PackedArrayQ

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

PackedArrayQ[expr]

Gives True if expr is a packed array -- a List stored as a dense machine-precision buffer -- else False. Provided under Mathematica's name; unlike NDArrayQ, a visible NDArray[...] object gives False.

Examples (7)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (4)

In[1]:= PackedArrayQ[ToPackedArray[{1., 2., 3.}]]
Out[1]= True

In[2]:= PackedArrayQ[NDArray[{1., 2., 3.}]]
Out[2]= False

In[3]:= NDArrayQ[NDArray[{1., 2., 3.}]]
Out[3]= True

In[4]:= PackedArrayQ[{a, b, c}]
Out[4]= False

Applications (3)

A packed List

In[5]:= PackedArrayQ[ToPackedArray[{1., 2., 3.}]]
Out[5]= True

A visible NDArray is not a packed List

In[6]:= PackedArrayQ[NDArray[{1., 2., 3.}]]
Out[6]= False

An ordinary unpacked List

In[7]:= PackedArrayQ[{a, b, c}]
Out[7]= False

Implementation notes

Algorithm. builtin_packedarrayq is a one-argument predicate that returns the symbol True when its argument is a packed array — a List the system stores as a dense machine-precision buffer — and False otherwise. The test is exactly is_packed_list, so it is deliberately narrower than NDArrayQ: NDArrayQ is True for either surface of an EXPR_NDARRAY (the packed-List surface or a visible NDArray[...] object), whereas PackedArrayQ is True only for the packed-List surface. A visible NDArray[...] is a distinct atom (AtomQ true, ListQ false) rather than a List that happens to be packed, so it gives False — matching Mathematica (Developer​PackedArrayQ), which has no visible NDArray head at all. The two predicates differ only on that one input.

For the answer to be correct the head must itself be packed-aware: PackedArrayQ is on both the AWARE and INT64_OK lists in src/pack.c. Otherwise the evaluator's transparency gate would materialise a packed argument into a plain List before the builtin runs, and it could only ever observe an already-unpacked list and answer False for every packed input.

Data structures. None of its own — it inspects the argument's tag (EXPR_NDARRAY with the packed-List presentation) and allocates only the returned True/False symbol.

Complexity / limits. O(1). Protected.

Attributes: Protected.

References

See also: List, NDArrayQ, AtomQ, ListQ, NDArray

• Source: src/ndarray.c
• Specification: docs/spec/builtins/packed-arrays.md
• Tests: tests/test_dirichletcharacter.c
• Tests: tests/test_dirichletl.c
• Tests: tests/test_elliptic_machine.c
• Tests: tests/test_expint_machine.c

Notes & additional examples

Notes

PackedArrayQ[expr] gives True when expr is a packed array — a List stored as a dense machine-precision buffer — and False otherwise. It is the Wolfram Language's name (Developer​PackedArrayQ) for that test.

It is deliberately narrower than NDArrayQ. NDArrayQ is True for either surface of the internal array object — a packed List or a visible NDArray[...]. PackedArrayQ is True only for the packed-List surface: a visible NDArray[...] is a distinct atom (AtomQ is True, ListQ is False), not a List that happens to be packed, so it gives False — matching the Wolfram Language, which has no visible NDArray head at all. The two predicates differ only on that one input.

Like NDArrayQ, PackedArrayQ is packed-aware, so the evaluator hands it a packed argument intact (for int64 buffers as well as float64) instead of unpacking it first — without that it could only ever observe an already-unpacked List and would answer False for every packed input. Protected.