In R: pass column name as argument and use it in function with dplyr::mutate() and lazyeval::interp()

Question

This question links to this SO answer except that here I want to use the variable specified as function arg in a mutate_(). It works if I don\'t make any \"calc

Answer 1

You have to use lazy evaluation (with the package lazyeval), for example like this:

library(lazyeval)
func2 <- function(df, varname){
     df %>%
       mutate_(v3=interp(~sum(x), x = as.name(varname)))
}
func2(data, "v1")
#  v1 v2 v3
#1  1  3  3
#2  2  4  3

Answer 2

With the devel version of dplyr (0.5.0) or in the new version (0.6.0 - awaiting release in April 2017), this can be done using slightly different syntax

library(dplyr)
funcN <- function(dat, varname){
 expr <- enquo(varname)
 dat %>%
     mutate(v3 = sum(!!expr))
     #or
     #mutate(v3 = sum(UQ(expr)))

} 

funcN(data, v1)
#  v1 v2 v3
#1  1  3  3
#2  2  4  3

Here, enquo takes the arguments and returns the value as a quosure (similar to substitute in base R) by evaluating the function arguments lazily and inside the summarise, we ask it to unquote (!! or UQ) so that it gets evaluated.