Infix to Postfix Conversion Using Stack in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an operator onto the stack.

DSA Python

stack.[1](ch)

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

Apop

Bpush

Cappend

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push' which is not a Python list method.

In Python, to add an element to the end of a list (used as a stack), we use append.

2fill in blank

medium

Complete the code to pop the top element from the stack.

DSA Python

top = stack.[1]()

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

Adelete

Bpop

Cremove

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' which removes by value, not by position.

To remove and get the last item from a list (stack), use pop().

3fill in blank

hard

Fix the error in the precedence function to return correct precedence for '+' operator.

DSA Python

if op == '[1]':
    return 1

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing '+' with '-' or '*'.

The '+' operator has precedence 1 in this function.

4fill in blank

hard

Fill both blanks to correctly check operator precedence and decide stack pop.

DSA Python

while stack and precedence(stack[-1]) [1] precedence(ch):
    postfix += stack.[2]()

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

A>=

B<=

Cpop

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' instead of '>='.

Using 'append' instead of 'pop'.

We pop from stack while top has higher or equal precedence, so use '>=' and pop().

5fill in blank

hard

Fill all three blanks to correctly handle closing parenthesis in the stack.

DSA Python

while stack and stack[-1] != [1]:
    postfix += stack.[2]()
stack.[3]()  # Remove '('

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

A'('

Bpop

Cappend

D')'

Attempts:

3 left

💡 Hint

Common Mistakes

Using ')' instead of '(' for comparison.

Using append instead of pop.

We pop until '(' is found, then pop '(' itself. So use '(' for comparison and pop() to remove.

Practice

1. What is the main purpose of using a stack in converting an infix expression to postfix?

easy

A. To reverse the order of operands

B. To temporarily hold operators and manage their precedence

C. To store operands permanently

D. To count the number of operators

Solution

1. Step 1: Understand the role of stack in infix to postfix

   The stack is used to hold operators temporarily while processing the expression to respect operator precedence and associativity.

2. Step 2: Clarify what the stack does not do

   The stack does not store operands permanently or reverse operands; it only manages operators.

3. Final Answer:

   To temporarily hold operators and manage their precedence -> Option B

4. Quick Check:

   Stack manages operators [OK]

Hint: Stack holds operators, not operands, during conversion [OK]

Common Mistakes:

• Thinking stack stores operands
• Confusing stack role with reversing expression
• Ignoring operator precedence handling

2. Which of the following is the correct way to push an operator onto a stack in Python?

easy

A. stack.append(operator)

B. stack.push(operator)

C. stack.insert(operator)

D. stack.add(operator)

Solution

1. Step 1: Recall Python list methods for stack operations

   In Python, a list is used as a stack and the method to add an element on top is append().

2. Step 2: Identify incorrect methods

   push() and add() are not valid list methods; insert() inserts at a specific index, not top of stack.

3. Final Answer:

   stack.append(operator) -> Option A

4. Quick Check:

   Use append() to push on stack [OK]

Hint: Use append() to push items on Python list stack [OK]

Common Mistakes:

• Using push() which is not a Python list method
• Using add() which is for sets, not lists
• Using insert() incorrectly for stack push

3. What is the postfix expression for the infix expression A + B * C?

medium

A. ABC*+

B. AB+C*

C. A+BC*

D. *ABC+

Solution

1. Step 1: Apply operator precedence rules

   Multiplication (*) has higher precedence than addition (+), so B * C is evaluated first.

2. Step 2: Convert to postfix respecting precedence

   Postfix places operands first, then operator: B C * for multiplication, then A and + last, so expression is A B C * +.

3. Final Answer:

   ABC*+ -> Option A

4. Quick Check:

   Multiplication before addition = ABC*+ [OK]

Hint: Multiply before add; postfix puts operator after operands [OK]

Common Mistakes:

• Ignoring operator precedence
• Placing operators before operands
• Mixing operand order

4. Given the following code snippet for infix to postfix conversion, what is the error?

def precedence(op):
    if op == '+' or op == '-':
        return 1
    if op == '*' or op == '/':
        return 2
    return 0

stack = []
postfix = ''
expr = 'A+B*C'

for char in expr:
    if char.isalpha():
        postfix += char
    else:
        while stack and precedence(stack[-1]) >= precedence(char):
            postfix += stack.pop()
        stack.append(char)

while stack:
    postfix += stack.pop()

print(postfix)

medium

A. The precedence function returns 0 for unknown operators, causing errors

B. The postfix string is not updated with operands

C. The stack is not initialized properly

D. The code does not handle parentheses in the expression

Solution

1. Step 1: Analyze code handling of operators and operands

   The code correctly appends operands and manages operator precedence using the stack.

2. Step 2: Check for missing features

   The code does not check or handle parentheses, which are common in infix expressions and affect precedence.

3. Final Answer:

   The code does not handle parentheses in the expression -> Option D

4. Quick Check:

   Missing parentheses handling [OK]

Hint: Remember to handle parentheses explicitly in infix to postfix [OK]

Common Mistakes:

• Assuming precedence 0 causes errors (it just means lowest)
• Ignoring parentheses handling
• Thinking stack initialization is wrong

5. Convert the infix expression (A+B)*(C-D)/E to postfix notation using stack. What is the correct postfix expression?

hard

A. AB+CD-E*/

B. AB+CD-E/*

C. AB+CD-*E/

D. AB+CD*-E/

Solution

1. Step 1: Handle parentheses and operator precedence

   Parentheses force A+B and C-D to be evaluated first, so their postfix forms are AB+ and CD- respectively.

2. Step 2: Apply multiplication and division in order

   After parentheses, multiply the two results: AB+ CD- * then divide by E: AB+CD-* E /.

3. Final Answer:

   AB+CD-*E/ -> Option C

4. Quick Check:

   Parentheses first, then * and / = AB+CD-*E/ [OK]

Hint: Convert inside parentheses first, then apply * and / left to right [OK]

Common Mistakes:

• Ignoring parentheses order
• Mixing operator order for * and /
• Placing operators before operands