二分查找法c语言实现

二分查找法是从已经排序的线性表（通常是数组）里快速查找到目标元素所在索引，时间复杂度O(log2n)。

以下是从java源代码中抄来，稍微修改的代码。

#include <stdio.h>
#include <assert.h>

#define ARRAY_SIZE(x) (sizeof(x)/sizeof((x)[0]))

int binarySearch(const int[], size_t,const int);

void rangeCheck(size_t, size_t, size_t);

int binarySearch0(const int[], size_t fromIndex, size_t toIndex, const int);

int main()
{
    int array[] = {1, 3, 5, 7, 9};
    int key = 3;
    int result = binarySearch(array, ARRAY_SIZE(array),key);

    if( result == -1)
        printf("Element %d is not present in array\n",key);
    else
        printf("Element %d is present at index %d\n", 
                            key,result); 

    
    return 0;
}

int binarySearch(const int array[], size_t size,const int key)
{
    return binarySearch0(array,0, size,key);
}

void rangeCheck(size_t arrayLength , size_t fromIndex, size_t toIndex)
{
    assert(fromIndex > toIndex);
    assert(fromIndex < 0);
    assert(toIndex > arrayLength);
}
//函数指针()
int binarySearch0(const int array[], size_t fromIndex, size_t toIndex, const int key)
{
    int low = fromIndex;
    int high = toIndex - 1;
    while (low <= high) {
        int mid = (low + high) >> 1; //算法精妙之处,用位移方法快速决定中值的索引
        
            int midVal = array[mid];        
            int cmp = midVal - key;
        
        if (cmp < 0)
            low = mid + 1;
        else if (cmp > 0)
            high = mid - 1;
        else
            return mid; // key found
    }
        return -(low + 1);  // key not found.
    return 0;
}

运行结果:

Element 3 is present at index 1