Heap Sort

Hi everyone I am having a little problem with a heap sort code that I am writing. I have found a few templates that all looked the same but am having problems writing it on my own because I do not understand them completely. I completely understand how the heap sort algorithm works in theory and can draw out what happens using a binary tree. When implemented in code however I do not understand certain declarations like (root*2). I just wish I had a better picture of it. Please let me know if anyone can explain this or can direct me to a book or website that has an explanation of it.

Thanks

Code:

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void heapSort(int numbers[], int array_size)
{
  int i, temp;

  for (i = (array_size / 2)-1; i >= 0; i--)
    siftDown(numbers, i, array_size);

  for (i = array_size-1; i >= 1; i--)
  {
    temp = numbers[0];
    numbers[0] = numbers[i];
    numbers[i] = temp;
    siftDown(numbers, 0, i-1);
  }
}


void siftDown(int numbers[], int root, int bottom)
{
  int done, maxChild, temp;

  done = 0;
  while ((root*2 <= bottom) && (!done))
  {
    if (root*2 == bottom)
      maxChild = root * 2;
    else if (numbers[root * 2] > numbers[root * 2 + 1])
      maxChild = root * 2;
    else
      maxChild = root * 2 + 1;

    if (numbers[root] < numbers[maxChild])
    {
      temp = numbers[root];
      numbers[root] = numbers[maxChild];
      numbers[maxChild] = temp;
      root = maxChild;
    }
    else
      done = 1;
  }
}

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no time to explain now so just look over this applet