edponce commented on a change in pull request #10395:
URL: https://github.com/apache/arrow/pull/10395#discussion_r656468614
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File path: cpp/src/arrow/compute/api_scalar.cc
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@@ -50,6 +50,11 @@ namespace compute {
SCALAR_ARITHMETIC_UNARY(AbsoluteValue, "abs", "abs_checked")
SCALAR_ARITHMETIC_UNARY(Negate, "negate", "negate_checked")
+Result<Datum> Sign(const Datum& arg, SignOptions options, ExecContext* ctx) {
+ auto func_name = options.with_signed_zero ? "sign_with_signed_zero" : "sign";
Review comment:
Also, I consider that `copysign` should not be exposed as a compute
function but rather used internally, and instead we should expose `sign` and
`sign_with_signed_zero` (implementations may use `copysign(x, 1)`. The
rationale for this is that `copysign(x, 1)` is not common in any SQL,
dataframes library but `sign` is.
A use case of `sign` is to build a custom ranking based on the signedness of
values.
For example, consider ranking values relative to a constant value (e.g., 3):
```
SELECT CASE
WHEN @a < 3 THEN 0
WHEN @a = 3 THEN 1
WHEN @a > 3 THEN 2
END
```
is not supported because expressions are not allowed in `CASE-WHEN`
constructs, so a workaround is
```
SELECT CASE SIGN(@a - 3)
WHEN -1 THEN 0
WHEN 0 THEN 1
WHEN 1 THEN 2
END
```
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