C++ Binary Search Not Working Correctly - Finds Element Not in Array

Question

I am running the binary search algorithm in C++ but it gives me spurious results. For example, searching for the value 21 gives me a

"Value is Found"

message but my array consists only of numbers from 0 to 20.

Any help is greatly appreciated.

#include <iostream>
#include <iomanip>
using namespace std;

int binarySearch(const int [], int, int, int, int ); // function prototype

int main()
{
    const int arraySize = 10;
    int arr[ arraySize ];
    int key;

    for( int i = 0; i <= arraySize; i++)  // generate data for array
       arr[i] = 2*i;

    cout << "The array being searched is: " << endl;

    for (int j = 0; j<=arraySize; j++)  // print subscript index of the array
    {
    cout << setw(5) << j << " ";
    }

    cout << endl;

    for (int z = 0; z<=arraySize; z++) // print elements of the array below index
    {
     cout << setw(5) << arr[z] << " ";
    }

    cout << "\n" <<"Enter value you want to search in array " << endl;
    cin >> key;

    int result = binarySearch(arr, key, 0, arraySize, arraySize); // function call

    if (result == 1)                  // print result of search
    cout << "Key is found " << endl;
    else
    cout << "Key not found " << endl;

    return 0;
} // end main function

int binarySearch(const int a[], int searchKey, int low, int high, int length)
{
    int middle;

    while (low <= high){

        middle = (low + high) / 2;

        if (searchKey == a[middle]) // search value found in the array, we have a match
        {
        return 1;
        break;
        }

        else
        {
        if( searchKey < a[middle] )  // if search value less than middle element
            high = middle - 1;      // set a new high element
        else
            low = middle + 1;       // otherwise search high end of the array
        }
    }
return -1;
}

When you used your debugger to step through your code, one line at a time, what did your debugger tell you is the reason your code produced the wrong result? — Sam Varshavchik
– Sam Varshavchik, Commented May 14, 2016 at 13:20
I went through line by line and could not see the issue, but I think the answer is the as per CodingBatman's answer below. — quidproquo
– quidproquo, Commented May 14, 2016 at 13:33

abhishek_naik · Accepted Answer · 2016-05-14 13:24:24Z

4

You are invoking undefined behavior because your for loop conditions are <=arraySize. Change it to <arraySize. On making this change, the code works perfectly for sample inputs.

By writing int arr[ arraySize ]; you are creating an array of 10 elements (i.e., from 0 to 9), while in the for loops, you start from 0 and move until 10.

Live Demo

answered May 14, 2016 at 13:24

abhishek_naik

1,2952 gold badges15 silver badges28 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

quidproquo Over a year ago

Thanks, the issues were as you said. Also the function call had to be adjusted. The 3rd argument in the call should be "arraySize - 1" as opposed to "arraySize" as per what you said.

Collectives™ on Stack Overflow

C++ Binary Search Not Working Correctly - Finds Element Not in Array

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related