[R] quote()/eval() question

[Duncan Murdoch]

On 08/09/2017 7:55 PM, Axel Urbiz wrote:
> Dear list,
> 
> For a reason it would take me long to explain, I need to do something along
> the lines of what's shown below -- i.e., create an object from
> dplyr::summarise, and then evaluate it on a data frame.
> 
> I know I could directly do:
> 
>   df %>% dplyr::summarise(x1_mean = mean(x1))
> 
> but this is not what I'm looking for.
> 
> 
> library(dplyr)
> 
> df <- data.frame(x1 = rnorm(100), x2 = rnorm(100))
> 
> foo <- function(df) {
> 
>    mySummary <- quote(dplyr::summarise(x1_mean = mean(x1)))
> 
>    df %>% eval(mySummary)

magrittr pipes are just syntactic sugar.  What your second line does is 
the same as

eval(df, mySummary)

which makes no sense.  These would work:

eval(df, expr = mySummary)
eval(mySummary, envir = df)

You could write the first as

df %>% eval(expr = mySummary)

and the second as

df %>% eval(mySummary, envir = .)

Duncan Murdoch

> 
> }
> 
> foo(df)
> 
> Error in eval(., mySummary) : invalid 'envir' argument of type 'language'
> 
> Thank you,
> Axel.
> 
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> 
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