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 a

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 "".