[C++] Enable Array creation from Scalar

[vyasr]

Describe the enhancement requested

Currently creating an ArrowArray from a Scalar requires going through the Builder interface. While this is fine as a general solution, a common special case is the creation of an Array from a single Scalar (possibly copying that Scalar N times). Abstracting out the handling of different types could be helpful for this case. If there is interest in this feature, there is code that I wrote for this task in cudf that could easily be extracted into arrow C++ (we're likely to remove that functionality from libcudf).

Concretely, this would be something like an API with the signature:

arrow::Array from_scalar(arrow::Scalar const& input, size_type N=1)

Apologies if there is such a function somewhere in the C++ and I missed it.

Component(s)

C++

[kou]

It seems that we have arrow::MakeArrayFromScalar():

arrow/cpp/src/arrow/array/util.h

Lines 49 to 55 in 6406676

	/// \brief Create an Array instance whose slots are the given scalar
	/// \param[in] scalar the value with which to fill the array
	/// \param[in] length the array length
	/// \param[in] pool the memory pool to allocate memory from
	ARROW_EXPORT
	Result<std::shared_ptr<Array>> MakeArrayFromScalar(
	const Scalar& scalar, int64_t length, MemoryPool* pool = default_memory_pool());

Could you take a look at it?

[vyasr]

That looks exactly like what I was envisioning. I've tested it and it works just as I'd expect. I can't find that function anywhere in the C++ documentation, though. Could we add it somewhere? Perhaps to a factories section on the Arrays page similar to the one that exists for Scalars?

[kou]

It makes sense. Could you work on this?

[vyasr]

@kou yup! I opened #40373 for this.

[pitrou]

Issue resolved by pull request 40373
#40373