FromPackedArray

Status: Stable

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

Description

FromPackedArray[expr]

is FromNDArray[expr]: it returns expr with any dense buffer storage undone, so a packed List becomes an ordinary List of separate elements and an NDArray[...] becomes the nested List of its entries. Provided under Mathematica's name for the same operation; see FromNDArray.

Examples (6)

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

Basic examples (3)

In[1]:= FromPackedArray[ToPackedArray[{1., 2., 3., 4.}]]
Out[1]= {1.0, 2.0, 3.0, 4.0}

In[2]:= NDArrayQ[FromPackedArray[ToPackedArray[{1., 2., 3., 4.}]]]
Out[2]= False

In[3]:= FromPackedArray[NDArray[{1., 2., 3.}]]
Out[3]= {1.0, 2.0, 3.0}

Applications (3)

Mathematica's name; undoes the buffer

In[4]:= FromPackedArray[ToPackedArray[{1., 2., 3., 4.}]]
Out[4]= {1.0, 2.0, 3.0, 4.0}

The result is an ordinary List

In[5]:= NDArrayQ[FromPackedArray[ToPackedArray[{1., 2., 3., 4.}]]]
Out[5]= False

Unpacks a visible NDArray too

In[6]:= FromPackedArray[NDArray[{1., 2., 3.}]]
Out[6]= {1.0, 2.0, 3.0}

Implementation notes

Algorithm. FromPackedArray is Mathematica's name (Developer​FromPackedArray) for FromNDArray and is the same C builtin (builtin_fromndarray) registered under a second name — an aliased builtin, not a DownValue that rewrites to the other, so it costs no extra evaluation pass and cannot be shadowed. It undoes both forms of buffer storage: a packed List becomes an ordinary List of separate element nodes (NDArrayQ then False), and a visible NDArray[...] becomes the nested List of its entries (ndarray_to_nested_list). Anything that is not an EXPR_NDARRAY is returned unchanged. It is the inverse of ToPackedArray.

Data structures. Reads the dense row-major buffer and allocates one boxed Expr leaf per element (Integer/Real/True/False by dtype), rebuilding the nested List from the stored int64 dims.

Complexity / limits. O(n), allocating an expression node per element — the per-element cost packing avoids, so this is where a large array stops being cheap to hold. The value is unchanged; only the representation differs.

Attributes: Protected.

References

See also: FromNDArray, ToPackedArray, List

• Source: src/pack.c
• Specification: docs/spec/builtins/packed-arrays.md
• Tests: tests/test_dirichletl.c
• Tests: tests/test_elliptic_machine.c
• Tests: tests/test_erf.c
• Tests: tests/test_erfc.c

Notes & additional examples

Notes

FromPackedArray is the Wolfram Language's name (Developer​FromPackedArray) for FromNDArray, and is the same builtin registered twice — not a rule that rewrites to the other — so it costs no extra evaluation pass and cannot be shadowed. It undoes both forms of buffer storage: a packed List becomes an ordinary List of separate elements (so NDArrayQ is then False), and a visible NDArray[...] becomes the nested List of its entries. Anything else is returned unchanged. It is the inverse of ToPackedArray.

The value is unchanged; only the representation differs. Like FromNDArray, it allocates one expression node per element — the per-element cost that packing avoids.