Why is it helpful to keep a sentinel element in our heap list?

Frequently Asked Questions Python FAQ
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Question

In the context of this exercise, why is it helpful to keep a sentinel element in our heap list?

Answer

There are a few reasons why having a “sentinel” element at the top of our heap list can be helpful.

One reason is that it lets us quickly check if a heap is empty. If the last element of our heap is the sentinel, that must mean there are no other elements, so the heap is empty.

Another reason is that using a sentinel helps simplify some of the methods. The parent_idx, left_child_idx, and right_child_idx methods in particular utilize the index values to calculate the parent, left child, and right child indices respectively. With the sentinel, the first index of the list is 1 instead of 0, and this can make some of these calculations simpler.

# without a sentinel
def parent_idx(self, idx):
  return (idx - 1) // 2

# with a sentinel
def parent_idx(self, idx):
  return idx // 2

Although this may appear like a small change, this can also help avoid issues like potential “off-by-one” errors when dealing with indices.