Lesson #1

namespace MyNamespace
{
    class MyClass
    {
        static void Main()
        {
            // Sum of arr1 all elements
            int[] arr1 = { 5, 9, 4, 13, 11 };
                      // { 14, 9, 4, 13, 11 } => step #1
                      // { 18, 9, 4, 13, 11 } => step #2
                      // { 31, 9, 4, 13, 11 } => step #3
                      // { 42, 9, 4, 13, 11 } => step #4

            /* METHOD #1
            int sum = 0;

            for (int i = 0; i < arr1.Length; i++)
            {
                // sum = sum + arr1[i];
                sum += arr1[i];

                // [0] => sum = 5
                // [1] => sum = 14
                // [2] => sum = 18
                // [3] => sum = 31
                // [4] => sum = 42
            }

            System.Console.WriteLine("Sum: " + sum);
            */

            // METHOD #2
            for (int i = 1; i < arr1.Length; i++)
            {
                arr1[0] = arr1[0] + arr1[i];
            }

            System.Console.WriteLine("Sum: " + arr1[0]);
        }
    }
}