#include <iostream>

using namespace std;

const int size = 10;

void search(const int a[],int first,int last,int key,bool& found,int& laction);

int main()

{
   
    int n;
   
    int a[size];
   
    int first;
   
    int last;
   
    int laction;
   
    bool found;
   
    int key;
   
    for(n=0;n<size;n++)
        
    {
        
        a[n]=n+1;
        
        cout<<a[n]<<"  ";
        
    }
   
    cout<<endl;
   
   
   
   
   
    last=size-1;
   
    cin>>key;
   
    search(a,0,last,key,found,laction);
   
    if(found)
        
        cout<<key<<"laction is :"<<laction<<endl;
   
    else
        
        cout<<key<<"is not laction"<<endl;
   
   
   
    return 0;
   
}



void search(const int a[],int first,int last,int key,bool& found,int& laction)

{
   
    int mid;
   
    if(first > last)
        
        found = false;
   
    else
        
    {
        
        mid=(first + last)/2;
        
        if(key = a[mid])
            
        {
            
            found=true;
            
            laction=mid;
            
        }
        
        else if (key > a[mid])
            
            search(a,mid+1,last,key,found,laction);
        
        else if(key < a[mid])
            
            search(a,first,mid-1,key,found,laction);
        
    }
   
}
这个是递归的二分法，我的输出总是4，求解决。。。

if(key = a[mid]) ? if( key == a[mid] )

同上！

不能都用int型

if(key = a[mid])  这句话错了 等号是 ==
我修改以后运行了一遍没问题

同上，一个等号是赋值，两个等号是判断

赋值和判断不同

这个是基础啊 - -  

#include<iostream>
#define N 10
using namespace std;
int main()
{
    int a[N],front,end,mid,x,i;
    cout<<"输入已排好序的a数组元素:"<<endl;
    for(i=0;i<N;i++)
     {a[i]=i+1;
    cout<<a[i]<<" ";
     }cin>>x;
    front=0;
    end=N-1;
    mid=(front+end)/2;
    while(front<end&&a[mid]!=x)
    {
        if(a[mid]<x)front=mid+1;
        if(a[mid]>x)end=mid-1;
        mid=front + (end - front)/2;
    }
    if(a[mid]!=x)
    cout<<"没找到！"<<endl;
    else
    cout<<"找到了！在第"<<mid+1<<"项里。"<<endl;
    return 0;
}