Question

write a program in java to print all the powers of 2 below a certain number and calculate their sum. Make sure your program conforms to the following requirements.

1. Accept the upper limit from the user (as an integer).

2. These numbers grow very fast. make sure the sum variable is of the "long" type. You can assume that the test output will be less that LONG MAX.

3.Print all numbers as a running sum, and finally print their sum. The input and output should match the sample run exactly.

SAMPLE RUN

Enter the upper limit: 500

1+2+4+8+16+32+64+128+256 = 511

Enter the upper limit: 1055

1+2+4+8+16+32+64+128+256+512+1024 = 2047

Answer 1

Note: Class name, function names and variable names are used in such a way that easily understandable. It can be changed according to the user. Comments are given in bold and italic.

Save the file name as same as the class name. Here class name is pow_of_two so the file has been saved as pow_of_two.java. It can be changed to the user's choice. For indentation and better understanding please refers to code snippets given.

Code : pow_of_two.java

import java.util.Scanner;

//for power function
import java.lang.Math;

public class pow_of_two
{

//function to calculate the powers of two
public static double cal_pow_two(double pow_num)
{
       //use of the math pow function to calculate power
       return Math.pow(2,pow_num);
      
   }

public static void main(String[] args)
   {
   //to store upper limit
       int up_limit;
      
       //counter variables
       double cntr=0;
      
       //to store return value of cal_pow_two() function
       double pow_result;
      
       //to store the final sum
       long fin_sum=0;
      
       Scanner val = new Scanner(System.in);
      
       System.out.print("Enter the upper limit: ");
      
       //getting the upper limit fromt the user
       up_limit=val.nextInt();
      
       do
       {
           //calling the cal_pow_two() function
           pow_result=cal_pow_two(cntr);
          
           //checking if power of 2 is less than the upper limit
           if(pow_result<=up_limit)
           {
           //This is condtion just to avoid printing of + sign in the end
               if(cntr==0)
               {
               System.out.print((int)pow_result);
fin_sum=fin_sum+(long)pow_result;
               }
               else
               {
               System.out.print("+"+(int)pow_result);
fin_sum=fin_sum+(long)pow_result;
               }
           }
           cntr++;
          
       }while(pow_result<=up_limit);
      
       //printing out the final sum
       System.out.print("="+fin_sum);
      
   }
}  

Code Snippets

Sample Output runs