I wanna print how many triangles exist having the perimeter of n.(with integer sides). Actually the formula I used is here. T(n): n=even --> n^2/48 | n=odd --> (n 3)^2/48. and here is my code:

        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            double m = 0;

            if(n%2==0)
            {
                m =Math.Round( n * n / 48.0);
            }
            else
            {
                m = Math.Round((n   3) * (n   3) / 48.0);
            }

            Console.Write(m);
        }

It actually gives me half of the score.

CodePudding user response：

It turned out the problem was the asker was supposed to handle values n up to 1_000_000. But for such inputs n * n which is done as integer multiplication, overflows silently and produces an unexpected result.

(Curiously, for these cases it could have worked to switch to double earlier, like saying n * (n / 48.0) or (double)n * n / 48.0 instead of n * n / 48.0.)

You can use a syntax like checked { /* statements here */} or (inside an expression) checked(n * n) to get an exception rather than a silent loss of significant bits that the fixed-width integer type cannot hold.

The asker fixed it by switching to long:

static void Main(string[] args)
{
    long n = long.Parse(Console.ReadLine());
    double m;

    if (n % 2 == 0)
    {
        m = Math.Round(n * n / 48.0);
    }
    else
    {
        m = Math.Round((n   3) * (n   3) / 48.0);
    }

    Console.Write(m);
}

If you have access to System.Numerics, you can write a solution that works for even bigger inputs:

static void Main(string[] args)
{
    BigInteger n = BigInteger.Parse(Console.ReadLine());
    BigInteger m;

    if (n % 2 == 0)
    {
        m = (n * n   24) / 48;
    }
    else
    {
        m = ((n   3) * (n   3)   24) / 48;
    }

    Console.Write(m);
}

where we use integer division /, and the trick of adding half the denominator to the numerator ensures correct rounding to nearest.