Why can't there be multiple `value.var` in` dcast`?

Question

Why can't multiple variables be passed in value.varto dcast? From ?dcast:

value.var is the name of the column in which values ​​are stored, see gt_value for default strategies to figure this out.

It does not explicitly state that only one variable can be passed as a value. If, however, I try this, then I get an error message:

> library("reshape2")
> library("MASS")
> 
> dcast(Cars93, AirBags ~ DriveTrain, mean, value.var=c("Price", "Weight"))
Error in .subset2(x, i, exact = exact) : subscript out of bounds
In addition: Warning message:
In if (!(value.var %in% names(data))) { :
  the condition has length > 1 and only the first element will be used

So is there any good reason to impose this restriction? And is it possible to get around this (possibly using reshape, etc.)?

Answer 1

.

@beginneR , " , ". , , , melt, , , .

, melt ing, 3 .

, :

C93L <- melt(Cars93, measure.vars = c("Price", "Weight"))
dcast(C93L, AirBags ~ DriveTrain + variable, mean, value.var = "value")
#              AirBags 4WD_Price 4WD_Weight Front_Price Front_Weight
# 1 Driver & Passenger       NaN        NaN    26.17273     3393.636
# 2        Driver only     21.38       3623    18.69286     2996.250
# 3               None     13.88       2987    12.98571     2703.036
#   Rear_Price Rear_Weight
# 1      33.20      3515.0
# 2      28.23      3463.5
# 3      14.90      3610.0

---

aggregate mean s, reshape dcast "" "". , reshape :

temp <- aggregate(cbind(Price, Weight) ~ AirBags + DriveTrain, 
                  Cars93, mean)
#              AirBags DriveTrain    Price   Weight
# 1        Driver only        4WD 21.38000 3623.000
# 2               None        4WD 13.88000 2987.000
# 3 Driver & Passenger      Front 26.17273 3393.636
# 4        Driver only      Front 18.69286 2996.250
# 5               None      Front 12.98571 2703.036
# 6 Driver & Passenger       Rear 33.20000 3515.000
# 7        Driver only       Rear 28.23000 3463.500
# 8               None       Rear 14.90000 3610.000

reshape(temp, direction = "wide", 
        idvar = "AirBags", timevar = "DriveTrain")
#              AirBags Price.4WD Weight.4WD Price.Front Weight.Front
# 1        Driver only     21.38       3623    18.69286     2996.250
# 2               None     13.88       2987    12.98571     2703.036
# 3 Driver & Passenger        NA         NA    26.17273     3393.636
#   Price.Rear Weight.Rear
# 1      28.23      3463.5
# 2      14.90      3610.0
# 3      33.20      3515.0

Answer 2

, : dcast .var, "" data.table dcast :

# multiple value.var
data.table::dcast(Cars93, AirBags ~ DriveTrain, mean, value.var=c("Price", "Weight"))

.

Answer 3

Try forcing data.frame into the data table using setDT or as.data.table.

dcast(setDT(C93L), AirBags ~ DriveTrain , mean, value.var=c("Price","Weight"))