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Diffstat (limited to 'doc/source/reference/arrays.indexing.rst')
| -rw-r--r-- | doc/source/reference/arrays.indexing.rst | 34 |
1 files changed, 23 insertions, 11 deletions
diff --git a/doc/source/reference/arrays.indexing.rst b/doc/source/reference/arrays.indexing.rst index 62d36e28c..8ec8d8330 100644 --- a/doc/source/reference/arrays.indexing.rst +++ b/doc/source/reference/arrays.indexing.rst @@ -3,6 +3,10 @@ Indexing ======== +.. seealso:: + + :ref:`Indexing basics <basics.indexing>` + .. sectionauthor:: adapted from "Guide to NumPy" by Travis E. Oliphant .. currentmodule:: numpy @@ -57,6 +61,17 @@ interpreted as counting from the end of the array (*i.e.*, if All arrays generated by basic slicing are always :term:`views <view>` of the original array. +.. note:: + + NumPy slicing creates a :term:`view` instead of a copy as in the case of + builtin Python sequences such as string, tuple and list. + Care must be taken when extracting + a small portion from a large array which becomes useless after the + extraction, because the small portion extracted contains a reference + to the large original array whose memory will not be released until + all arrays derived from it are garbage-collected. In such cases an + explicit ``copy()`` is recommended. + The standard rules of sequence slicing apply to basic slicing on a per-dimension basis (including using a step index). Some useful concepts to remember include: @@ -111,9 +126,10 @@ concepts to remember include: [5], [6]]]) -- :const:`Ellipsis` expand to the number of ``:`` objects needed to - make a selection tuple of the same length as ``x.ndim``. There may - only be a single ellipsis present. +- :const:`Ellipsis` expands to the number of ``:`` objects needed for the + selection tuple to index all dimensions. In most cases, this means that + length of the expanded selection tuple is ``x.ndim``. There may only be a + single ellipsis present. .. admonition:: Example @@ -513,14 +529,10 @@ only the part of the data in the specified field. Also :ref:`record array <arrays.classes.rec>` scalars can be "indexed" this way. Indexing into a structured array can also be done with a list of field names, -*e.g.* ``x[['field-name1','field-name2']]``. Currently this returns a new -array containing a copy of the values in the fields specified in the list. -As of NumPy 1.7, returning a copy is being deprecated in favor of returning -a view. A copy will continue to be returned for now, but a FutureWarning -will be issued when writing to the copy. If you depend on the current -behavior, then we suggest copying the returned array explicitly, i.e. use -x[['field-name1','field-name2']].copy(). This will work with both past and -future versions of NumPy. +*e.g.* ``x[['field-name1','field-name2']]``. As of NumPy 1.16 this returns a +view containing only those fields. In older versions of numpy it returned a +copy. See the user guide section on :ref:`structured_arrays` for more +information on multifield indexing. If the accessed field is a sub-array, the dimensions of the sub-array are appended to the shape of the result. |