Problems with binary search function.

Question

Having trouble with the binary_search function listed at the top. not sure where to go with it. I'm not very familiar with binary searching.

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

void get_input(ifstream& fin, int a[], int size, int & array_size);

void binary_search (int a[], int & array_size)
{
    cout << "Please enter the element you would like to search for \n";
    int element;
    cin >> element;

    int lastindex=array_size-1, startindex=0;

    while (startindex <= lastindex)
    {
        int midindex=(array_size/2);
        if(element > a[midindex])
        {
            startindex=midindex;
        }
        else if (element < a[midindex])
        {
            lastindex=midindex-1;
        }

    }

}

int main()
{
    int array_size=-1;
    int a[100];

    ifstream fin;

    get_input (fin, a, 100, array_size);

    binary_search (a, array_size);

    return 0;
}

void get_input (ifstream& fin, int a[], int size, int & array_size)
{
    fin.open("numbers.txt");
    if (fin.fail())
    {
        cout << "File failed to open";
        exit(1);
    }


    for(int i = 0; i < size; i++)
    {
        a[i] = 0;
    }

    cout << "The numbers in the array are: \n\n";

    for (int i = 0; i < size; i++)
    {
        if (!fin.eof())
        {
            fin >> a[i];
            array_size ++;
        }
    }

    for (int i = 0; i < array_size; i++)
    {
            cout << a[i] << "  ";
    }

    cout << "\n\n\n";
    cout << "The numbers in the array sorted are: \n\n";

   for(int i = 0; i < array_size; ++i )
   {
        int temp2 = a[i];

        for (int j = i+1; j < array_size; ++j )
        {

            if( a[j] < temp2)
            {
                temp2 = a[j];

                int temp = a[i];
                a[i]    = a[j];
                a[j]    = temp;
            }
        }
    }





    for (int i = 0; i < array_size; i++)
    {
            cout << a[i] << "  ";
    }

    cout << "\n\n\n";

    fin.close();
}

when done the program is suppose to take an input from a file assign it to an array then sort the array. After this i need to use a binary search to find a number given by the user and display its place in the array to the user.

update: getting wrong output for the index found.... should i just add one to midindex?

void binary_search (int a[], int & array_size)
{
    cout << "Please enter the element you would like to search for \n";
    int element;
    cin >> element;

    int lastindex=array_size-1, startindex=0;

    while (startindex <= lastindex)
    {
        int midindex= startindex + (lastindex - startindex) / 2;

        if(element > a[midindex])
        {
            startindex=midindex+1;
        }
        else if (element < a[midindex])
        {
            lastindex=midindex-1;
        }
        else if (element == a[midindex])
        {
            cout<<"Element "<<element<<" found at index "<<midindex<<endl;
            return;
        }



    }

}

Answer 1

Try changing

startindex=midindex;

to:

startindex=midindex + 1;

and

int midindex=(array_size/2);

to

int midindex= startindex + (lastindex - startindex) / 2

and most importantly you are doing nothing when you find the element !!

if(element == a[midindex]) {
  cout<<"Element "<<element<<" found at index "<<midindex<<endl;
  return;
}

Answer 2

My first reaction is to change the line

int midindex=(array_size/2);

to

int midindex = startindex + (lastindex - startindex) / 2;

Also, don't you want to report if the sought element was found or not? To detect the case when the element is found, another if branch like the following

if( element == a[midindex] )

can be inserted. That can have a return element; or return midindex inside it coupled with a return failure; outside the loop.

---

EDIT: I made a casual attempt to write a version of binary search. I don't claim it to be correct, as binary search is (in)famous for getting incorrect. Some code with test cases and output is uploaded at codepad.

Snippet:

int *
mybsearch( int const *  const a, size_t const n, int const key ) {

    int * lo = const_cast< int * >( a );
    int * hi = lo + n;

    while( lo <= hi ) {

        int * const mid = lo + (hi - lo) / 2;
        int const midelem = *mid;

        if( key == midelem ) {
            return mid;
        }
        else if( key < midelem ) {
            hi = mid - 1;
        }
        else {
            lo = mid + 1;
        }
    }

    return NULL;
}

The main and test code:

int main() {

    int const arr[] = {10, 20, 30, 40, 50, 60, 70, 80, 90};
    size_t const num = sizeof( arr ) / sizeof( int );

    int * pos20 = mybsearch( arr, num, 20 );
    assert( pos20 && (*pos20 == 20) );

    int * pos25 = mybsearch( arr, num, 25 );
    assert( !pos25 );

    int * pos5 = mybsearch( arr, num, 5 );
    assert( !pos5 );

    int * pos105 = mybsearch( arr, num, 105 );
    assert( !pos105 );
}

Answer 3

Binary search works nicely as a recursive algorithm. Pass in the array and length, check the middle value, and recurse on the upper / lower half of the array, as appropriate.

Answer 4

Consider carefully what is not right about int midindex=(array_size/2); when array_size = 1. Then generalize to array_size = 3. Then to any odd number. This will require small run simulations in your head or on paper.

Answer 5

You're close. You want to do something like this:

int binary_search ...

so you can return the index of the element

while (startindex < lastindex)    
{
    int midindex=(startindex+endindex)/2;
    if(element = a[midindex]) {
        return midindex;
    }
    else if(element > a[midindex])
    {
        startindex=midindex+1;