Implement str_which

Fixes #129

Author: hadley

NAMESPACE

@@ -40,6 +40,7 @@ export(str_trim)
 export(str_trunc)
 export(str_view)
 export(str_view_all)
+export(str_which)
 export(str_wrap)
 export(word)
 import(stringi)

NEWS.md

@@ -14,6 +14,8 @@
   vector. If `replacement` is `NA_character_` it replaces the complete string 
   with replaces with `NA` (#124).
 
+* New `str_which()` mimics `grep()` (#129).
+
 * All functions that take a locale (e.g. `str_to_lower()` and `str_sort()`)
   default to "en" (English) to ensure that the default is consistent across
   platforms.

R/subset.R

@@ -1,6 +1,10 @@
-#' Keep strings matching a pattern.
+#' Keep strings matching a pattern, or find positions.
+#'
+#' \code{str_subset()} is a wrapper around \code{x[str_detect(x, pattern)]},
+#' and is equivalent to \code{grep(pattern, x, value = TRUE)}.
+#' \code{str_which()} is a wrapper around \code{which(str_detect(x, pattern))},
+#' and is equivalent to \code{grep(pattern, x)}.
 #'
-#' This is a convenient wrapper around \code{x[str_detect(x, pattern)]}.
 #' Vectorised over \code{string} and \code{pattern}
 #'
 #' @inheritParams str_detect
@@ -11,13 +15,16 @@
 #' @examples
 #' fruit <- c("apple", "banana", "pear", "pinapple")
 #' str_subset(fruit, "a")
+#' str_which(fruit, "a")
+#'
 #' str_subset(fruit, "^a")
 #' str_subset(fruit, "a$")
 #' str_subset(fruit, "b")
 #' str_subset(fruit, "[aeiou]")
 #'
-#' # Missings are silently dropped
+#' # Missings never match
 #' str_subset(c("a", NA, "b"), ".")
+#' str_which(c("a", NA, "b"), ".")
 str_subset <- function(string, pattern) {
   switch(type(pattern),
     empty = ,
@@ -27,3 +34,9 @@ str_subset <- function(string, pattern) {
     regex = stri_subset_regex(string, pattern, omit_na = TRUE, opts_regex = opts(pattern))
   )
 }
+
+#' @export
+#' @rdname str_subset
+str_which <- function(string, pattern) {
+  which(str_detect(string, pattern))
+}