jorisvandenbossche commented on a change in pull request #8990:
URL: https://github.com/apache/arrow/pull/8990#discussion_r601492842
##########
File path: cpp/src/arrow/compute/kernels/scalar_string_test.cc
##########
@@ -428,6 +433,26 @@ TYPED_TEST(TestStringKernels,
StrptimeDoesNotProvideDefaultOptions) {
ASSERT_RAISES(Invalid, CallFunction("strptime", {input}));
}
+TYPED_TEST(TestStringKernels, Join) {
+ auto separator = this->scalar("--");
+ CheckScalarBinary(
+ "binary_join", list(this->type()),
+ R"([["a", "bb", "ccc"], [], null, ["dd"], ["eee", null], ["ff", ""]])",
separator,
+ this->type(), R"(["a--bb--ccc", "", null, "dd", null, "ff--"])");
Review comment:
Looking at how pandas / R does this right now:
Pandas actually results in a missing value:
```
In [7]: s = pd.Series([["a", None, "b"], ["c", "d", "e"]])
In [8]: s.str.join("-")
Out[8]:
0 NaN
1 c-d-e
dtype: object
```
but I doubt that this is necessarily intentional (when it comes to missing
values). The docstring basically says that if there any non-string element in a
list, the result is NaN for that list. And I assume this is just because in
that case the stdlib `"-".join(..)` which is used under the hood fails.
For R, I am not fully sure what's the most "typical" way to do this, but
with the basic `paste`, you actually get the string representation of NA:
```
> paste("a", NA, "b", sep="-")
[1] "a-NA-b"
```
(but `paste` is more general in converting input to strings first).
Using a more specific string function from `stringr`, it actually results in
NA:
```
> str_c("a", "b", sep="-")
[1] "a-b"
> str_c("a", NA, "b", sep="-")
[1] NA
```
where the docstring explicitly mentions this (*"Like most other R functions,
missing values are "infectious": whenever a missing value is combined with
another string the result will always be missing"*). So that could be an
argument for returning null. But since in Arrow the default is that nulls are
*not* infectious, it could also be an argument to skip it ;)
cc @ianmcook
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