Fix weirdness with collector_value methods for number and skip cols (#573)

* Fix weirdness with collector_value methods for number and skip cols

* Add NEWS bullet

Author: jennybc

NAMESPACE

@@ -13,7 +13,8 @@ S3method(collector_value,collector_factor)
 S3method(collector_value,collector_guess)
 S3method(collector_value,collector_integer)
 S3method(collector_value,collector_logical)
-S3method(collector_value,collector_numeric)
+S3method(collector_value,collector_number)
+S3method(collector_value,collector_skip)
 S3method(collector_value,collector_time)
 S3method(output_column,POSIXt)
 S3method(output_column,character)

NEWS.md

@@ -1,5 +1,7 @@
 # vroom (development version)
 
+* Columns specified as having type "number" (requested via `col_number()` or `"number"` or `'n'`) or "skip" (requested via `col_skip()` or `"skip"` or `_` or `-`) now work in the case where 0 rows of data are parsed (#427, #540, #548).
+
 # vroom 1.6.7
 
 * `locale(encoding =)` now warns, instead of errors, when the encoding cannot be found in `iconvlist()` return value. This removes an unnecessary blocker on platforms like Alpine Linux where the output doesn't reflect actual capabilities.

R/col_types.R

@@ -569,7 +569,7 @@ collector_value.collector_integer <- function(x, ...) {
 }
 
 #' @export
-collector_value.collector_numeric <- function(x, ...) {
+collector_value.collector_number <- function(x, ...) {
   numeric()
 }
 
@@ -607,6 +607,11 @@ collector_value.collector_guess <- function(x, ...) {
   character()
 }
 
+#' @export
+collector_value.collector_skip <- function(x, ...) {
+  NULL
+}
+
 #' @export
 summary.col_spec <- function(
   object,

R/vroom.R

@@ -282,13 +282,24 @@ vroom <- function(
     progress = progress
   )
 
-  # If no rows expand columns to be the same length and names as the spec
+  # If no rows, expand columns to be the same length and names as the spec
+  # Skipped columns present a bit of a wrinkle: they appear in the spec,
+  # but not in the result
   if (NROW(out) == 0) {
     cols <- attr(out, "spec")[["cols"]]
-    for (i in seq_along(cols)) {
-      out[[i]] <- collector_value(cols[[i]])
+    nms <- names(cols)
+
+    out_i <- 1
+    for (cols_i in seq_along(cols)) {
+      value <- collector_value(cols[[cols_i]])
+      if (is.null(value)) {
+        # this is a skipped column
+        next
+      }
+      out[[out_i]] <- value
+      names(out)[out_i] <- nms[cols_i]
+      out_i <- out_i + 1
     }
-    names(out) <- names(cols)
   }
 
   out <- tibble::as_tibble(out, .name_repair = identity)

tests/testthat/test-col_types.R

@@ -33,3 +33,19 @@ test_that("all col_types can be reported with color", {
   )
   expect_snapshot(spec(dat))
 })
+
+# https://github.com/tidyverse/vroom/issues/548
+test_that("col_number() works with empty data", {
+  out <- vroom(I(""), col_types = "in")
+  expect_equal(out, tibble::tibble(X1 = integer(), X2 = numeric()))
+})
+
+# https://github.com/tidyverse/vroom/issues/540
+test_that("col_skip works with empty data (#540)", {
+  out <- vroom(I("a\tb\tc\n"), delim = "\t", col_types = "c-d", skip = 1L)
+  expect_equal(out, tibble::tibble(X1 = character(), X3 = numeric()))
+
+  # All columns skipped
+  out <- vroom(I("a\tb\n"), delim = "\t", col_types = "--", skip = 1L)
+  expect_equal(out, tibble::tibble())
+})