[R] using dcast with a function of multiple arguments

[Bos, Roger]

I am trying to figure out how to use dcast.data.table with a function with multiple arguments.  Here is my reproducible example for a simple function of one argument:

require(data.table)
dt <- as.data.table(mtcars)
dcast.data.table(dt, carb ~ cyl, value.var='mpg', fun=mean)

If I instead want to use, say, weighted.mean(x, w), how do I do so?

The docs say
...
Any other arguments that maybe passed to the aggregating function.

So I tried:

> dcast.data.table(dt, carb ~ cyl, value.var='mpg', fun=weighted.mean, w="wt")
Error in weighted.mean.default(data[[value.var]][0], ...) :
  'x' and 'w' must have the same length

The docs also say that value.var can be a list, so I tried that:

 In cases where value.var is a list, the function should be able to handle a list input and provide a single value or list of length one as output.

> dcast.data.table(dt, carb ~ cyl, value.var=list('mpg','wt'), fun=weighted.mean)
Error in dcast.data.table(dt, carb ~ cyl, value.var = list("mpg", "wt"),  :
  'value.var' must be a character vector of length 1.

I didn't actually expect that to work, but without an example I don't know what else to try.  Any hints would be greatly appreciated.

Thanks,

Roger



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