CODEWITHSUNDEEP
rdplyr
Robert
Robert

Reputation: 103

passing data frame to mutate within function

I would like to pass a data frame and its columns to be processed by dplyr's mutate within a function.

Here is an example

multifun <- function(dataf,vari){
 mutate(dataf,newvar=vari*2)
}

multifun(mtcars,gear)

The problem with this function is that the variable 'gear' is not a recognized object. More specifically I get the error

Error in mutate_impl(.data, named_dots(...), environment()) object 'gear' not found

This is a problem with the environment where dplyr's mutate is looking for the variable in question.

I understand that 

multifun(mtcars,mtcars$gear)

will give me the answer that I want, namely

    mpg  cyl  disp  hp   drat  wt   qsec  vs am   gear carb newvar
1  21.0   6   160.0 110  3.90 2.620 16.46  0  1    4    4      8
2  21.0   6   160.0 110  3.90 2.875 17.02  0  1    4    4      8
3  22.8   4   108.0  93  3.85 2.320 18.61  1  1    4    1      8

but I would like to see if there is a way of avoiding the need to reference each variable used from the data frame in the function call.

I am also aware that taking mutate out of the function call works without problems. Namely, mutate(mtcars,newvar=gear*2) does the job. However, I am trying to understand how dplyr's mutate is looking for the variable in question in the different environments when placed inside a function.

Upvotes: 10

Views: 6624

Answers (4)

acylam
acylam

Reputation: 18691

With dplyr 0.7.0, this can now be done with tidyeval:

multifun <- function(dataf,vari){
  mutate(dataf,newvar = UQ(enquo(vari))*2)
}

multifun(mtcars,gear)

enquo quotes the symbol referring to the function argument and bundles it with the environment in which the function is being called into a quosure. UQ or !! can then be used to unquote the quosure and evaluate it immediately within mutate.

Upvotes: 3

akrun
akrun

Reputation: 887831

Or

multifun1 <- function(dataf, vari){
eval(substitute(mutate(dataf, newvar=vari*2), list(vari=as.name(vari))))
}

multifun1(mtcars,"gear")

To use unquoted value, it would be better to use @Ananda Mahto's suggestion

multifun1 <- function(dataf, vari){
vari <- deparse(substitute(vari))
eval(substitute(mutate(dataf, newvar=vari*2), list(vari=as.name(vari))))
}

multifun1(mtcars,gear)

Upvotes: 2

David Arenburg
David Arenburg

Reputation: 92300

Looking at @Anandas solution, this seems to be simplest hack 

multifun <- function(dataf, vari){   
dataf <- mutate(dataf, newvar = dataf[, vari]*2);   
return(dataf) 
}

multifun(mtcars,"gear")

Again, taking incount @Anandas suggestion, you could also do

multifun <- function(dataf, vari){  
  vari <- deparse(substitute(vari))
  dataf <- mutate(dataf, newvar = dataf[, vari]*2)   
  return(dataf) 
}

multifun(mtcars, gear)

Upvotes: 4

A5C1D2H2I1M1N2O1R2T1
A5C1D2H2I1M1N2O1R2T1

Reputation: 193677

This is really ugly to me, but seems to work. Basically, I tried using get but it didn't seem to know where to look, so I specified the environment.

multifun <- function(dataf, vari){
  vari <- deparse(substitute(vari))
  mutate(dataf, newvar = get(vari, envir = as.environment(dataf)) * 2)
}

Output:

multifun(mtcars, gear)
#                      mpg cyl  disp  hp drat    wt  qsec vs am gear carb newvar
# Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4      8
# Mazda RX4 Wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4      8
# Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1      8
# <<<SNIP>>>
# Ferrari Dino        19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6     10
# Maserati Bora       15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8     10
# Volvo 142E          21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2      8

Upvotes: 4

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