How do I get the sum of the first two sequences when the user enters "2" as "n"?

Question

The sequence is: sum = 1 + 1/2 + 1/4 + 1/9 + 1/16 + 1/25+...

When I enter "2" it just gives me the sum of 1.25. How do you get it so when "2" is entered, it is adding 1 + 1/2?

Oh and I'm in an entry level java course so I we cant use arrays or anything that advance yet.

Thanks in advance!

import java.util.Scanner;
public class Sum
{
    public static void main(String[] args)
    {
        //declarations
        Scanner scan = new Scanner(System.in);
        double sum = 0;
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();

        //process
        for(int counter = 1; counter <= n; counter += 1)
        {
            sum += 1.0 / (counter*counter);
        }

        System.out.println("The sum is: " + sum);
    }
}

Answer 1

The following code will solve your problem, i have used Math.pow() to get the sequence running and not multiplying it twice.

public static void main(String[] args) throws UnknownHostException {
        //declarations
        Scanner scan = new Scanner(System.in);
        //to generate this sequence we take the first two as constants and start generating from the third onwards
        double first = 1;//first number in the sequence
        double second = 0.5;//second number in the sequence
        double sum = first+second;//adding first and second
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();
        if(n==1){
           System.out.println("The sum is: " + first);
           return;
        }
        if(n==2){
           System.out.println("The sum is: " + sum);
           return;
        }
        //process
        for(int counter = 2; counter <n; counter += 1)
        {
            sum += 1.0 / Math.pow(counter, 2);//will be computed when you enter values when n is more than 3

        }

        System.out.println("The sum is: " + sum);
     }

Answer 2

Since you say that 1/2 must be part of the sequence, so be it. (But that's a bizarre sequence and I strongly suggest that you double check this with your professor.) I'll assume that the remainder of the sequence is defined by 1/i². Note that with these assumptions, the sequence terminates at 1/(n-1)² rather than 1/n².

You'll need special handling for the cases n == 1 and n > 1. One possibility is to initialize sum to 1 if n == 1; initialize it to 1.5 if n > 1; and otherwise initialize it to 0. Then start the loop at counter = 2 and change the loop termination condition to counter < n (instead of <=).

Answer 3

If that's your sequence then you should really start by setting the sum equal to 1.5 and then rest of it will work. Your sequence should be a geometric sequence 1/n^2 I think it's a mistake.

public static void main(String[]args) {
     Scanner scan = new Scanner(System.in);
        double sum = 1.5;
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();
        if(n==1)
          System.out.println("The sum is: " + 1);
        //process
        for(int counter = 2; counter <n; counter++) {
            double mul = counter*counter;
            sum += 1.0/mul ;
        }

        System.out.println("The sum is: " + sum);

}

Output :

    Enter n: 
    2
    The sum is: 1.5
    Enter n: 
    3
    The sum is: 1.75

Answer 4

You need to manage "1" and "2" as special cases.

import java.util.Scanner;
public class Sum
{
    public static void main(String[] args)
    {
    //declarations

        Scanner scan = new Scanner(System.in);
        double sum = 0;
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();

        //process
        for(int counter = 1; counter <= n; counter += 1)
        {
          if (counter == 1)
            sum = 1;
          else if (counter == 2 )
            sum += 1.0/((counter-1)+(counter-1));
          else 
            sum += 1.0 / ((counter-1)*(counter-1)); 
        }

        System.out.println("The sum is: " + sum);
    }
}

Answer 5

As per your for loop, the sequence generated will be 1 + 1/(2*2) + 1/(3*3)+ ......

So, when you enter 2 => 1+1/(2*2) = 1+0.25=1.25.

Otherwise, your logic is Good. you can implement few exceptions, but as you mentioned that you re new to Java, you ll slowly encounter them.

Happy Learning Java :)