• Examines the element in the middle of the array. Is it the sought item? If so, stop searching.

• Is the middle element too small? Then start looking in second half of array.

• Is the middle element too large? Then begin looking in first half of the array.

• Repeat the process in the half of the list that should be examined next.

• Stop when item is found, or when there is nowhere else to look and item has not been found.

void  SortedType::RetrieveItem ( ItemType&  item,   bool&  found )  
//  Pre: Key member of item is initialized.
//  Post: If found, item’s key matches an element’s key in the list
//	 and a copy of that element has been stored in item; otherwise,
//  item is unchanged.
{  int midPoint ;
	int   first  =  0;
   int	last  = length - 1 ;
	bool  moreToSearch  =  ( first  <=  last ) ;

   found = false ;
	while ( moreToSearch  &&  !found )
	{	midPoint  =  ( first + last ) / 2 ;	// INDEX OF MIDDLE ELEMENT
		switch ( item.ComparedTo( info [ midPoint ] ) )
    	{     
		   case   LESS    :	. . .  // LOOK IN FIRST HALF NEXT 
		   case  GREATER  :   	. . .  // LOOK IN SECOND HALF NEXT
		   case  EQUAL    :  	. . .  // ITEM HAS BEEN FOUND
   	}
   }
}

 Binary Search Trace

Second step

3rd Step

What is the final algoritm