r - 为什么不能在 `value.var` 中有几个 `dcast` ？

Question

为什么不能将多个变量传递给 value.var在 dcast ?来自 ?dcast :

value.var name of column which stores values, see guess_value for default strategies to figure this out.

它没有明确表示只能传递一个变量作为值。但是，如果我尝试这样做，则会出现错误:

> 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

那么是否有充分的理由强加这种限制？是否有可能解决这个问题(也许使用 reshape 等)？

Answer 1

这个问题与your other question from earlier today非常相关.

@beginneR 在评论中写道:“只要现有数据已经​​是长格式的，我就没有看到任何一般需要在转换之前将其融化。”在我在你的另一个问题上发布的回答中，我举了一个例子，说明何时 melt将需要，或者更确切地说，如何确定您的数据是否足够长。

这里的这个问题是另一个例子，当进一步melt自 my answer 中的第 3 点起就需要 ing不满意。

要获得您想要的行为，请尝试以下操作:

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

另一种方法是使用 aggregate计算 mean s，然后使用 reshape或 dcast从“长”到“宽”。两者都是必需的，因为 reshape不执行任何聚合:

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