projects / OpenSPARC-T2-DV / blame
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| 54 | ||
| 55 | <H1><A NAME="SECTION009300000000000000000"></A><A NAME="for"></A> | |
| 56 | <BR> | |
| 57 | 7.3 The <tt class="keyword">for</tt> statement | |
| 58 | </H1> | |
| 59 | <a id='l2h-581' xml:id='l2h-581'></a><a id='l2h-582' xml:id='l2h-582'></a> | |
| 60 | <P> | |
| 61 | The <tt class="keyword">for</tt> statement is used to iterate over the elements of a | |
| 62 | sequence (such as a string, tuple or list) or other iterable object: | |
| 63 | <a id='l2h-583' xml:id='l2h-583'></a> | |
| 64 | <P> | |
| 65 | <dl><dd class="grammar"> | |
| 66 | <div class="productions"> | |
| 67 | <table> | |
| 68 | <tr> | |
| 69 | <td><a id='tok-for_stmt' xml:id='tok-for_stmt'>for_stmt</a></td> | |
| 70 | <td>::=</td> | |
| 71 | <td>"for" <a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> "in" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> | |
| 72 | ":" <a class='grammartoken' href="compound.html#tok-suite">suite</a></td></tr> | |
| 73 | <tr> | |
| 74 | <td></td> | |
| 75 | <td></td> | |
| 76 | <td><code>["else" ":" <a class='grammartoken' href="compound.html#tok-suite">suite</a>]</code></td></tr> | |
| 77 | </table> | |
| 78 | </div> | |
| 79 | <a class="grammar-footer" | |
| 80 | href="grammar.txt" type="text/plain" | |
| 81 | >Download entire grammar as text.</a> | |
| 82 | </dd></dl> | |
| 83 | ||
| 84 | <P> | |
| 85 | The expression list is evaluated once; it should yield an iterable | |
| 86 | object. An iterator is created for the result of the | |
| 87 | <code>expression_list</code>. The suite is then executed once for each | |
| 88 | item provided by the iterator, in the | |
| 89 | order of ascending indices. Each item in turn is assigned to the | |
| 90 | target list using the standard rules for assignments, and then the | |
| 91 | suite is executed. When the items are exhausted (which is immediately | |
| 92 | when the sequence is empty), the suite in the <tt class="keyword">else</tt> clause, if | |
| 93 | present, is executed, and the loop terminates. | |
| 94 | <a id='l2h-584' xml:id='l2h-584'></a><a id='l2h-585' xml:id='l2h-585'></a><a id='l2h-586' xml:id='l2h-586'></a> | |
| 95 | <P> | |
| 96 | A <tt class="keyword">break</tt> statement executed in the first suite terminates the | |
| 97 | loop without executing the <tt class="keyword">else</tt> clause's suite. A | |
| 98 | <tt class="keyword">continue</tt> statement executed in the first suite skips the rest | |
| 99 | of the suite and continues with the next item, or with the <tt class="keyword">else</tt> | |
| 100 | clause if there was no next item. | |
| 101 | <a id='l2h-587' xml:id='l2h-587'></a><a id='l2h-588' xml:id='l2h-588'></a> | |
| 102 | <P> | |
| 103 | The suite may assign to the variable(s) in the target list; this does | |
| 104 | not affect the next item assigned to it. | |
| 105 | ||
| 106 | <P> | |
| 107 | The target list is not deleted when the loop is finished, but if the | |
| 108 | sequence is empty, it will not have been assigned to at all by the | |
| 109 | loop. Hint: the built-in function <tt class="function">range()</tt> returns a | |
| 110 | sequence of integers suitable to emulate the effect of Pascal's | |
| 111 | <code>for i := a to b do</code>; | |
| 112 | e.g., <code>range(3)</code> returns the list <code>[0, 1, 2]</code>. | |
| 113 | <a id='l2h-589' xml:id='l2h-589'></a><a id='l2h-590' xml:id='l2h-590'></a> | |
| 114 | <P> | |
| 115 | <span class="warning"><b class="label">Warning:</b> | |
| 116 | There is a subtlety when the sequence is being modified | |
| 117 | by the loop (this can only occur for mutable sequences, i.e. lists). | |
| 118 | An internal counter is used to keep track of which item is used next, | |
| 119 | and this is incremented on each iteration. When this counter has | |
| 120 | reached the length of the sequence the loop terminates. This means that | |
| 121 | if the suite deletes the current (or a previous) item from the | |
| 122 | sequence, the next item will be skipped (since it gets the index of | |
| 123 | the current item which has already been treated). Likewise, if the | |
| 124 | suite inserts an item in the sequence before the current item, the | |
| 125 | current item will be treated again the next time through the loop. | |
| 126 | This can lead to nasty bugs that can be avoided by making a temporary | |
| 127 | copy using a slice of the whole sequence, e.g., | |
| 128 | <a id='l2h-591' xml:id='l2h-591'></a></span> | |
| 129 | ||
| 130 | <P> | |
| 131 | <div class="verbatim"><pre> | |
| 132 | for x in a[:]: | |
| 133 | if x < 0: a.remove(x) | |
| 134 | </pre></div> | |
| 135 | ||
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| 172 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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