Searching in O(logn)

#include<iostream>
using namespace std;
int main()
{
           int a[20],m,n,x,pos=-1,j;
           cout<<"Enter the size of the array:";
           cin>>m;
           cout<<"Enter the array:\n";
           for(int i=0;i<m;i++)
                      cin>>a[i];
           for(int i=1;i<m;i*=2)
           {
                      if(a[i]==0)
                      {
                                 j=i-1;
                                 while(a[j]==0)
                                            j--;
                      }
           }
           n=j;
           cout<<"Enter the element to be searched:";
           cin>>x;
           if(x==0)
                      cout<<"The given element is present in the array !!";
           else
           {
                      int beg=0;
                      int last=n;
                      while(beg<=last)
                      {
                                 int mid=((beg+last)/2);
                                 if(a[mid]==x)
                                 {
                                            pos=mid;
                                            break;
                                 }
                                 else if(x<a[mid])
                                            last=mid-1;
                                 else
                                            beg=mid+1;
                      }
                      if(pos==-1)
                                 cout<<"The required element is not present in the array !!";
                      else
                                 cout<<"The given element is present in the array at "<<pos<<" position";
           }
           return 0;
}