Dplyr write a function with column names as inputs

Question

I'm writing a function that I'm going to use on multiple columns in dplyr, but I'm having trouble passing column names as inputs to functions for dplyr.

Here's an example of what I want to do:

df<-tbl_df(data.frame(group=rep(c("A", "B"), each=3), var1=sample(1:100, 6), var2=sample(1:100, 6)))


example<-function(colname){
  df %>%
    group_by(group)%>%
    summarize(output=mean(sqrt(colname)))%>%
    select(output)
}
example("var1")

Output should look like

df %>%
  group_by(group)%>%
  summarize(output=mean(sqrt(var1)))%>%
  select(output)

I've found a few similar questions, but nothing that I could directly apply to my problem, so any help is appreciated. I've tried some solutions involving eval, but I honestly don't know what exactly I'm supposed to be passing to eval.

Answer 1

Is this what you expected?

df<-tbl_df(data.frame(group=rep(c("A", "B"), each=3), var1=sample(1:100, 6), var2=sample(1:100, 6)))

example<-function(colname){
  df %>%
    group_by(group)%>%
    summarize(output=mean(sqrt(colname)))%>%
    select(output)
}
example( quote(var1) )
#-----
Source: local data frame [2 x 1]

    output
1 7.185935
2 8.090866

Answer 2

The accepted answer does not work anymore in R 3.6 / dplyr 0.8.

As suggested in another answer , one can use !!as.name()

This works for me:

df<-tbl_df(data.frame(group=rep(c("A", "B"), each=3), var1=sample(1:100, 6), var2=sample(1:100, 6)))

example<-function(colname){
  df %>%
    group_by(group)%>%
    summarize(output=mean(sqrt(!!as.name(colname)))%>%
    select(output)
}
example( quote(var1) )

If one additionally wants to have the column names to assign to in a mutate , then the easiest is to use the assignment := . For example to replace colname with its square root.

example_mutate<-function(colname){
  df %>%
    mutate(!!colname := sqrt(!!as.name(colname)))
}
example_mutate( quote(var1) )

quote() can of course be replaced with quotation marks "" .