
# 2  2     10       5      3       3      9       6
# 3  3     10       4      7       6      7       7

value.var is optional in this function call as dcast will automatically guess it.