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| <h3 class="section">9.3 Array Sort Function</h3> |
| |
| <p><a name="index-sort-function-_0028for-arrays_0029-828"></a><a name="index-quick-sort-function-_0028for-arrays_0029-829"></a><a name="index-array-sort-function-830"></a> |
| To sort an array using an arbitrary comparison function, use the |
| <code>qsort</code> function. The prototype for this function is in |
| <samp><span class="file">stdlib.h</span></samp>. |
| <a name="index-stdlib_002eh-831"></a> |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: void <b>qsort</b> (<var>void *array, size_t count, size_t size, comparison_fn_t compare</var>)<var><a name="index-qsort-832"></a></var><br> |
| <blockquote><p>The <var>qsort</var> function sorts the array <var>array</var>. The array contains |
| <var>count</var> elements, each of which is of size <var>size</var>. |
| |
| <p>The <var>compare</var> function is used to perform the comparison on the |
| array elements. This function is called with two pointer arguments and |
| should return an integer less than, equal to, or greater than zero |
| corresponding to whether its first argument is considered less than, |
| equal to, or greater than its second argument. |
| |
| <p><a name="index-stable-sorting-833"></a><strong>Warning:</strong> If two objects compare as equal, their order after |
| sorting is unpredictable. That is to say, the sorting is not stable. |
| This can make a difference when the comparison considers only part of |
| the elements. Two elements with the same sort key may differ in other |
| respects. |
| |
| <p>If you want the effect of a stable sort, you can get this result by |
| writing the comparison function so that, lacking other reason |
| distinguish between two elements, it compares them by their addresses. |
| Note that doing this may make the sorting algorithm less efficient, so |
| do it only if necessary. |
| |
| <p>Here is a simple example of sorting an array of doubles in numerical |
| order, using the comparison function defined above (see <a href="Comparison-Functions.html#Comparison-Functions">Comparison Functions</a>): |
| |
| <pre class="smallexample"> { |
| double *array; |
| int size; |
| ... |
| qsort (array, size, sizeof (double), compare_doubles); |
| } |
| </pre> |
| <p>The <code>qsort</code> function derives its name from the fact that it was |
| originally implemented using the “quick sort” algorithm. |
| |
| <p>The implementation of <code>qsort</code> in this library might not be an |
| in-place sort and might thereby use an extra amount of memory to store |
| the array. |
| </p></blockquote></div> |
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