Question

The code below is already completed using C++. Could you please explain why the input is

3 : 2 :
3 : 6 : 4 : -1 :
3 : 1 : 0 :? Please let me know really short explanation.

#include <iostream>

using namespace std ;

int binarySearch(const int array[], int numElems, int value)

{

int first = 0, // First array element

last = numElems - 1, // Last array element

middle, // Midpoint of search

position = -1; // Position of search value

bool found = false; // Flag

while (!found && first <= last)

{

middle = (first + last) / 2; // Calculate midpoint

cout << array[middle] << " : " ;

if (array[middle] == value) // If value is found at mid

{

found = true;

position = middle;

}

else if (array[middle] > value) // If value is in lower half

last = middle - 1;

else

first = middle + 1; // If value is in upper half

}

cout << position << " : " ;

return position;

}

int main()

{

int arr1[] = { 1 , 2 , 3 , 4 , 6 , 9 } ;

binarySearch( arr1 , 6, 3 ) ;

cout << endl ;

binarySearch( arr1 , 6, 5 ) ;

cout << endl ;

binarySearch( arr1 , 6, 1 ) ;

cout << endl ;

return ( 0 ) ;

}

Answer 1

#include <iostream>
using namespace std ;

int binarySearch(const int array[], int numElems, int value)

{

int first = 0, // First array element

last = numElems - 1, // Last array element

middle, // Midpoint of search

position = -1; // Position of search value

bool found = false; // Flag

while (!found && first <= last)

{

middle = (first + last) / 2; // Calculate midpoint

cout << array[middle] << " : " ;

if (array[middle] == value) // If value is found at mid

{

found = true;

position = middle;

}

else if (array[middle] > value) // If value is in lower half

last = middle - 1;

else

first = middle + 1; // If value is in upper half

}

cout << position << " : " ;

return position;

}

int main()

{

int arr1[] = { 1 , 2 , 3 , 4 , 6 , 9 } ;

binarySearch( arr1 , 6, 3 ) ;
//note binary search searches the list by everytime exculding half of the elements of the list
//here we are searching 3
//first =0, last =6-1=5
//the mid index is : (first+last)/2 =(0+5)/2=5/2=2 (works as floor)
//now a[2]=3 //mid value
//so 3: is printed
//since we are searching for 3 ,position is update with 2(index of 3), then while loop stops
//then position is printed
//2:
//so, 3:2: printed
cout << endl ;

binarySearch( arr1 , 6, 5 ) ;

//searching value 5
//first =0
//last=5
//mid = 5/2=2
//a[mid]=3
//3: printed
//since 5 is greater than 3
//the new mid is
//mid =(3+5)/2=8/2=4 //since first =middle +1
//the value at index four is 6
//hence 6: printed
//now 5 is less than 6
//now last=middle-1=3
//now mid =(3+3)/2=3
//value at index 3 is 4
//so 4: printed
//and 4 is less than 5
//so now first =middle+1=3+1=4
//the loop stops
//since position is not upadeted
//-1: printed
//so
//3 : 6 : 4 : -1 : printed
cout << endl ;

binarySearch( arr1 , 6, 1 ) ;

//search element 1
//first =0
//last=5
//mid =2
//value at index 2 is 3
//3: printed
//since 3 is greater than 1
//last=middle-1=2-1=1
//mid = (0+1)/2 = 0
//value at index 0 is 1
//1: printed
//since the searching value is found position is updated with index 0
//then
//0: printed
//so
//3:1:0: printed

cout << endl ;

return ( 0 ) ;

}