Question

Write a C++ function binsearch that carries out the binary search algorithm on a sorted array of integers. Your function takes as parameters an array of integers (sorted in increasing order), the size of the array, and a target integer to search for. The function returns the index of the target in the array, and returns -1 if the target is not in the array.

The file main1.txt on the webpage contains code that generates a specific array of integers in increasing order. (Do not change the code that generates the array.) Add your binsearch function to this C++ program. Use your function to search for the elements {39, 47, 208, 288, 427, 685, 676, 972}.

Your program should print out each of these elements, and the return value of the function for that element, that is, the index in the array a where the element was found, or the integer -1 if the element was not found.

#main1.txt

#include <iostream>
# include <cstdlib>

using namespace std;

int main()
{
    int a[100];
    srand(300);
    for ( int k = 0; k < 100; ++k){
        a[k] = 10*k + rand()%10;
    }

}

Answer 1

The following code of the program is explained with comments.

I have included the print_array function to print the array contents to visually check whether the element searched using bin_search exists in the array or not.

Please let me know if you have any doubts regarding the code.

Code:

#include <iostream>
#include <cstdlib>
using namespace std;

// This function searches a element in a array using binary search
int binsearch(int a[],int size,int target_int)
{
    // This variable keeps track of indexes of left half array
    int left_half_index = 0;
    // This variable keeps track of indexes of right half array
    int right_half_index = size - 1;
    // Until left_half_index is less than or equal to right_half_index, this loop is iterated
    while(left_half_index <= right_half_index)
    {

        // Calculating the mid index of the array
        int mid_index = (left_half_index + right_half_index) / 2;

        // If the value at index mid_index in array is less than target_int
        if(a[mid_index] < target_int)
        {
            // Update the value of left_half_index
            left_half_index = mid_index + 1;
        }

        // Else If the value at index mid_index in array is greater than target_int
        else if(a[mid_index] > target_int)
        {
            // Update the value of right_half_index
            right_half_index = mid_index - 1;
        }

        // Else the target_int is found
        else
        {
            // So return the mid_index
            return mid_index;
        }
    }
    // If this step is reached, it means that the target_int is not in the array. So return -1
    return -1;
}

// This function prints the array
void print_array(int a[], int size)
{
    cout<<"Array contents:\n";

    for(int i = 0;i<size;i++)
    {
        // If 10 elements are printed, print a new line
        if((i+1)%10 == 0)
        {
            cout<<endl;
        }

        // Else print the element
        else
        {
            cout<<a[i]<<" ";
        }
    }
    // Print a newline
    cout<<endl;
}

// This is the main function
int main()
{
    // Creating an array of type int and of size 100
    int a[100];
    // Setting the seed of random function
    srand(300);

    // Generating 100 random values and inserting them into the array
    for ( int k = 0; k < 100; ++k)
    {
        a[k] = 10*k + rand()%10;
    }

    // This is an array of elements whose every element is to be searched in the array a
    int values_searched[] =  {39, 47, 208, 288, 427, 685, 676, 972};
    // Calculating size of the array a
    int a_size = sizeof(a) / sizeof(a[0]);

    // Printing the array
    print_array(a,a_size);

    // Calculating size of the array values_size
    int values_size = sizeof(values_searched) / sizeof(values_searched[0]);
    // Iterating over values_searched array
    for(int k=0;k<values_size;k++)
    {
        // Searching if the element exists in the array a
        int result = binsearch(a,a_size,values_searched[k]);
        // If the element exists
        if(result != -1)
        {
            // Print the element and it's index
            cout<<"Value "<<values_searched[k]<<" is present in the array at index "<<result<<endl;
        }
        // Else
        else
        {
            // Print that element doesn't exist in the array
            cout<<"Value "<<values_searched[k]<<" is not present in the array"<<endl;
        }
    }
}

Sample Output: