Count items on a stack
Updated
Size is how many values are currently on the stack. You maintain it on each push (+1) and pop (−1), or you read the backing length. Empty is 0.
What to focus on
Size is not the top. Size 3 with top 9 is [?, ?, 9]. You can know size without knowing buried values, if you store a counter.
Worked examples
three
a = [4, 1, 9]- three pushes, no pops
Result: size 3
after pop
a = [4, 1, 9], pop- size was 3
- now 2
Result: size 2
empty
a = []- no items
Result: size 0
Try it
Write size(a). a is the stack, written [bottom, ..., top]. Return how many values are on the stack. Size tracks push and pop, not the top value. On [4, 1, 9] return 3. After one pop return 2. On [] return 0.
Values are integers. The stack may be empty.