ITPEC FE Subject A April 2024 Question 3
ITPEC FE Subject A April 2024 — Question 3 of 60
Postfix (Reverse Polish) Notation — evaluate an expression using a stack.
Expression: 100 5 6 2 + * 12 4 / - - 36 -
Step-by-step stack evaluation:
- •
100→[100] - •
5→[100, 5] - •
6 2 +→ 6+2 =8→[100, 5, 8] - •
*→ 5×8 =40→[100, 40] - •
12 4 /→ 12÷4 =3→[100, 40, 3] - •
-→ 40−3 =37→[100, 37] - •
-→ 100−37 =63→[63] - •
36→[63, 36] - •
-→ 63−36 = 27
Why not others:
- (a) 5 — incorrect operator ordering
- (b) 18 — likely error in subtraction order
- (c) 21 — arithmetic mistake
Key rule: When an operator is encountered, pop two operands: the first popped is the right operand, the second popped is the left. For - and /, order matters: left - right, left / right.
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