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https://issues.apache.org/jira/browse/ARROW-13259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17374908#comment-17374908
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David Li commented on ARROW-13259:
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Maybe we could add a SliceOptions::kEnd constant just to make it clear what to
do? (Not sure that'd help R?)
> [C++] Enable slicing to end of string using "utf8_slice_codeunits" when
> string length unknown or different lengths
> -------------------------------------------------------------------------------------------------------------------
>
> Key: ARROW-13259
> URL: https://issues.apache.org/jira/browse/ARROW-13259
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Nic Crane
> Priority: Major
>
> We're currently trying to write bindings from the C++ function
> "utf8_slice_codeunits" to R, specifically trying to replicate the behaviour
> of R's string::str_sub
> In both the R and C++ implementations, I can use negative indices to count
> back from the end of a string (show below in R, but the latter directly
> invokes the C++ implementation):
>
> {code:java}
> # stringr version
> > stringr::str_sub("Apache Arrow", -5, -2)
> [1] "Arro"
> # C++ version
> > call_function("utf8_slice_codeunits", Scalar$create("Apache Arrow"),
> > options = list(start=-5L, stop=-1L))
> Scalar
> Arro{code}
> Note that in the C++ implementation, I have to add 1 to the stop value as the
> final value is non-inclusive.
> The problem is when I'm trying to use negative indices to refer to the final
> values in a string:
>
> {code:java}
> stringr version
> > stringr::str_sub("Apache Arrow", -5, -1)
> [1] "Arrow"
> # C++ version
> > call_function("utf8_slice_codeunits", Scalar$create("Apache Arrow"),
> > options = list(start=-5L, stop=0L))
> Scalar
> {code}
> The result is blank as the 'stop' value 0 refers to the start of the string,
> effective walking backwards, which isn't possible (except via the step
> argument which I can't get working but I don't think is what I want anyway).
> I've tried to get around this by attempting to write some code that
> calculates the length of the string and supply that to the stop argument, but
> it didn't work.
> I do have a possible workaround that involves reversing the string,
> extracting the substring using inverted values of swapped stop/start values,
> and then reversing the result, but before I go down that path, I was
> wondering if there is anything that can (and should! the answer may be a
> simple "nope!") be changed in the C++ code to make it possible to do this a
> different way?
>
>
>
>
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