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Diffstat (limited to 'doc/source/reference/arrays.nditer.rst')
| -rw-r--r-- | doc/source/reference/arrays.nditer.rst | 81 |
1 files changed, 55 insertions, 26 deletions
diff --git a/doc/source/reference/arrays.nditer.rst b/doc/source/reference/arrays.nditer.rst index fa8183f75..7dab09a71 100644 --- a/doc/source/reference/arrays.nditer.rst +++ b/doc/source/reference/arrays.nditer.rst @@ -115,13 +115,18 @@ context is exited. array([[ 0, 2, 4], [ 6, 8, 10]]) +If you are writing code that needs to support older versions of numpy, +note that prior to 1.15, :class:`nditer` was not a context manager and +did not have a `close` method. Instead it relied on the destructor to +initiate the writeback of the buffer. + Using an External Loop ---------------------- In all the examples so far, the elements of `a` are provided by the iterator one at a time, because all the looping logic is internal to the -iterator. While this is simple and convenient, it is not very efficient. A -better approach is to move the one-dimensional innermost loop into your +iterator. While this is simple and convenient, it is not very efficient. +A better approach is to move the one-dimensional innermost loop into your code, external to the iterator. This way, NumPy's vectorized operations can be used on larger chunks of the elements being visited. @@ -156,41 +161,29 @@ element in a computation. For example, you may want to visit the elements of an array in memory order, but use a C-order, Fortran-order, or multidimensional index to look up values in a different array. -The Python iterator protocol doesn't have a natural way to query these -additional values from the iterator, so we introduce an alternate syntax -for iterating with an :class:`nditer`. This syntax explicitly works -with the iterator object itself, so its properties are readily accessible -during iteration. With this looping construct, the current value is -accessible by indexing into the iterator, and the index being tracked -is the property `index` or `multi_index` depending on what was requested. - -The Python interactive interpreter unfortunately prints out the -values of expressions inside the while loop during each iteration of the -loop. We have modified the output in the examples using this looping -construct in order to be more readable. +The index is tracked by the iterator object itself, and accessible +through the `index` or `multi_index` properties, depending on what was +requested. The examples below show printouts demonstrating the +progression of the index: .. admonition:: Example >>> a = np.arange(6).reshape(2,3) >>> it = np.nditer(a, flags=['f_index']) - >>> while not it.finished: - ... print("%d <%d>" % (it[0], it.index), end=' ') - ... it.iternext() + >>> for x in it: + ... print("%d <%d>" % (x, it.index), end=' ') ... 0 <0> 1 <2> 2 <4> 3 <1> 4 <3> 5 <5> >>> it = np.nditer(a, flags=['multi_index']) - >>> while not it.finished: - ... print("%d <%s>" % (it[0], it.multi_index), end=' ') - ... it.iternext() + >>> for x in it: + ... print("%d <%s>" % (x, it.multi_index), end=' ') ... 0 <(0, 0)> 1 <(0, 1)> 2 <(0, 2)> 3 <(1, 0)> 4 <(1, 1)> 5 <(1, 2)> - >>> it = np.nditer(a, flags=['multi_index'], op_flags=['writeonly']) - >>> with it: - .... while not it.finished: - ... it[0] = it.multi_index[1] - it.multi_index[0] - ... it.iternext() + >>> with np.nditer(a, flags=['multi_index'], op_flags=['writeonly']) as it: + ... for x in it: + ... x[...] = it.multi_index[1] - it.multi_index[0] ... >>> a array([[ 0, 1, 2], @@ -199,7 +192,7 @@ construct in order to be more readable. Tracking an index or multi-index is incompatible with using an external loop, because it requires a different index value per element. If you try to combine these flags, the :class:`nditer` object will -raise an exception +raise an exception. .. admonition:: Example @@ -209,6 +202,42 @@ raise an exception File "<stdin>", line 1, in <module> ValueError: Iterator flag EXTERNAL_LOOP cannot be used if an index or multi-index is being tracked +Alternative Looping and Element Access +-------------------------------------- + +To make its properties more readily accessible during iteration, +:class:`nditer` has an alternative syntax for iterating, which works +explicitly with the iterator object itself. With this looping construct, +the current value is accessible by indexing into the iterator. Other +properties, such as tracked indices remain as before. The examples below +produce identical results to the ones in the previous section. + +.. admonition:: Example + + >>> a = np.arange(6).reshape(2,3) + >>> it = np.nditer(a, flags=['f_index']) + >>> while not it.finished: + ... print("%d <%d>" % (it[0], it.index), end=' ') + ... it.iternext() + ... + 0 <0> 1 <2> 2 <4> 3 <1> 4 <3> 5 <5> + + >>> it = np.nditer(a, flags=['multi_index']) + >>> while not it.finished: + ... print("%d <%s>" % (it[0], it.multi_index), end=' ') + ... it.iternext() + ... + 0 <(0, 0)> 1 <(0, 1)> 2 <(0, 2)> 3 <(1, 0)> 4 <(1, 1)> 5 <(1, 2)> + + >>> with np.nditer(a, flags=['multi_index'], op_flags=['writeonly']) as it: + ... while not it.finished: + ... it[0] = it.multi_index[1] - it.multi_index[0] + ... it.iternext() + ... + >>> a + array([[ 0, 1, 2], + [-1, 0, 1]]) + Buffering the Array Elements ---------------------------- |