Reject peek on empty
Updated
Peek on [] is the same class of error as pop on []: no top exists. Returning 0 is wrong. Returning a sentinel is only ok if the problem defines one.
What to focus on
Call isEmpty first, or let peek throw. After a drain loop, the next peek is illegal. One-item peek is legal.
Worked examples
empty
a = []- no top
Result: illegal
after drain
a = [7], pop, then peek- now empty
Result: illegal
zero is data
a = [0]- peek 0
Result: legal — 0 is a value
Try it
Write peek(a). a is the stack, written [bottom, ..., top]. Empty means size 0 and there is no top. peek([]) reports an error, not 0. After popping [7], peek is illegal. peek([0]) is 0. [] is illegal. [7] then pop then peek is illegal. [0] peeks 0.
The stack may be empty. Values are integers, including 0. Returning 0 is not a valid empty peek.