How does the equation for min heap array indexing work? I've been reading about min heaps, currently looking at this article, and I am very confused by something.

The article makes the following statement:


  If a given node is located at index 'x' in the array, its left child
  exists at index = 2x, and its right child exists at index = 2x + 1. 
  Each node's parent exists at index = x / 2 (rounded down).


It gives this example:



This min heap is represented in the array [100, 19, 36, 17, 3, 25, 1, 2, 7].

But this doesn't jive with the preceding statement about indexing. 36 can be found at index 2, and 2 * 2 is 4. But index 4 is 3-- the right child of the 19 node, not the left child of the 36 node.

What am I missing here?
Start indexing at $1$, then everything will be fine.

Want to start at $0$? That is no problem, but one has to adjust the formula's. Children of $x$ can be found at $2x+1$ and $2x+2$.