Error when using dplyr inside of a function

Question

I'm trying to put together a function that creates a subset from my original data frame, and then uses dplyr's SELECT and MUTATE to give me the number of large/small entries, based on the sum of the width and length of sepals/petals.

filter <- function (spp, LENGTH, WIDTH) {
  d <- subset (iris, subset=iris$Species == spp) # This part seems to work just fine
  large <- d %>%                       
    select (LENGTH, WIDTH) %>%   # This is where the problem arises.
    mutate (sum = LENGTH + WIDTH) 
  big_samples <- which(large$sum > 4)
 return (length(big_samples)) 
}

Basically, I want the function to return the number of large flowers. However, when I run the function I get the following error -

filter("virginica", "Sepal.Length", "Sepal.Width")

 Error: All select() inputs must resolve to integer column positions.
The following do not:
*  LENGTH
*  WIDTH 

What am I doing wrong?

Answer 1

You are running into NSE/SE problems, see the vignette for more info.

Briefly, dplyr uses a non standard evaluation (NSE) of names, and passing names of columns into functions breaks it, without using the standard evaluation (SE) version.

The SE versions of the dplyr functions end in _. You can see that select_ works nicely with your original arguments.

However, things get more complicated when using functions. We can use lazyeval::interp to convert most function arguments into column names, see the conversion of the mutate to mutate_ call in your function below and more generally, the help: ?lazyeval::interp

Try:

filter <- function (spp, LENGTH, WIDTH) {
    d <- subset (iris, subset=iris$Species == spp) 
    large <- d %>%                       
        select_(LENGTH, WIDTH) %>%  
        mutate_(sum = lazyeval::interp(~X + Y, X = as.name(LENGTH), Y = as.name(WIDTH))) 
    big_samples <- which(large$sum > 4)
    return (length(big_samples)) 
}