Evaluate Postfix Expression Using Stack in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push operands onto the stack.

DSA Python

if ch.isdigit():
    stack.[1](int(ch))

Drag options to blanks, or click blank then click option'

Apush

Bremove

Cpop

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of append() to add elements.

Using push() which is not a Python list method.

In Python, list's append() method adds an element to the end of the list, which is used here to push onto the stack.

2fill in blank

medium

Complete the code to pop the top two operands from the stack.

DSA Python

op2 = stack.[1]()
op1 = stack.pop()

Drag options to blanks, or click blank then click option'

Apop

Bappend

Cremove

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using append() which adds elements instead of removing.

Using remove() which removes by value, not by position.

pop() removes and returns the last element from the list, which is the top of the stack.

3fill in blank

hard

Fix the error in the operation application line to correctly compute the result.

DSA Python

if ch == '+':
    res = op1 [1] op2

Drag options to blanks, or click blank then click option'

A*

B-

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' or '*' instead of '+'.

Using '/' which performs division.

For addition, the operator '+' must be used between operands.

4fill in blank

hard

Fill both blanks to correctly handle multiplication and division operations.

DSA Python

elif ch == '*':
    res = op1 [1] op2
elif ch == '/':
    res = op1 [2] op2

Drag options to blanks, or click blank then click option'

A*

B+

C/

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up operators like using '+' for multiplication.

Using '-' for division.

Multiplication uses '*', division uses '/' operators between operands.

5fill in blank

hard

Fill all three blanks to complete the postfix evaluation function with correct stack operations and return.

DSA Python

def evaluate_postfix(expr):
    stack = []
    for ch in expr:
        if ch.isdigit():
            stack.[1](int(ch))
        else:
            op2 = stack.[2]()
            op1 = stack.pop()
            if ch == '+':
                res = op1 + op2
            elif ch == '-':
                res = op1 - op2
            elif ch == '*':
                res = op1 * op2
            elif ch == '/':
                res = op1 / op2
            stack.append(res)
    return stack.[3]()

Drag options to blanks, or click blank then click option'

Aappend

Bpop

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() which removes by value, not position.

Mixing append and pop incorrectly.

Operands are pushed using append(), popped using pop(), and the final result is popped from the stack.

Practice

1. What does evaluating a postfix expression using a stack mainly involve?

easy

A. Using recursion to evaluate the expression

B. Converting the postfix expression to infix before evaluation

C. Pushing operands onto the stack and popping two operands for each operator to apply the operation

D. Ignoring operators and only summing operands

Solution

1. Step 1: Understand postfix evaluation process

   In postfix evaluation, operands are pushed onto a stack. When an operator is encountered, two operands are popped, the operation is applied, and the result is pushed back.

2. Step 2: Identify correct method

   Pushing operands onto the stack and popping two operands for each operator to apply the operation correctly describes this stack-based process. Other options describe unrelated or incorrect methods.

3. Final Answer:

   Pushing operands onto the stack and popping two operands for each operator to apply the operation -> Option C

4. Quick Check:

   Postfix evaluation uses stack push/pop [OK]

Hint: Remember: push operands, pop two for operator, push result [OK]

Common Mistakes:

• Thinking postfix needs conversion to infix first
• Trying to evaluate from left to right without stack
• Ignoring the order of operands when popping

2. Which of the following Python code snippets correctly pushes operands and evaluates a postfix expression using a stack?

easy

A. stack.append(int(char)) when char is digit; pop two, apply operator, push result

B. stack.push(char) for operands; pop one operand for operator

C. Use stack.pop() to push operands; stack.append() to pop

D. Directly evaluate expression without stack

Solution

1. Step 1: Check correct stack operations

   In Python, list's append() adds elements (push), pop() removes last element (pop). For postfix, push operands as integers, pop two for operator.

2. Step 2: Match with options

   stack.append(int(char)) when char is digit; pop two, apply operator, push result correctly uses stack.append(int(char)) for digits and pops two operands for operators. Others misuse push/pop or skip stack.

3. Final Answer:

   stack.append(int(char)) when char is digit; pop two, apply operator, push result -> Option A

4. Quick Check:

   Use append to push, pop to remove [OK]

Hint: Use append() to push and pop() to remove from stack [OK]

Common Mistakes:

• Using stack.push() which is not a Python list method
• Popping only one operand for operator
• Not converting char to int before pushing

3. What is the output of the following Python code that evaluates the postfix expression '231*+9-'?

def evaluate_postfix(expr):
    stack = []
    for char in expr:
        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('231*+9-'))

medium

A. -4

B. -2

C. -3

D. 4

Solution

1. Step 1: Evaluate expression step-by-step

   Expression: '2 3 1 * + 9 -' - Push 2, 3, 1 - '*' operator: pop 1 and 3, multiply 3*1=3, push 3 - '+' operator: pop 3 and 2, add 2+3=5, push 5 - Push 9 - '-' operator: pop 9 and 5, subtract 5-9=-4, push -4

2. Step 2: Final result

   Pop final value from stack: -4

3. Final Answer:

   -4 -> Option A

4. Quick Check:

   Postfix '231*+9-' = -4 [OK]

Hint: Evaluate stepwise: push digits, apply operators on last two [OK]

Common Mistakes:

• Mixing operand order when popping (a, b vs b, a)
• Using float division instead of integer division
• Misreading the expression order

4. The following code is intended to evaluate a postfix expression but throws an error. What is the error and how to fix it?

def eval_postfix(expr):
    stack = []
    for char in expr:
        if char.isdigit():
            stack.append(char)
        else:
            a = stack.pop()
            b = stack.pop()
            if char == '+':
                stack.append(a + b)
            elif char == '-':
                stack.append(a - b)
    return stack.pop()

print(eval_postfix('23+'))

medium

A. SyntaxError due to missing colon; fix by adding colon after if

B. TypeError due to adding strings; fix by converting char to int before pushing

C. IndexError due to popping from empty stack; fix by checking stack size

D. No error; output is '5'

Solution

1. Step 1: Identify error cause

   The code pushes characters as strings onto stack. When popping, it tries to add strings, causing TypeError.

2. Step 2: Fix by converting to int

   Convert char to int before pushing: stack.append(int(char)) to allow arithmetic operations.

3. Final Answer:

   TypeError due to adding strings; fix by converting char to int before pushing -> Option B

4. Quick Check:

   Push integers, not strings, to avoid TypeError [OK]

Hint: Always convert digit chars to int before pushing [OK]

Common Mistakes:

• Pushing strings instead of integers
• Swapping operand order when popping
• Ignoring operator cases other than '+' and '-'

5. Given the postfix expression '56+82/-', what is the result after evaluating it using a stack? Show the step-by-step evaluation.

hard

A. 5

B. 1

C. 9

D. 7

Solution

1. Step 1: Evaluate postfix expression stepwise

   Expression: '5 6 + 8 2 / -' - Push 5, 6 - '+' operator: pop 6 and 5, add 5+6=11, push 11 - Push 8, 2 - '/' operator: pop 2 and 8, integer divide 8//2=4, push 4 - '-' operator: pop 4 and 11, subtract 11-4=7, push 7

2. Step 2: Final result

   Pop final value from stack: 7

3. Final Answer:

   7 -> Option D

4. Quick Check:

   Postfix '56+82/-' = 7 [OK]

Hint: Process operators on last two stack values in order [OK]

Common Mistakes:

• Dividing operands in wrong order (2/8 instead of 8/2)
• Using float division instead of integer division
• Mixing operand order when subtracting