thisisnic commented on a change in pull request #10624:
URL: https://github.com/apache/arrow/pull/10624#discussion_r670256573
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File path: r/R/dplyr-functions.R
##########
@@ -280,6 +284,81 @@ nse_funcs$str_trim <- function(string, side = c("both",
"left", "right")) {
Expression$create(trim_fun, string)
}
+nse_funcs$substr <- function(string, start, stop) {
+ assert_that(
+ length(start) == 1,
+ msg = "`start` must be length 1 - other lengths are not supported in Arrow"
+ )
+ assert_that(
+ length(stop) == 1,
+ msg = "`stop` must be length 1 - other lengths are not supported in Arrow"
+ )
+
+ if (start <= 0) {
+ start <- 1
+ }
+
+ if (stop < start) {
+ stop <- 0
+ }
+
+ Expression$create(
+ "utf8_slice_codeunits",
+ string,
+ options = list(start = start - 1L, stop = stop)
Review comment:
we don't need to subtract 1 from `end` as R counts inclusively (i.e.
returned string includes the item at position `end`) whereas C++ counts
exclusively (i.e. returned string includes up to the item at position `end`
which effectively cancels out the difference due to indexing
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