CODEWITHSUNDEEP
javaalgorithmsum
user1166232
user1166232

Reputation:

Adding integers in Range not working for negative numbers

I am trying to do this exercise:

Write a program that asks the user for N and M and adds up the integers between N and M using the formula

SUM(N to M) = SUM( 1 to M ) - SUM( 1 to N-1 )

I can get this to work for positive numbers but not negative numbers.

static int method2(int n, int m) {
        int sum = 0;
        int sum2 = 0;

        for (int i = 1; i <= m; i++) {
            sum = sum + i;
        }

        for (int i = 1; i <= n - 1; i++) {
            sum2 = sum2 + i;
        }
        System.out.println("sum: " + sum + ", sum2: " + sum2);

        return sum = sum - sum2;
    }

e.g.

  1. using n = -1, m = 1 returns sum = 1.
  2. Using n = -5, m = 5 returns sum = 15.
  3. Using n = 5, m = -5 returns sum = -10. These should all return 0.

e.g.

  1. Using n = -2, m = 3, returns sum = 6.
  2. Using n = -2, m = 4, returns sum = 10.

The problem is with for (int i = 1; i <= n - 1; i++), specifically i <= n - 1 because when n-1 <= 0 this will not run. I just can't think of a way around it.

Upvotes: 0

Views: 125

Answers (3)

vladmihaisima
vladmihaisima

Reputation: 2248

If you really have to use that formula you could use instead of:

for (int i = 1; i <= m; i++) {

the following code which changes the index either by 1 or by -1

for (int i = 1; i <= m; i+=(int)Math.signum(m-1+0.1)) {

(added 0.1 such that in case m is 1 the result is positive and not 0)

Ofc you should do the same for n.

Upvotes: 0

Ogiez
Ogiez

Reputation: 107

You could always check before if n < 0.

And then do another reverse loop for negative numbers.

e.g.

int sum = 0;
if(m < 0){
 for(int i = 0; i >= m; i--) {
  sum += i;
 }
} else {
 for (int i = 1; i <= m; i++) {
  sum += i;
 }
}

Upvotes: 0

SQL Hacks
SQL Hacks

Reputation: 1332

Your formula

SUM(N to M) = SUM( 1 to M ) - SUM( 1 to N-1 )

Doesn't really make sense for negative values. If you give that up you can make your program simpler. We very often start for loops at 0 or 1 but that doesn't have to be the case. You could instead start your loop at a n which might be negative:

   static int method2(int n, int m) {
        int sum = 0;
        for (int i = n; i <= m; i++) {
            sum = sum + i;
        }
        System.out.println("sum: " + sum);
        return sum;
    }

Upvotes: 1

Related Questions