Check out example codes for "sum in c#". It will help you in understanding the concepts better.

Code Example 1

int[] arr = new int[] { 1, 2, 3 };
int sum = 0;
for (int i = 0; i < arr.Length; i++)
{
    sum += arr[i];
}

int[] arr = { 3, 6, 4, 1, 6, 8 };

// returns 1
Array.IndexOf(arr, 6);

int maxValue = anArray.Max();
int maxIndex = anArray.ToList().IndexOf(maxValue);

Code Example 2

int num = 123;
int sum = 0;
while(num > 0)
{
  sum += number % 10;
  num /= 10;
}
Console.WriteLine(sum); // output: 6

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