array.Take(13).Skip(x) is subtracting the take

Question

This is for Project Euler, problem 8.

I am trying to foreach through the array of numbers, each time skipping the last number and pulling the next 13 adjacent numbers in the array.

My code:

for(int x = 0; x < 987; x++)
{
    foreach(int number in numbers.Take(13).Skip(x))
    {
        hold = hold * number;
        adjacent[index] = number;
        index++;
    }

    if (hold > product)
    {
        product = hold;
    }

    Array.Clear(adjacent, 0, adjacent.Length);
    index = 0;
    hold = 1;
}

The problem I'm running into is, every time it enumerates through the array, it subtracts the amount that x is, from how many times it goes through the list, which is 13.

So when x is 5, it only goes through the array 8 times.

How do I fix it where it traverses 13 numbers at a time?

Answer 1

Your code takes 13 first, and then skips the next x elements out of the 13 taken. Instead, skip first and then take:

  for (int x = 0; x < 987; x++)
    {
        foreach(int number in numbers.Skip(x).Take(13))
        {
            hold = hold * number;
            adjacent[index] = number;
            index++;
        }

        if (hold > product)
        {
            product = hold;
        }

        Array.Clear(adjacent, 0, adjacent.Length);
        index = 0;
        hold = 1;
    }

Answer 2

Put Skip and Take in the right order: first Skip, then Take, e.g. for the 100 digit number

  string source = string.Concat(
      @"73167176531330624919225119674426574742355349194934
        96983520312774506326239578318016984801869478851843
        85861560789112949495459501737958331952853208805511
        12540698747158523863050715693290963295227443043557
        66896648950445244523161731856403098711121722383113
        62229893423380308135336276614282806444486645238749
        30358907296290491560440772390713810515859307960866
        70172427121883998797908792274921901699720888093776
        65727333001053367881220235421809751254540594752243
        52584907711670556013604839586446706324415722155397
        53697817977846174064955149290862569321978468622482
        83972241375657056057490261407972968652414535100474
        82166370484403199890008895243450658541227588666881
        16427171479924442928230863465674813919123162824586
        17866458359124566529476545682848912883142607690042
        24219022671055626321111109370544217506941658960408
        07198403850962455444362981230987879927244284909188
        84580156166097919133875499200524063689912560717606
        05886116467109405077541002256983155200055935729725
        71636269561882670428252483600823257530420752963450"
     .Where(c => char.IsDigit(c)));

Linq implementation can be

  int size = 13; // or size = 4; for testing

  long result = Enumerable
    .Range(0, source.Length - size)
    .Select(index => source
      .Skip(index)
      .Take(size) 
      .Select(c => (long) (c - '0'))
      .Aggregate((x, y) => x * y))
    .Max();

   // 5832 if size = 4 (test)
   Console.Write(result);