ITPEC FE Morning April 2020 Question 3

Postfix Expression Evaluation — evaluate a postfix (reverse Polish notation) expression using a stack.

Expression: A B 2 × + D B ÷ C × − where A=4, B=3, C=5, D=6.

Step-by-step stack trace:

• •Push A(4), B(3), 2 → stack: [4, 3, 2]
• •× → 3 × 2 = 6 → stack: [4, 6]
• •+ → 4 + 6 = 10 → stack: [10]
• •Push D(6), B(3) → stack: [10, 6, 3]
• •÷ → 6 ÷ 3 = 2 → stack: [10, 2]
• •Push C(5) → stack: [10, 2, 5]
• •× → 2 × 5 = 10 → stack: [10, 10]
• •− → 10 − 10 = 0 → stack: [0]

Result: 0

Why not others:
- (a) −3 — incorrect operand order in subtraction or division
- (c) 3 — arithmetic error in intermediate steps
- (d) 40/3 — fails to evaluate B × 2 before addition

Key rule: In postfix, read left to right; when you hit an operator, pop two operands (first popped = right operand, second popped = left operand), apply the operator, push the result.

AI-generated — may contain errors