projects / OpenSPARC-T2-SAM / blame
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| 53 | <H1><A NAME="SECTION000700000000000000000"> | |
| 54 | 6 PEP 322: Reverse Iteration</A> | |
| 55 | </H1> | |
| 56 | ||
| 57 | <P> | |
| 58 | A new built-in function, <tt class="function">reversed(<var>seq</var>)</tt>, takes a sequence | |
| 59 | and returns an iterator that loops over the elements of the sequence | |
| 60 | in reverse order. | |
| 61 | ||
| 62 | <P> | |
| 63 | <div class="verbatim"><pre> | |
| 64 | >>> for i in reversed(xrange(1,4)): | |
| 65 | ... print i | |
| 66 | ... | |
| 67 | 3 | |
| 68 | 2 | |
| 69 | 1 | |
| 70 | </pre></div> | |
| 71 | ||
| 72 | <P> | |
| 73 | Compared to extended slicing, such as <code>range(1,4)[::-1]</code>, | |
| 74 | <tt class="function">reversed()</tt> is easier to read, runs faster, and uses | |
| 75 | substantially less memory. | |
| 76 | ||
| 77 | <P> | |
| 78 | Note that <tt class="function">reversed()</tt> only accepts sequences, not arbitrary | |
| 79 | iterators. If you want to reverse an iterator, first convert it to | |
| 80 | a list with <tt class="function">list()</tt>. | |
| 81 | ||
| 82 | <P> | |
| 83 | <div class="verbatim"><pre> | |
| 84 | >>> input = open('/etc/passwd', 'r') | |
| 85 | >>> for line in reversed(list(input)): | |
| 86 | ... print line | |
| 87 | ... | |
| 88 | root:*:0:0:System Administrator:/var/root:/bin/tcsh | |
| 89 | ... | |
| 90 | </pre></div> | |
| 91 | ||
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| 93 | <div class="seealso"> | |
| 94 | <p class="heading">See Also:</p> | |
| 95 | ||
| 96 | <dl compact="compact" class="seerfc"> | |
| 97 | <dt><a href="http://www.python.org/peps/pep-0322.html" | |
| 98 | title="Reverse Iteration" | |
| 99 | >PEP 322, <em>Reverse Iteration</em></a> | |
| 100 | <dd>Written and implemented by Raymond Hettinger. | |
| 101 | </dl> | |
| 102 | ||
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