The sum of 1+(1-2)+(1-2+3)+(1-2+3-n)… where even integers are -k and odd integers are +k

I want to write a program where a user tells me an integer(n) and i calculate The sum of 1+(1-2)+(1-2+3)+(1-2+3-n)… where even integers are -k and odd integers are +k.

Ive made a function which does that But the sum is never correct. For example for n=2 it should be sum=0 but shows sum=-1 for n=3 should be sum=+2 but i shows sum=3. (Ignore the debugging printfs)

#include <stdio.h>

int athroismaAkolouthias(int n); // i sinartisi me tin opoia ypologizete to athroisma akolouthias 1+(1-2)+(1-2+3)+(1-2+3-4).....

int main(){
    int n;
    printf("give n: ");
    scanf("%d", &n);
    printf("the sum is %d", athroismaAkolouthias(n));
}

int athroismaAkolouthias(int n){
    int sum1=0, sum2=0,sum=0;
    int i, temp, j;
    for (i=1; i<=n; i++){
        for (j=1; j<=i; j++){
            temp=j;
        }
        if (i%2==0){sum=sum-temp; printf("test1 %d%d",sum,temp);}
        else{sum=temp; printf("test2 %d%d",sum,temp);}
    }
    return sum;
}

   

   

>Solution :

Your issue is with our loop which iterate with j, it should update the inner_sum based on j even/odd condition as follows:

#include <stdio.h>

int akl(int n) {
  int sum = 0;

  for (int i = 1; i <= n; i++) {
    int inner_sum = 0;
    for (int j = 1; j <= i; j++) {
      if (j % 2 == 0) {
        inner_sum -= j;
      } else {
        inner_sum += j;
      }
    }
    sum += inner_sum;
  }

  return sum;
}

int main() {
  int n;

  scanf("%d", &n);

  printf("%d\n", akl(n));
}

You only need two variables for sum that I named them inner_sum and sum which shows the sum of each term and sum over all terms.