这是一个二进制搜索代码，我无法找出问题所在

Question

Here is my code.The code is running but not getting the right results. For ex- searching 3 in the program is getting output "not found" even though present.

 #include<stdlib.h>
 #include<iostream>
using namespace std;
int Bsearch(int arr[],int item,int n)
{
 int low,mid,up;
 low=0;up=n-1;
 while(low<=up && item!=arr[mid])
 {
     mid=(low+up)/2;
     if(item==arr[mid])
        return mid;
     else if(item<arr[mid])
        low=mid+1;
     else
        up=mid-1;
 }
 return -1;


}
int main()
{
    int index,item;;
    int arr[]={1,2,3,4,5,6,7,8,9};

  cout<<"enter search item\n";
  cin>>item;
    index=Bsearch(arr,item,9);
    if(index!=-1)
    cout<<"element found at position"<<(index+1);
    else
    cout<<"element not found";

    return 0;
}

Answer 1

Here is the problem:

if(item==arr[mid])
        return mid;
     else if(item<arr[mid])
        low=mid+1;
     else
        up=mid-1;

just change the conditon of choosing low and up index according the value of mid index arr[mid] like:

 if(item==arr[mid])
    return mid;
 else if(item<arr[mid])
    up=mid-1;
 else
    low=mid+1;

OR change this condition else if(item<arr[mid]) into else if(item > arr[mid]) and your code will work fine.

if(item==arr[mid])
        return mid;
     else if(item > arr[mid])
        low=mid+1;
     else
        up=mid-1;

NB: while(low<=up && item!=arr[mid]) in this line, you're using mid in arr[mid] before it has been assigned a value, resulting in Undefined Behavior. (Also, the && item!=arr[mid] part of the while test is unnecessary.) Credit: 1201ProgramAlarm