docs up

docs/build/html/_sources/usage/indexing.rst

@@ -0,0 +1,123 @@
+.. _indexing:
+
+Indexing Arrays
+===============
+
+.. currentmodule:: mlx.core
+
+For the most part, indexing an MLX :obj:`array` works the same as indexing a
+NumPy :obj:`numpy.ndarray`. See the `NumPy documentation
+<https://numpy.org/doc/stable/user/basics.indexing.html>`_ for more details on
+how that works.
+
+For example, you can use regular integers and slices (:obj:`slice`) to index arrays:
+
+.. code-block:: shell
+
+  >>> arr = mx.arange(10)
+  >>> arr[3]
+  array(3, dtype=int32)
+  >>> arr[-2]  # negative indexing works
+  array(8, dtype=int32)
+  >>> arr[2:8:2] # start, stop, stride
+  array([2, 4, 6], dtype=int32)
+
+For multi-dimensional arrays, the ``...`` or :obj:`Ellipsis` syntax works as in NumPy:
+
+.. code-block:: shell
+
+  >>> arr = mx.arange(8).reshape(2, 2, 2)
+  >>> arr[:, :, 0]
+  array(3, dtype=int32)
+  array([[0, 2],
+         [4, 6]], dtype=int32
+  >>> arr[..., 0]
+  array([[0, 2],
+         [4, 6]], dtype=int32
+
+You can index with ``None`` to create a new axis:
+
+.. code-block:: shell
+
+  >>> arr = mx.arange(8)
+  >>> arr.shape
+  [8]
+  >>> arr[None].shape
+  [1, 8]
+
+
+You can also use an :obj:`array` to index another :obj:`array`:
+
+.. code-block:: shell
+
+  >>> arr = mx.arange(10)
+  >>> idx = mx.array([5, 7]) 
+  >>> arr[idx]
+  array([5, 7], dtype=int32)
+
+Mixing and matching integers, :obj:`slice`, ``...``, and :obj:`array` indices
+works just as in NumPy.
+
+Other functions which may be useful for indexing arrays are :func:`take` and
+:func:`take_along_axis`.
+
+Differences from NumPy
+----------------------
+
+.. Note::
+
+  MLX indexing is different from NumPy indexing in two important ways:
+
+  * Indexing does not perform bounds checking. Indexing out of bounds is
+    undefined behavior.
+  * Boolean mask based indexing is not yet supported.
+
+The reason for the lack of bounds checking is that exceptions cannot propagate
+from the GPU. Performing bounds checking for array indices before launching the
+kernel would be extremely inefficient.
+
+Indexing with boolean masks is something that MLX may support in the future. In
+general, MLX has limited support for operations for which outputs
+*shapes* are dependent on input *data*. Other examples of these types of
+operations which MLX does not yet support include :func:`numpy.nonzero` and the
+single input version of :func:`numpy.where`.
+
+In Place Updates 
+----------------
+
+In place updates to indexed arrays are possible in MLX. For example:
+
+.. code-block:: shell
+
+  >>> a = mx.array([1, 2, 3])
+  >>> a[2] = 0
+  >>> a
+  array([1, 2, 0], dtype=int32)
+
+Just as in NumPy, in place updates will be reflected in all references to the
+same array:
+
+.. code-block:: shell
+
+  >>> a = mx.array([1, 2, 3])
+  >>> b = a
+  >>> b[2] = 0
+  >>> b
+  array([1, 2, 0], dtype=int32)
+  >>> a
+  array([1, 2, 0], dtype=int32)
+
+Transformations of functions which use in-place updates are allowed and work as
+expected. For example:
+
+.. code-block:: python
+
+   def fun(x, idx):
+       x[idx] = 2.0
+       return x.sum()
+
+   dfdx = mx.grad(fun)(mx.array([1.0, 2.0, 3.0]), mx.array([1]))
+   print(dfdx)  # Prints: array([1, 0, 1], dtype=float32)
+
+In the above ``dfdx`` will have the correct gradient, namely zeros at ``idx``
+and ones elsewhere.