[R] using dcast with a function of multiple arguments

[Thierry Onkelinx]

Here is a solution using dplyr

require(data.table)
dt <- as.data.table(mtcars)
library(dplyr)
library(tidyr)
dt %>%
  group_by(carb, cyl) %>%
  summarise(WM = weighted.mean(x = mpg, w = wt)) %>%
  spread(cyl, WM)

Best regards,

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium

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ensure that a reasonable answer can be extracted from a given body of data.
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2015-07-20 20:52 GMT+02:00 Bos, Roger <roger.bos op rothschild.com>:

> 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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