r - Using dplyr functions within another function

Question

I've been struggling with this issue which is quite similar to a question raised here before. Somehow I can't translate the solution given in that question to my own problem.

I start off with making an example data frame:

test.df <- data.frame(col1 = rep(c('a','b'), each=5), col2 = runif(10))
str(test.df)

The following function should create a new data frame with the mean of a "statvar" based on groups of a "groupvar".

test.f <- function(df, groupvar, statvar) {
  df %>% 
    group_by_(groupvar) %>% 
    select_(statvar) %>%
    summarise_(
      avg = ~mean(statvar, na.rm = TRUE)
    )
} 

test.f(df = test.df,
       groupvar = "col1",
       statvar = "col2")

What I would like this to return is a data frame with 2 calculated averages (one for all a values in col1 and one for all b values in col1). Instead I get this:

  col1 avg
1    a  NA
2    b  NA
Warning messages:
1: In mean.default("col2", na.rm = TRUE) :
  argument is not numeric or logical: returning NA
2: In mean.default("col2", na.rm = TRUE) :
  argument is not numeric or logical: returning NA

I find this strange cause I'm pretty sure col2 is numeric:

str(test.df)
'data.frame':   10 obs. of  2 variables:
 $ col1: Factor w/ 2 levels "a","b": 1 1 1 1 1 2 2 2 2 2
 $ col2: num  0.4269 0.1928 0.7766 0.0865 0.1798 ...

Answer 1

library(lazyeval)
library(dplyr)

test.f <- function(df, groupvar, statvar) {
  test.df %>% 
    group_by_(groupvar) %>% 
    select_(statvar) %>%
    summarise_(
      avg = (~mean(statvar, na.rm = TRUE)) %>%
        interp(statvar = as.name(statvar))
    )
} 

test.f(df = test.df,
       groupvar = "col1",
       statvar = "col2")

Your issue is that "col2" is being substituted for statvar, and the mean("col2") is undefined