ITPEC FE Morning October 2023 Question 7

Stack Operations (PUSH / POP / READ) — trace stack state to find what the last READ returns.

Given operations on an empty stack:

PUSH 2 → READ → PUSH 3 → PUSH 6 → POP → READ → PUSH 4 → READ → PUSH 7 → PUSH 5 → POP → POP → READ

Step-by-step trace (top of stack on the right):

• •PUSH 2 → [2]
• •READ → peek 2
• •PUSH 3 → [2, 3]
• •PUSH 6 → [2, 3, 6]
• •POP → remove 6 → [2, 3]
• •READ → peek 3
• •PUSH 4 → [2, 3, 4]
• •READ → peek 4
• •PUSH 7 → [2, 3, 4, 7]
• •PUSH 5 → [2, 3, 4, 7, 5]
• •POP → remove 5 → [2, 3, 4, 7]
• •POP → remove 7 → [2, 3, 4]
• •READ → peek `4`

Answer: c) 4

Why not others:
- (a) 2 — sits at the bottom of the stack, never reached by the last READ
- (b) 3 — below 4 at the time of the last READ
- (d) 6 — was already removed by POP in step 5

Key rule: READ (peek) returns the top element without removing it; POP removes the top element. Trace the full sequence to know what sits on top at any given moment.

AI-generated — may contain errors