#include <iostream>

int main()
{
    int n,i;
    std::cin>>n;
    int sum=0;
    if(n<0)
    {
        for(i=n;i<=1;i++)
            sum+=i;
        std::cout<<sum;

    }
    else
    {

        for(i=n;i>=1;i--)
            sum+=i;
        std::cout<<sum;
    }
return 0;}

You outputs 0 on test 0 when should be 1.

try so ;)


#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    int a, sum = 0;
    cin >> a;

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

    else
    {
        for (int i = 1; i <= abs(a); i++)
        sum += i;
        sum = 1 - sum;
    }

    cout << sum;
    return 0;
}