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9.2 Array Search Function

Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in search.h.

— Function: void * lfind (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)

The lfind function searches in the array with *nmemb elements of size bytes pointed to by base for an element which matches the one pointed to by key. The function pointed to by compar is used decide whether two elements match.

The return value is a pointer to the matching element in the array starting at base if it is found. If no matching element is available NULL is returned.

The mean runtime of this function is *nmemb/2. This function should only be used if elements often get added to or deleted from the array in which case it might not be useful to sort the array before searching.

— Function: void * lsearch (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)

The lsearch function is similar to the lfind function. It searches the given array for an element and returns it if found. The difference is that if no matching element is found the lsearch function adds the object pointed to by key (with a size of size bytes) at the end of the array and it increments the value of *nmemb to reflect this addition.

This means for the caller that if it is not sure that the array contains the element one is searching for the memory allocated for the array starting at base must have room for at least size more bytes. If one is sure the element is in the array it is better to use lfind so having more room in the array is always necessary when calling lsearch.

To search a sorted array for an element matching the key, use the bsearch function. The prototype for this function is in the header file stdlib.h.

— Function: void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)

The bsearch function searches the sorted array array for an object that is equivalent to key. The array contains count elements, each of which is of size size bytes.

The compare function is used to perform the comparison. 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. The elements of the array must already be sorted in ascending order according to this comparison function.

The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified.

This function derives its name from the fact that it is implemented using the binary search algorithm.