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https://issues.apache.org/jira/browse/ARROW-13259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17377194#comment-17377194
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Eduardo Ponce commented on ARROW-13259:
---------------------------------------
[In C++ by default {{SliceOptions}} has the {{stop}} option set to
{{std::numeric_limits<int64_t>::max()}}|https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/api_scalar.h#L205-L206].
Therefore, if you want to slice to end of string simply omit a value for
{{stop}} or set it to a value >= len(string).
{code:c++}
// start=-5, stop=std::numeric_limits<int64_t>::max(), step=1
SliceOptions opts(-5);
auto result = CallFunction("utf8_slice_codeunits", {Datum("Apache Arrow")},
&opts);
if (result.ok()) {
Datum slice = std::move(result).ValueOrDie();
// Prints "Arrow"
std::cout << slice.scalar()->ToString() << std::endl;
} else {
ARROW_LOG(ERROR) << result.status();
}
{code}
In R you should be able to do the following,
{code:r}
# C++ version
> call_function("utf8_slice_codeunits", Scalar$create("Apache Arrow"), options
> = list(start=-5L))
[1] "Arrow"
{code}
[~jorisvandenbossche]
The issue in PyArrow arises because the [interface for {{SliceOptions}} does
not sets the default value for {{stop}} option (only for {{step}}
option)|https://github.com/apache/arrow/blob/master/python/pyarrow/_compute.pyx#L798].
Therefore, these are required arguments.
{code:python}
>>> string = 'Apache Arrow'
>>> pc.utf8_slice_codeunits(string, start=-5, stop=len(string))
<pyarrow.StringScalar: 'Arrow'>
{code}
[By providing {{sys.maxsize}} as default {{stop}}
option|https://github.com/edponce/arrow/blob/ARROW-13259-Enable-slicing-to-end-of-string-using-ut/python/pyarrow/_compute.pyx#L800-L802],
we can do the following:
{code:python}
>>> string = 'Apache Arrow'
>>> pc.utf8_slice_codeunits(string, start=-5)
<pyarrow.StringScalar: 'Arrow'>
{code}
The question that naturally follows from this JIRA is: *Are all the default
options in PyArrow and R bindings consistent with C++ defaults?*
> [C++] Enable slicing to end of string using "utf8_slice_codeunits" when
> string length unknown or different lengths
> -------------------------------------------------------------------------------------------------------------------
>
> Key: ARROW-13259
> URL: https://issues.apache.org/jira/browse/ARROW-13259
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Nic Crane
> Priority: Major
>
> We're currently trying to write bindings from the C++ function
> "utf8_slice_codeunits" to R, specifically trying to replicate the behaviour
> of R's string::str_sub
> In both the R and C++ implementations, I can use negative indices to count
> back from the end of a string (show below in R, but the latter directly
> invokes the C++ implementation):
>
> {code:java}
> # stringr version
> > stringr::str_sub("Apache Arrow", -5, -2)
> [1] "Arro"
> # C++ version
> > call_function("utf8_slice_codeunits", Scalar$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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