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| <h3 class="section">9.1 Defining the Comparison Function</h3> |
| |
| <p><a name="index-Comparison-Function-819"></a> |
| In order to use the sorted array library functions, you have to describe |
| how to compare the elements of the array. |
| |
| <p>To do this, you supply a comparison function to compare two elements of |
| the array. The library will call this function, passing as arguments |
| pointers to two array elements to be compared. Your comparison function |
| should return a value the way <code>strcmp</code> (see <a href="String_002fArray-Comparison.html#String_002fArray-Comparison">String/Array Comparison</a>) does: negative if the first argument is “less” than the |
| second, zero if they are “equal”, and positive if the first argument |
| is “greater”. |
| |
| <p>Here is an example of a comparison function which works with an array of |
| numbers of type <code>double</code>: |
| |
| <pre class="smallexample"> int |
| compare_doubles (const void *a, const void *b) |
| { |
| const double *da = (const double *) a; |
| const double *db = (const double *) b; |
| |
| return (*da > *db) - (*da < *db); |
| } |
| </pre> |
| <p>The header file <samp><span class="file">stdlib.h</span></samp> defines a name for the data type of |
| comparison functions. This type is a GNU extension. |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <p><a name="index-comparison_005ffn_005ft-820"></a> |
| <pre class="smallexample"> int comparison_fn_t (const void *, const void *); |
| </pre> |
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