Infix to Postfix Conversion Using Stack in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an operator onto the stack.

DSA C#

stack.[1](ch);

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

APop

BPeek

CClear

DPush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push removes an element instead of adding.

Using peek only looks at the top element without adding.

The push method adds an element to the top of the stack.

2fill in blank

medium

Complete the code to check if the stack is empty.

DSA C#

if (stack.[1] == 0) { /* stack is empty */ }

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

ACount

BLength

CCapacity

DSize

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length or Size which are not properties of Stack in C#.

Using Capacity which is for collections like List, not Stack.

The Count property returns the number of elements in the stack.

3fill in blank

hard

Fix the error in the code to get the top element without removing it.

DSA C#

char top = stack.[1]();

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

APop

BPush

CPeek

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop removes the element, which is not desired here.

Using push or top are not valid Stack methods.

The Peek method returns the top element without removing it.

4fill in blank

hard

Fill both blanks to correctly check operator precedence.

DSA C#

if (GetPrecedence(ch) [1] GetPrecedence(stack.[2]())) { /* pop stack */ }

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

A<

B>

CPeek

DPop

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' reverses the logic and causes wrong popping.

Using pop() removes the element prematurely.

We compare if current operator has lower precedence than top of stack using '<' and get top with Peek.

5fill in blank

hard

Fill all three blanks to correctly build the postfix expression.

DSA C#

postfix += [1].ToString();
stack.[2]();
stack.[3](ch);

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

Astack.Peek()

BPop

CPush

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using top instead of Peek() is invalid.

Using push instead of pop removes wrong element.

Add top of stack to postfix, pop it, then push current operator.

Practice

1. What is the main purpose of using a stack in Infix to Postfix Conversion?

easy

A. To temporarily hold operators and manage their precedence

B. To store the final postfix expression

C. To reverse the infix expression

D. To count the number of operands

Solution

1. Step 1: Understand the role of stack in conversion

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

2. Step 2: Differentiate stack usage from other tasks

   The postfix expression is built separately, and the stack is not used to store operands or reverse the expression.

3. Final Answer:

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

4. Quick Check:

   Stack holds operators = A [OK]

Hint: Stack holds operators to manage precedence [OK]

Common Mistakes:

• Thinking stack stores operands
• Confusing stack with output storage
• Assuming stack reverses expression

2. Which of the following C# code snippets correctly pushes an operator onto a stack named operatorStack?

easy

A. operatorStack.Push('+');

B. operatorStack.Add('+');

C. operatorStack.Insert('+');

D. operatorStack.Append('+');

Solution

1. Step 1: Recall stack method to add elements

   In C#, the method to add an element to a stack is Push().

2. Step 2: Verify other methods are invalid for Stack

   Add(), Insert(), and Append() are not valid methods for the Stack class.

3. Final Answer:

   operatorStack.Push('+'); -> Option A

4. Quick Check:

   Stack push method = Push() [OK]

Hint: Use Push() to add elements to stack [OK]

Common Mistakes:

• Using Add() instead of Push()
• Confusing List methods with Stack methods
• Trying to use Append() on Stack

3. What is the postfix expression output for the infix expression A + B * C using standard operator precedence?

medium

A. *ABC+

B. AB+C*

C. ABC*+

D. A+BC*

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 * then A +, resulting in ABC*+.

3. Final Answer:

   ABC*+ -> Option C

4. Quick Check:

   Multiplication before addition = ABC*+ [OK]

Hint: Multiply before add in postfix [OK]

Common Mistakes:

• Ignoring operator precedence
• Placing operators before operands
• Confusing postfix order

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

Stack<char> stack = new Stack<char>();
string infix = "A+B*C";
string postfix = "";
foreach (char ch in infix)
{
  if (char.IsLetter(ch))
    postfix += ch;
  else
    stack.Push(ch);
}
while (stack.Count > 0)
  postfix += stack.Pop();
Console.WriteLine(postfix);

medium

A. Operands are not added to postfix

B. Operators are pushed without checking precedence

C. Stack is not initialized

D. Postfix string is not printed

Solution

1. Step 1: Analyze operator handling in code

   The code pushes operators directly onto the stack without checking precedence or popping higher precedence operators first.

2. Step 2: Understand impact on postfix output

   This causes incorrect postfix order because operator precedence is ignored.

3. Final Answer:

   Operators are pushed without checking precedence -> Option B

4. Quick Check:

   Missing precedence check = B [OK]

Hint: Always check operator precedence before pushing [OK]

Common Mistakes:

• Ignoring operator precedence rules
• Assuming stack auto-sorts operators
• Not popping operators when needed

5. Convert the infix expression (A+B)*(C-D)/E to postfix using stack-based conversion. 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, then multiply results, then divide by E.

2. Step 2: Convert stepwise to postfix

   A+B becomes AB+, C-D becomes CD-, then multiply: AB+CD-*, then divide by E: AB+CD-*E/.

3. Final Answer:

   AB+CD-*E/ -> Option D

4. Quick Check:

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

Hint: Process parentheses first, then operators by precedence [OK]

Common Mistakes:

• Ignoring parentheses order
• Mixing operator precedence
• Placing division before multiplication