折半查找

//折半查找：

#include<stdio.h>
#include<stdlib.h>

int binFind(int A[],int len,int x)   //折半查找
{
	if(A == NULL)
		exit(1);
	else
	{
		int low = 0,high = len-1;            //low用來記錄低下標，high用來記錄高下標
		int mid;                             //low值爲中間下標，用來與x比較（因此成爲折半查找）
		while(low <= high)
		{
			mid = ((low + high)/2);
			if(A[mid]<x)                 //如果x大於中間值，則x一定在右半段
			{
				low = mid+1;         //改變低下標爲中間下標的下一個
				mid = (low + high)/2;
			}
			else if(A[mid]>x)            //如果x小於中間值，x一定存在於左半段
			{
				high = mid-1;        //改變高下標爲中間下標的前一個
				mid = (low + high)/2;
			}
			else
				return mid;          //否則，x與當前中間值相等，則返回中間下標
		}
	}
	return -1;

}

int main()
{
	int arr[] = {1,2,3,5,9,15,36,46,78,95},ph = 0,item = 0;
	for(int i = 0;i<sizeof(arr)/sizeof(arr[0]);i++)
		printf("%d ",arr[i]);
	printf("\n請輸入要查找的值：");
	scanf("%d",&item);
	ph = binFind(arr,sizeof(arr)/sizeof(arr[0]),item);
	if(-1 != ph)
		printf("a[%d] = %d\n",ph,arr[ph]);
	else
		printf("沒有找到%d\n",item);
	return 0;

}