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https://github.com/TheAlgorithms/C
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80 lines
2.8 KiB
C
80 lines
2.8 KiB
C
/**
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* @file
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* @brief [Linear Search with Sentinel](https://en.wikipedia.org/wiki/Linear_search#With_a_sentinel) algorithm implementation
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* @details
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* This algorithm saves the last element of the array,
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* then replaces it with the value to be found and sets it as the sentinel.
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* When searching, compares each element with the sentinel.
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* If the same, returns the index. If the index is the index of the sentinel, it means it was not found.
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* Of course, if the value to be found is the last element, we return the index of the last element.
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* @author [Regan Yue](https://github.com/ReganYue)
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* Time Complexity: O(N)
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*/
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#include <stdio.h> /// for IO operations
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#include <assert.h> /// for assert
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/**
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* @brief Utility function to search for an element in the array and return the index of the element
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* @details
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* The so-called "sentinel" is to use a special value as the boundary key of the array.
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* One less judgment statement can be used.
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* The purpose is to avoid checking whether the entire array is searched at each step in the search
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* process, so as to improve the efficiency of the program.
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* We can use the last value of the array as the "sentinel", the data storage index i
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* starts from 0 and ends at len-1, then the position where the index of arr is n-1 indicates
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* that there is no data temporarily, which is the "sentinel" key.
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* If the last element of the array is equal to the key, directly return the index of the last element.
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* Before setting the last element of the array as the key, we hand over the last element of the array to temp for temporary storage.
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* Then go to the array to find the key. If the key is found, stop the search, and then compare the found element index with len-1.
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* If it is equal, it means it was not found. If it is not equal, it is found.
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* @param arr this is an array containing elements
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* @param len this is the number of elements in the array
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* @param key the value we want to search
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* @return i if found, otherwise -1 is returned.
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*/
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int sentinel_linear_search( int arr[], int len, int key ){
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if(key == arr[len-1]){
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return len-1;
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}
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int temp = arr[len-1];
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arr[len-1] = key;
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int i = 0;
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while (arr[len-1] != arr[i]) {
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i++;
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}
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arr[len-1] = temp;
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return i != len-1 ? i : -1;
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}
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/**
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* @brief Self-test implementations
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* @returns void
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*/
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static void test(){
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int n,i;
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n = 5;
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/* init array */
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int arr[] = { 1, 2, 2, 6, 99, 100, 999 };
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assert(sentinel_linear_search( arr, n, 1 )==0);
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assert(sentinel_linear_search( arr, n, 2 )==1);
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assert(sentinel_linear_search( arr, n, 6 )==3);
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assert(sentinel_linear_search( arr, n, 101 )==-1);
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printf("All test cases have successfully passed!\n");
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}
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/**
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* @brief Main function
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* @returns 0 on exit
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*/
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int main(){
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test(); // run self-test implementations
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return 0;
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}
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