Evaluate Postfix Expression Using Stack in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Evaluate Postfix Expression Output

What is the output of the following Python code that evaluates a postfix expression using a stack?

DSA Python

def evaluate_postfix(expression):
    stack = []
    for char in expression:
        if char.isdigit():
            stack.append(int(char))
        else:
            b = stack.pop()
            a = stack.pop()
            if char == '+':
                stack.append(a + b)
            elif char == '-':
                stack.append(a - b)
            elif char == '*':
                stack.append(a * b)
            elif char == '/':
                stack.append(int(a / b))
    return stack.pop()

result = evaluate_postfix("231*+9-")
print(result)

A -4

B 7

C 8

D 2

💡 Hint

Trace the stack operations step-by-step for each character in the expression.

The postfix expression "231*+9-" is evaluated as follows: push 2, push 3, push 1, multiply top two (3*1=3), stack becomes [2,3], add top two (2+3=5), stack becomes [5], push 9, subtract top two (5-9=-4), final result is -4.

🧠 Conceptual

intermediate

Stack Usage in Postfix Evaluation

Why is a stack the best data structure to evaluate postfix expressions?

A Because it stores all operands in sorted order automatically.

B Because it uses less memory than other data structures.

C Because it can store operators and operands separately.

D Because it allows easy access to the last two operands needed for each operator.

💡 Hint

Think about how postfix expressions are evaluated step-by-step.

A stack allows pushing operands and popping the last two operands when an operator is encountered, which matches the postfix evaluation process perfectly.

🔧 Debug

advanced

Identify the Error in Postfix Evaluation Code

What error will the following code raise when evaluating the postfix expression "82/"? ```python def evaluate_postfix(expression): stack = [] for char in expression: if char.isdigit(): stack.append(int(char)) else: b = stack.pop() a = stack.pop() if char == '+': stack.append(a + b) elif char == '-': stack.append(a - b) elif char == '*': stack.append(a * b) elif char == '/': stack.append(a // b) return stack.pop() print(evaluate_postfix("82/")) ```

A No error, output is 4

B IndexError

C TypeError

D ZeroDivisionError

💡 Hint

Check the division operation and the stack contents carefully.

The expression "82/" means 8 divided by 2, which is 4. The code uses integer division (//), which works correctly here. No error occurs.

❓ Predict Output

advanced

Output of Postfix Evaluation with Multi-digit Numbers

What will be the output of this code that attempts to evaluate a postfix expression with multi-digit numbers separated by spaces?

DSA Python

def evaluate_postfix(expression):
    stack = []
    tokens = expression.split()
    for token in tokens:
        if token.isdigit():
            stack.append(int(token))
        else:
            b = stack.pop()
            a = stack.pop()
            if token == '+':
                stack.append(a + b)
            elif token == '-':
                stack.append(a - b)
            elif token == '*':
                stack.append(a * b)
            elif token == '/':
                stack.append(int(a / b))
    return stack.pop()

print(evaluate_postfix("12 3 4 * + 6 -"))

A SyntaxError

B 18

C 14

D 24

💡 Hint

Evaluate the expression step-by-step: 3*4=12, 12+12=24, 24-6=18.

The expression "12 3 4 * + 6 -" evaluates as: push 12, push 3, push 4, multiply top two (3*4=12), stack: [12,12], add top two (12+12=24), stack: [24], push 6, subtract top two (24-6=18), final result 18.

🧠 Conceptual

expert

Handling Division in Postfix Evaluation

In postfix evaluation, why might using integer division (//) instead of float division (/) cause issues in some cases?

A Because integer division converts operands to strings before dividing.

B Because integer division raises an error when dividing by zero.

C Because integer division truncates the result, losing decimal precision which may be needed.

D Because integer division always rounds up the result.

💡 Hint

Think about the difference between truncating and keeping decimals.

Integer division truncates the decimal part, which can lead to incorrect results if the exact decimal value is important in the expression evaluation.