Evaluate Postfix Expression Using Stack in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push a number onto the stack.

DSA C#

stack.[1](num);

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

APeek

BPop

CPush

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop instead of Push

Using Peek instead of Push

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

2fill in blank

medium

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

DSA C#

int val = stack.[1]();

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

APush

BPop

CPeek

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Push instead of Pop

Using Peek instead of Pop

The Pop method removes and returns the top element from the stack.

3fill in blank

hard

Fix the error in the operator check condition.

DSA C#

if (token == '+' || token == '-' || token == '*' || token == [1]) {

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

A/

B%

C&

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using modulus % instead of division /

Using bitwise & instead of division /

The division operator '/' is used in postfix expressions along with +, -, and *.

4fill in blank

hard

Fill both blanks to correctly perform the operation on two popped values.

DSA C#

int b = stack.[1]();
int a = stack.[2]();

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

APop

BPush

CPeek

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Push or Peek instead of Pop

Using different methods for each blank

Both values must be removed from the stack using Pop before applying the operator.

5fill in blank

hard

Fill all three blanks to correctly compute and push the result for the '+' operator.

DSA C#

if (token == '+') {
    int result = a [1] b;
    stack.[2](result);
} else if (token == '-') {
    int result = a [3] b;
    stack.Push(result);
}

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

A+

BPush

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operators

Not pushing the result back

Mixing operators in wrong places

Use + to add a and b, then Push the result. For subtraction, use -.

Practice

1. What is the main data structure used to evaluate a postfix expression?

easy

A. Array

B. Queue

C. Stack

D. Linked List

Solution

1. Step 1: Understand postfix evaluation process

   Postfix expressions are evaluated by pushing operands onto a stack and popping them when an operator is encountered. The stack allows last-in, first-out access which is perfect for postfix evaluation.

2. Final Answer:

   Stack -> Option C

3. Quick Check:

   Postfix evaluation uses stack [OK]

Hint: Postfix needs last-in-first-out structure: stack [OK]

Common Mistakes:

• Confusing stack with queue
• Using array without stack logic
• Thinking linked list is primary structure

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

easy

A. stack.Push(operand);

B. stack.Add(operand);

C. stack.Insert(operand);

D. stack.Enqueue(operand);

Solution

1. Step 1: Recall C# Stack.Push method

   In C#, the method to add an element to a stack is Push. Enqueue is for queues; Add and Insert are not stack methods.

2. Final Answer:

   stack.Push(operand); -> Option A

3. Quick Check:

   Use Push() to add to stack [OK]

Hint: Use Push() to add items to stack in C# [OK]

Common Mistakes:

• Using Enqueue which is for queues
• Using Add or Insert which are not stack methods
• Confusing stack with list methods

3. What is the output of the following C# code that evaluates the postfix expression "23+5*"?

string expr = "23+5*";
Stack<int> stack = new Stack<int>();
foreach (char c in expr)
{
    if (char.IsDigit(c))
        stack.Push(c - '0');
    else
    {
        int b = stack.Pop();
        int a = stack.Pop();
        if (c == '+') stack.Push(a + b);
        else if (c == '*') stack.Push(a * b);
    }
}
Console.WriteLine(stack.Pop());

medium

A. 25

B. 15

C. 10

D. Error

Solution

1. Step 1: Trace code execution

   Push 2, push 3; '+' pops b=3, a=2, push 5. Push 5; '*' pops b=5, a=5, push 25. Final pop prints 25.

2. Final Answer:

   25 -> Option A

3. Quick Check:

   (2+3)*5 = 25 [OK]

Hint: Calculate stepwise: operands pushed, operators pop two values [OK]

Common Mistakes:

• Mixing operand order when popping
• Confusing '+' and '*' operations
• Forgetting to convert char digits to int

4. Identify the error in the following C# code snippet that evaluates a postfix expression:

string expr = "82/";
Stack<int> stack = new Stack<int>();
foreach (char c in expr)
{
    if (char.IsDigit(c))
        stack.Push(c - '0');
    else
    {
        int a = stack.Pop();
        int b = stack.Pop();
        if (c == '/') stack.Push(b / a);
    }
}
Console.WriteLine(stack.Pop());

medium

A. Stack is not initialized

B. Operands are popped in wrong order

C. Division operator is not handled

D. Digits are not converted to integers

Solution

1. Step 1: Trace popping and computation for division

   Push 8, push 2 (top=2). For '/', b=Pop()=2 (right), a=Pop()=8 (left), but pushes b/a=2/8=0 instead of a/b=8/2=4. Operands popped correctly but used/applied in wrong order for non-commutative division.

2. Final Answer:

   Operands are popped in wrong order -> Option B

3. Quick Check:

   Pop right first (b), left second (a), compute a / b [OK]

Hint: Pop second operand first, then first operand for correct order [OK]

Common Mistakes:

• Swapping operand order causing wrong division
• Not handling division operator
• Forgetting to convert char to int

5. Given the postfix expression "6523+8*+3+*", what is the final result after evaluation using a stack?

hard

A. 2880

B. Incorrect expression

C. None (runtime error)

D. 288

Solution

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

   Push 6,5,2,3; '+' pops 3&2 push 5 (stack:6,5,5); push 8; '*' pops 8&5 push 40 (6,5,40); '+' pops 40&5 push 45 (6,45); push 3 (6,45,3); '+' pops 3&45 push 48 (6,48); '*' pops 48&6 push 288.

2. Final Answer:

   288 -> Option D

3. Quick Check:

   Stepwise postfix evaluation = 288 [OK]

Hint: Evaluate postfix stepwise, push operands, apply operators [OK]

Common Mistakes:

• Misordering operands during pop
• Skipping steps in complex expressions
• Confusing operator precedence in postfix