Why is Java rounding down my double value?

Question

I am trying to write a Java program that finds the sum of the following series using an input for the value of n:

formula

The code I have written is as follows:

package sum_of_series1;
import java.util.Scanner;

public class Sum_of_series1 
{
    public static void main(String[] args) 
    {
        Scanner sc = new Scanner (System.in);
        int n, i;
        double sum;
        sum = 1;
        System.out.println("Enter the value of n:");
        n = sc.nextInt();
        i = 0;
        while (i<n) 
        {
            i++;
            sum = sum + ((double)(i/(i+1)));
        }
        System.out.println("The sum of the series is " + sum);
    }
    
}

However, the output is the following for every input of n:

The sum of the series is 1.0

Clearly, this is incorrect (for say n = 4). After some inspection, I realised that ((double)(i/(i+1))) is being calculated as 0.0 every time. Why is it not, for example, calculating 2/3 as 0.6666666666666666 and how can I fix this problem?

Strangely, however, when I write System.out.println((double) 2/3), the output is 0.6666666666666666. I am rather new to programming and extremely confused. Any help would be much appreciated!

`i` is an integer and division between integer yields integer. So `1/2` is `0`, for example. You need to convert it to a float or double before the divide. — Ted Klein Bergman, May 22 '21 at 19:07

Daniel · Answer 1 · 2021-05-22T20:56:06.767

1

The problem lies here:

sum = sum + ((double)(i/(i+1)));

You cast only the rounded integer. You can think of it like this:

int a = i / (i + 1);
double b = (double) a; // = a.0

Just move the cast to the next parenthesis:

sum = sum + ( (double) i / (i+1) );

edited May 22 '21 at 20:56

answered May 22 '21 at 19:08

Daniel

669
3
19

2

You mean `int a = i / (i+1)` . Due the precedence rules in the grammar (and common to nearly all languages other than APL and Forth) `(i / i + 1)` is the same as `(i/i) + 1` which for nonzero `i` is the same as `(1+1)` or `2`. – dave_thompson_085 May 22 '21 at 20:09

score 1 · Accepted Answer · edited May 23 '21 at 21:03

1

This happens because you are dividing using integers and then casting the value to double. You should cast one of the integers before dividing them by each other, like: ((double)i)/(i + 1). When you are dividing using integers, the JVM always returns an integer by rounding down the value, so you have to cast one of them before dividing.

edited May 23 '21 at 21:03

learning123

103
4

answered May 22 '21 at 19:11

Programmer

531
2
10

Why is Java rounding down my double value?

2 Answers2