R help - Sep 2017 - quote()/eval() question

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)

}

foo(df)

Error in eval(., mySummary) : invalid 'envir' argument of type
'language'

Thank you,
Axel.

	[[alternative HTML version deleted]]

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.
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

> On Sep 8, 2017, at 4:55 PM, Axel Urbiz <axel.urbiz at gmail.com>
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)
> 
> }
> 
> foo(df)
> 
> Error in eval(., mySummary) : invalid 'envir' argument of type
'language'
You might get a more informative error message if you didn't use `df` as
your local variable. It's the name of a function.

-- 
David.
> 
> Thank you,
> Axel.
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius
Alameda, CA, USA

'Any technology distinguishable from magic is insufficiently advanced.' 
-Gehm's Corollary to Clarke's Third Law