C language: Write a function to realize the binary search of an integer ordered array

topic:

Write a function to implement a binary search of an integer ordered array , and find out the corresponding subscript of the number you are looking for in the array .

                    

 =========================================================================

                       

Ideas:

general idea:

(1) Custom function part:

                

（1）.

parameter:

int arr[] --  the first address of the array

 int k -- the number to find in the array

 int sz -- array length

              

Define the left and right subscripts ;

                   

（2）.

use binary search ;

               

(2) Main function part:

           

Define an ordered array , set the value to find , and find the number of elements in the array .

               

Call a custom function .

               

Determine the return value of the custom function and print the corresponding situation .

                

---

                 

Custom function part:

first step:

(1). Setting of formal parameters :

int arr[] -- the first address of the array ;

int k -- the number to find in the array ;

int sz -- the length of the array .

             

(2). Define the left and right subscripts -- left and right

                     

Implementation code:

#include <stdio.h>

int binary_search(int arr[], int k, int sz)//形参
{
	int left = 0; //左下标
	int right = sz - 1; //右下标


}

int main()
{

	return 0;
}

Realize the picture:

                 

---

                 

Step two:

Use binary search method :

         

(1). Use while loop .

          

(2). Generate the intermediate value subscript mid :

int mid = left + (right - left) / 2;

left is the smaller side , (right - left) is the difference between the two , (right - left)/2 is half the difference ,

left + (right - left) / 2 , that is, the smaller side plus half of the difference ,

At this time, both sides are the same , and either side is the average value ( middle value )

              

（3）.

If the middle value is less than the value you are looking for ,

Discard the middle value and all numbers to the left of the middle value , and adjust the left subscript left : left = mid + 1 .

If the middle value is greater than the value you are looking for ,

Discard the middle value and all numbers to the right of the middle value , and adjust the right subscript right : right = mid - 1 .

If the middle value is equal to  the value you are looking for ,

Returns the intermediate value subscript mid .

               

(4). Return -1 if not found .

                     

Implementation code:

int binary_search(int arr[], int k, int sz)//形参
{
	int left = 0; //左下标
	int right = sz - 1; //右下标

	//使用while循环
	while (left <= right)
	{
		//生成中间值下标 mid ：
		int mid = left + (right - left) / 2;

		//二分查找:
		if (arr[mid] < k)//中间值 小于 要找的值
		{
			left = mid + 1;
			//舍弃中间值和中间值左边的所有数，
			//调整 左下标left ：left = mid + 1。
		}
		else if (arr[mid] > k)//中间值 大于 要找的值
		{
			right = mid - 1;
			//舍弃中间值和中间值右边的所有数，
			//调整 右下标right ：right = mid - 1。
		}
		else if (arr[mid] == k)//中间值 等于 要找的值
		{
			return mid;
			//返回中间值下标mid。
		}
	} 
	if (left > right)
	{
		return -1; //找不到则返回-1
	}
}

Realize the picture:

                 

---

                 

Main function part:

(1). Define an ordered array , set the value to be searched , and find the number of elements in the array .

               

(2). Call the custom function .

                

(3). Judge the return value of the custom function and print the corresponding situation .

                     

Implementation code:

#include <stdio.h>

int binary_search(int arr[], int k, int sz)//形参
{
	int left = 0; //左下标
	int right = sz - 1; //右下标

	//使用while循环
	while (left <= right)
	{
		//生成中间值下标 mid ：
		int mid = left + (right - left) / 2;

		//二分查找:
		if (arr[mid] < k)//中间值 小于 要找的值
		{
			left = mid + 1;
			//舍弃中间值和中间值左边的所有数，
			//调整 左下标left ：left = mid + 1。
		}
		else if (arr[mid] > k)//中间值 大于 要找的值
		{
			right = mid - 1;
			//舍弃中间值和中间值右边的所有数，
			//调整 右下标right ：right = mid - 1。
		}
		else if (arr[mid] == k)//中间值 等于 要找的值
		{
			return mid;
			//返回中间值下标mid。
		}
	} 
	if (left > right)
	{
		return -1; //找不到则返回-1
	}
}

int main()
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,10 }; //定义有序数组
	int k = 7; //设置要查找的值
	int sz = sizeof(arr) / sizeof(arr[0]); //求出数组元素个数
	// 整个数组大小 / 单个数组元素大小 = 数组元素个数

	//调用自定义函数：
	int ret = binary_search(arr, k, sz); //ret接收返回的下标

	//判断自定义函数的返回值，打印相应的情况：
	if (ret == -1) //未找到，返回-1
	{
		printf("找不到\n");
	}
	else
	{
		printf("找到了，下标是：%d\n", ret);
	}

	return 0;
}

Realize the picture:

                    

Final code and implementation effect

Final code:

#include <stdio.h>

int binary_search(int arr[], int k, int sz)//形参
{
	int left = 0; //左下标
	int right = sz - 1; //右下标

	//使用while循环
	while (left <= right)
	{
		//生成中间值下标 mid ：
		int mid = left + (right - left) / 2;

		//二分查找:
		if (arr[mid] < k)//中间值 小于 要找的值
		{
			left = mid + 1;
			//舍弃中间值和中间值左边的所有数，
			//调整 左下标left ：left = mid + 1。
		}
		else if (arr[mid] > k)//中间值 大于 要找的值
		{
			right = mid - 1;
			//舍弃中间值和中间值右边的所有数，
			//调整 右下标right ：right = mid - 1。
		}
		else if (arr[mid] == k)//中间值 等于 要找的值
		{
			return mid;
			//返回中间值下标mid。
		}
	} 
	if (left > right)
	{
		return -1; //找不到则返回-1
	}
}

int main()
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,10 }; //定义有序数组
	int k = 7; //设置要查找的值
	int sz = sizeof(arr) / sizeof(arr[0]); //求出数组元素个数
	// 整个数组大小 / 单个数组元素大小 = 数组元素个数

	//调用自定义函数：
	int ret = binary_search(arr, k, sz); //ret接收返回的下标

	//判断自定义函数的返回值，打印相应的情况：
	if (ret == -1) //未找到，返回-1
	{
		printf("找不到\n");
	}
	else
	{
		printf("找到了，下标是：%d\n", ret);
	}

	return 0;
}

Realize the effect: