Problems with binary search function - c++

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;