1

I'm trying to write a wrapper for some ggplot2 graphs and am trying to use ellipsis' to make the function flexible. I want to save the user (n = 1 me!) from having to explicitly pass the axis titles so thought it might be possible to parse the ... arguments and set the axis titles appropriately. I've read in several Stackoverflow threads (e.g. 1, e.g. 2 or even the R Documentation) that ellipsis arguments can be converted to a list using args <- list(...) so have knocked up a simplified example...

test <- function(...){
    args <- list(...)
    is.list(args) %>% print()
    if(grepl('a', args)){
        title <- 'A'
    }
    else if(grepl('b', args)){
        title <- 'B'
    }
    return(title)
}

Testing the function I get what I expect when supplying a single a as an argument...

> test(a)
[1] TRUE
[1] "A"

But when I try passing other arguments including multiple one via ellipsis I don't understand what is happening. One non-a argument 

> test(b)
Error in test(b) (from #2) : object 'b' not found

...then the first argument as a with secondary ones...

> test(a, c, d)
Error in test(a, c, d) (from #2) : object 'd' not found

...or non a at first but something further down the line which should match....

> test(c, b, d)
Error in test(c, b, d) (from #2) : object 'b' not found

The problem is cropping up at args <- list(...) because the logical test to see if args is a list isn't printed, but this doesn't fit with what I've read list(...) does (which is turn the ellipsis arguments into a list). I expect I may need to use something like args <- list(...) %>% unlist() in order to convert the list into a vector which can then be used as an argument in grepl() (and have actually tried it but as far as I can tell the error is occurring before getting to the if()) but I don't understand whats going on and would be grateful for any explanations.

EDIT :

In light of comments it looks like this is a problem of my own creation as I'm mixing Standard and Non-Standard Evaluation. I had been trying to write a wrapper to ggplot2 and have been using the NSE Vignette and lazyeeval vignette to learn/guide me (as well as various threads here on SO), but was faltering when trying to pick out specific variables from the ellipsis (...) to pass to the ggplot call I was making.

Downside is work want results and don't afford much time for learning/improving our coding practices so I'll switch to using Standard Evaulation and have another stab at properly understanding Non-Standard Evaluation in the future.

slackline
  • 2,295
  • 4
  • 28
  • 43
  • Seems like you have a variable named `a` defined. And there is a built in function called `c`. It's not clear what your intention is just passing in `a` vs `"a"`, the former being a symbol and the latter being a character. `ggplot` functions use a lot of non-standard evaluation which is difficult to re-create in user defined functions. Better to stick to standard evolution. In this case just wrap your parameters in quotes `test("a", "c", "d")` and use `args <- c(...)` (no need for a list) – MrFlick Feb 28 '17 at 14:55
  • 1
    Do you mean `if(any(grepl('a', args)))`? Assuming you use a vector as commented by @MrFlick. – Roland Feb 28 '17 at 15:24
  • Thanks both for your comments, as clocked I'm in the process of getting my head around Standard Evaluation v's Non-Standard Evaluation and guess I'm currently adrift between the two. I currently am using non-standard evaluation but encountered problems further along with my ggplot call so opted to use `dplyr::select_(df, .dots = lazyeval::lazy_dots(...))` to subset my data, rename the variables and then call ggplot with the same name all the time (quite a fudge really). I'm now being bitten in trying to set other components. Annoying I don't have time to learn NSE properly. – slackline Feb 28 '17 at 15:40

0 Answers0