From 2db3fb25f67d20cfbc8c1a59c2b5dc8a3f4da96f Mon Sep 17 00:00:00 2001 From: Stefan Behnel Date: Sat, 13 Dec 2008 09:24:46 +0100 Subject: [PATCH] clarification: prefer for-in-range over for-from --- docs/language_basics.rst | 25 +++++++++++-------------- 1 file changed, 11 insertions(+), 14 deletions(-) diff --git a/docs/language_basics.rst b/docs/language_basics.rst index 12348fdc..082d2102 100644 --- a/docs/language_basics.rst +++ b/docs/language_basics.rst @@ -295,13 +295,21 @@ Python and C, and that Cython uses the Python precedences, not the C ones. Integer for-loops ------------------ -You should be aware that a for-loop such as:: +Cython recognises the usual Python for-in-range integer loop pattern:: for i in range(n): ... -won't be very fast if ``i`` is not a :keyword:`cdef` integer type. -For iterating over ranges of integers, Cython has another form of for-loop:: +If ``i`` is declared as a :keyword:`cdef` integer type, it will +optimise this into a pure C loop. This restriction is required as +otherwise the generated code wouldn't be correct due to potential +integer overflows on the target architecture. If you are worried that +the loop is not being converted correctly, use the annotate feature of +the cython commandline (``-a``) to easily see the generated C code. +See :ref:`automatic-range-conversion` + +For backwards compatibility to Pyrex, Cython also supports another +form of for-loop:: for i from 0 <= i < n: ... @@ -313,17 +321,6 @@ or:: where ``s`` is some integer step size. -If the loop variable and the lower and upper bounds are all C integers, this -form of loop will be much faster, because Cython will translate it into pure C -code. - -.. note:: - This is not necessary if ``i`` is a C integer type and ``n`` can be - determined at compile time. Just use the idiomatic :func:`range` loop, if - you are worried that the loop is not being converted correctly use the - annotate feature of the cython commandline (``-a``) to easily see the - generated C code. See :ref:`automatic-range-conversion` - Some things to note about the for-from loop: * The target expression must be a variable name. -- 2.26.2