Merge pull request #23957 from jakevdp:choice-doc

PiperOrigin-RevId: 679657248

jax/_src/numpy/lax_numpy.py

@@ -4388,9 +4388,85 @@ def column_stack(tup: np.ndarray | Array | Sequence[ArrayLike]) -> Array:
   return concatenate(arrs, axis=1)
 
 
-@util.implements(np.choose, skip_params=['out'])
-def choose(a: ArrayLike, choices: Sequence[ArrayLike],
+def choose(a: ArrayLike, choices: Array | np.ndarray | Sequence[ArrayLike],
            out: None = None, mode: str = 'raise') -> Array:
+  """Construct an array by stacking slices of choice arrays.
+
+  JAX implementation of :func:`numpy.choose`.
+
+  The semantics of this function can be confusing, but in the simplest case where
+  ``a`` is a one-dimensional array, ``choices`` is a two-dimensional array, and
+  all entries of ``a`` are in-bounds (i.e. ``0 <= a_i < len(choices)``), then the
+  function is equivalent to the following::
+
+     def choose(a, choices):
+       return jnp.array([choices[a_i, i] for i, a_i in enumerate(a)])
+
+  In the more general case, ``a`` may have any number of dimensions and ``choices``
+  may be an arbitrary sequence of broadcast-compatible arrays. In this case, again
+  for in-bound indices, the logic is equivalent to::
+
+     def choose(a, choices):
+       a, *choices = jnp.broadcast_arrays(a, *choices)
+       choices = jnp.array(choices)
+       return jnp.array([choices[a[idx], *idx] for idx in np.ndindex(a.shape)])
+
+  The only additional complexity comes from the ``mode`` argument, which controls
+  the behavior for out-of-bound indices in ``a`` as described below.
+
+  Args:
+    a: an N-dimensional array of integer indices.
+    choices: an array or sequence of arrays. All arrays in the sequence must be
+      mutually broadcast compatible with ``a``.
+    out: unused by JAX
+    mode: specify the out-of-bounds indexing mode; one of ``'raise'`` (default),
+      ``'wrap'``, or ``'clip'``. Note that the default mode of ``'raise'`` is
+      not compatible with JAX transformations.
+
+  Returns:
+    an array containing stacked slices from ``choices`` at the indices
+    specified by ``a``. The shape of the result is
+    ``broadcast_shapes(a.shape, *(c.shape for c in choices))``.
+
+  See also:
+    - :func:`jax.lax.switch`: choose between N functions based on an index.
+
+  Examples:
+    Here is the simplest case of a 1D index array with a 2D choice array,
+    in which case this chooses the indexed value from each column:
+
+    >>> choices = jnp.array([[ 1,  2,  3,  4],
+    ...                      [ 5,  6,  7,  8],
+    ...                      [ 9, 10, 11, 12]])
+    >>> a = jnp.array([2, 0, 1, 0])
+    >>> jnp.choose(a, choices)
+    Array([9, 2, 7, 4], dtype=int32)
+
+    The ``mode`` argument specifies what to do with out-of-bound indices;
+    options are to either ``wrap`` or ``clip``:
+
+    >>> a2 = jnp.array([2, 0, 1, 4])  # last index out-of-bound
+    >>> jnp.choose(a2, choices, mode='clip')
+    Array([ 9,  2,  7, 12], dtype=int32)
+    >>> jnp.choose(a2, choices, mode='wrap')
+    Array([9, 2, 7, 8], dtype=int32)
+
+    In the more general case, ``choices`` may be a sequence of array-like
+    objects with any broadcast-compatible shapes.
+
+    >>> choice_1 = jnp.array([1, 2, 3, 4])
+    >>> choice_2 = 99
+    >>> choice_3 = jnp.array([[10],
+    ...                       [20],
+    ...                       [30]])
+    >>> a = jnp.array([[0, 1, 2, 0],
+    ...                [1, 2, 0, 1],
+    ...                [2, 0, 1, 2]])
+    >>> jnp.choose(a, [choice_1, choice_2, choice_3], mode='wrap')
+    Array([[ 1, 99, 10,  4],
+           [99, 20,  3, 99],
+           [30,  2, 99, 30]], dtype=int32)
+  """
   if out is not None:
     raise NotImplementedError("The 'out' argument to jnp.choose is not supported.")
   util.check_arraylike('choose', a, *choices)

jax/numpy/__init__.pyi

@@ -218,7 +218,7 @@ def cbrt(x: ArrayLike, /) -> Array: ...
 cdouble: Any
 def ceil(x: ArrayLike, /) -> Array: ...
 character = _np.character
-def choose(a: ArrayLike, choices: Sequence[ArrayLike],
+def choose(a: ArrayLike, choices: Array | _np.ndarray | Sequence[ArrayLike],
            out: None = ..., mode: str = ...) -> Array: ...
 def clip(
     x: ArrayLike | None = ...,