Using arrays with recursion

Question

Now that I am using recursion to calcuate the sum of numbers, I want to do something slightly different. The following is my code that will sum the numbers 1,2,3,4,5. How would I modify my code to place the numbers 1, 2, 3, 4, 5 into an array and then use it in the recursion method? I have tried so many different attempts and I am apparently missing something. I know that in the recursion method, that I want to use the Length property of the array to control it.

Susan

   static void Main(string[] args)
    {
        Console.WriteLine(Sum(5));
        Console.Read();
    }

    static int Sum(int value)
    {
        if (value > 0) 
        {
            return value + Sum(value - 1);
        }
        else
        {
            return 0;
        }
    }

Answer 1

Sniff, sniff, smells like homework to me.

However, a hint. Adding all the elements of an array of length 'n' is the same as adding all the elements of an array of length 'n-1', then adding the value of element 'n'.

The result of adding all the elements of an array of length '1' is just the value of the one element

Answer 2

I believe that you need to pass the array and the index in your Sum function to control the recursivity.

Answer 3

What about using a Stack?:

Stack<int> stack = new Stack<int>(new int [] {1,2,3,4,5});
Console.WriteLine(SumStack(stack));

public static int SumStack(Stack<int> input)
{
    return input.Count > 0 ? input.Pop() + SumStack(input) : 0;
}

Answer 4

You're wanting to perform an operation that is commonly referred to as "fold" a.k.a. "reduce".

Luckily, the .NET team did your (home)work for you!

    static int sum(int[] values)
    {
        return values.Aggregate<int>((hd, tl) => hd + tl);            
    }

You can rewrite it for easier readability if you like. ;)

EDIT: usage example

int[] values = new int[]{1,2,3,4,5};
Console.WriteLine(sum(values));

Answer 5

Of if your no good with lambda expressions or you only have .net 2.0, this is simple enough;

static void Main(string[] args)
{

    int[] myArray = new int[5] {1,2,3,4,5 };

    Console.WriteLine(Sum1(myArray));
    Console.Read();

}

private static int Sum1(int[] myArray)
{

    if (myArray.Length > 0)
    {
        int lengthZeroAdjusted = myArray.Length - 1;
        int element = myArray[lengthZeroAdjusted];

        Array.Resize<int>(ref myArray, lengthZeroAdjusted);
        return element + Sum1(myArray);
    }
    else
    {
        return 0;
    }
}

Answer 6

let seq = [1;2;3;4;5]

let rec samp nums = 
  match nums with
  | []      -> 0
  | h::t    -> h + samp t

Answer 7

Recursion essentially means doing the small part, and giving the larger part to somebody else. Now if you have to 'n' elements of array, you ask somebody to give you sum of last n-1 elements, and just first one yourself ... just handle the case where there is nothing to be done ...

a pseudo code would be :

   sum array start_index   =  if ( start_index >= length(array) ) return 0
                              else return array[start_index] = sum array (start_index + 1)

   print (sum array 0 )