Cannot calculate high number of iterations like 1,000,000,000

Question

This code calculates the function f(x) = -1 + 2 + -3 + 4 + ... + (( - 1)^n)*n But when the input(n) is too big, like 1000000000, java doesn't display an output.

What can I do to solve this problem?

import java.util.Scanner;

public class Calculating_function {
    public static void main(String[] args) {



        Scanner input_taker = new Scanner(System.in);

        String n_string = input_taker.nextLine();
        long n = Long.parseLong(n_string);



        System.out.println(fonk(n));


    }

    public static long fonk(long n) {
        long total = 0;
        for(long i = 1; i <= n; i++) {

            total += (long)Math.pow(-1, i) * i; 
        }
        return total;
    }

Do you get an overflow exception or does the process terminate with no further output? Please clarify — meowgoesthedog, Jul 12 '18 at 17:10
How long does it take for 100, 10000, 10000000, 100000000? Is there a pattern which just means that for 1000000000 it will take a year but not forever? — Yunnosch, Jul 12 '18 at 17:13
Is there a particular reason you want to solve this using a loop? — VeeArr, Jul 12 '18 at 17:27
@Jon Cokisler Yes, observe that f(n) = n/2 for any even n, and then you just need to account for what to do in the case where n is odd. — VeeArr, Jul 12 '18 at 19:14

score 2 · Answer 1 · answered Jul 12 '18 at 17:08

As the value of i increases, your exponent calculation Math.pow(-1, i) will take longer and longer to calculate.

As you're using Math.pow(-1, i) to simply swap the sign, you can optimize it as Math.pow(-1, i%2). Better yet, map the multiplier to the values 1 and -1 and use i%2 as your key.

Additionally, you are risking overflows as your total has an upper bound of 500,000,000,000,000,000 so use BigInteger instead to store the total.

Cannot calculate high number of iterations like 1,000,000,000

1 Answers1