Fix selection when col_names = FALSE

If `col_names = FALSE` we pass `character()` to
`col_types_standardise()`, not `NULL`. So we need to check the length
rather than explicitly for `NULL`.

Fixes #381

Author: jimhester

NEWS.md

@@ -1,5 +1,7 @@
 # vroom (development version)
 
+* `vroom(col_select=)` now works if `col_names = FALSE` as intended (#381)
+
 * `vroom(n_max=)` now correctly handles cases when reading from a connection and the file does _not_ end with a newline (https://github.com/tidyverse/readr/issues/1321)
 
 * `vroom()` no longer issues a spurious warning when the parsing needs to be restarted due to the presence of embedded newlines (https://github.com/tidyverse/readr/issues/1313)

R/col_types.R

@@ -382,7 +382,7 @@ col_types_standardise <- function(spec, num_cols, col_names, col_select, name_re
     }
   }
 
-  if (is.null(col_names)) {
+  if (length(col_names) == 0) {
     col_names <- make_names(NULL, num_cols)
   }
 

tests/testthat/test-select.R

@@ -63,3 +63,11 @@ test_that("col_select can select the id column", {
     c("path", "model", "mpg")
   )
 })
+
+test_that("col_select works with col_names = FALSE", {
+  res <- vroom(I("foo\tbar\n1\t2\n"), col_names = FALSE, col_select = 1, col_types = list())
+  expect_equal(res[[1]], c("foo", "1"))
+
+  res2 <- vroom(I("foo\tbar\n1\t2\n"), col_names = FALSE, col_select = c(X2), col_types = list())
+  expect_equal(res2[[1]], c("bar", "2"))
+})