- operator with gather using non standard evaluation

Question

I'd like to write a function that takes a quosure as an argument, appends - to the quosure, and passes that to gather, like so:

library(tidyverse)
my_gather <- function(d, not.columns) {
  dg <- tidyr::gather(d, key = k, value = v, .dots = -!!!not.columns)
  dg
}

de <- my_gather(mtcars, not.columns = quos(mpg, cyl, disp))

> Error in `-`(~mpg, ~cyl, ~disp) : operator needs one or two arguments

This is clearly because I need to append each element of the quosure with -, rather than appending the entire quosure with -. But in my work, it won't be easy to create this quosure in the form of quos(-mpg, -cyl, -disp) - so how can I modify quos(mpg, cyl, disp) to add the -?

I would like to see a result that is identical to gather(mtcars, key = k, value = v, -mpg, -cyl, -disp), the first 3 rows of which are

   mpg cyl disp  k   v
1 21.0   6  160 hp 110
2 21.0   6  160 hp 110
3 22.8   4  108 hp  93

There's a similar question here, but it is unanswered and doesn't seem to deal with the issue of quos() rather than quo().

Can you describe in more detail what you expect to see as a result of this? — IRTFM, Oct 29 '17 at 16:21

akrun · Accepted Answer · 2017-10-29T17:03:39.017

3

We can do

my_gather <- function(d, not.columns) {
  tidyr::gather(d, key = k, value = v, .dots =  -c(UQS(not.columns)))
  #or use !!! instead of UQS
  #tidyr::gather(d, key = k, value = v, .dots =  -c(!!!(not.columns)))

}
de <- my_gather(mtcars, not.columns = quos(mpg, cyl, disp))
head(de, 3)
#   mpg cyl disp  k   v
#1 21.0   6  160 hp 110
#2 21.0   6  160 hp 110
#3 22.8   4  108 hp  93

Checking with the output without the function

de1 <- gather(mtcars, key = k, value = v, -mpg, -cyl, -disp)
identical(de, de1)
#[1] TRUE

edited Oct 29 '17 at 17:03

answered Oct 29 '17 at 16:47

akrun

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How does this differ from OP’s attempt? `UQS` = `!!!`; the `c` shouldn’t make a difference here. – Konrad Rudolph Oct 29 '17 at 17:00
2

@KonradRudolph Yes, the `c` is the difference, But, I like to place it inside the braces – akrun Oct 29 '17 at 17:01
OK. Mind explaining why? It’s very much not obvious – Konrad Rudolph Oct 29 '17 at 17:08
3

@KonradRudolph I think the difference can be explained by comparing two commands: `'-'(1, 2, 3)` (gives error because function `-` needs one argument to change sign) and `'-'(c(1, 2, 3))` (gives expected result). So `c` wraps column names to become one argument. – echasnovski Oct 29 '17 at 19:09

score 2 · Answer 2 · answered Oct 29 '17 at 16:38

I can offer "Answer the problem not the question" type of answer. You actually need a way to specify gathered columns having information about not used columns. Here is my way:

library(tidyverse)

negate_columns <- function(.tbl, not.columns) {
  not_columns <- colnames(select(.tbl, !!!not.columns))

  setdiff(colnames(.tbl), not_columns)
}

my_gather <- function(d, not.columns) {
  columns <- negate_columns(d, not.columns)

  tidyr::gather(d, key = k, value = v, !!!syms(columns))
}

This way it works as expected:

my_gather(mtcars, not.columns = quos(mpg, cyl, disp)) %>%
    head(3)
#>    mpg cyl disp  k   v
#> 1 21.0   6  160 hp 110
#> 2 21.0   6  160 hp 110
#> 3 22.8   4  108 hp  93

- operator with gather using non standard evaluation

2 Answers2