Evaluate Postfix Expression Using Stack in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push a number onto the stack.

DSA Typescript

stack.[1](num);

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

Apop

Bpeek

Cpush

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push to add elements.

To add an element to the top of the stack, we use the push method.

2fill in blank

medium

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

DSA Typescript

const top = stack.[1]();

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

Apop

Bshift

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop to remove elements.

The pop method removes and returns the last element from the stack.

3fill in blank

hard

Fix the error in the code to correctly check if a character is an operator.

DSA Typescript

if (['+', '-', '*', '/'].[1](char)) {

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

Aincludes

BindexOf

Ccontains

Dhas

Attempts:

3 left

💡 Hint

Common Mistakes

Using contains or has which are not valid array methods in TypeScript.

The includes method checks if an array contains a specific element and returns true or false.

4fill in blank

hard

Fill both blanks to correctly perform the operation on two operands.

DSA Typescript

const result = operand1 [1] operand2; // [2] operator

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using + or - when the comment says multiplication.

The * operator multiplies operand1 and operand2.

5fill in blank

hard

Fill all three blanks to correctly evaluate the postfix expression step by step.

DSA Typescript

const operand2 = stack.[1]();
const operand1 = stack.[2]();
const result = operand1 [3] operand2;

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

Apop

Bpush

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop to get operands.

Using - instead of + for addition.

We pop operand2 and operand1 from the stack, then add them using +.

Practice

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

easy

A. Linked List

B. Queue

C. Array

D. Stack

Solution

1. Step 1: Understand postfix evaluation process

   Postfix expressions are evaluated by scanning from left to right and using a data structure to store operands temporarily.

2. Step 2: Identify the suitable data structure

   A stack allows pushing operands and popping them when an operator is encountered, making it ideal for postfix evaluation.

3. Final Answer:

   Stack -> Option D

4. Quick Check:

   Postfix evaluation uses Stack [OK]

Hint: Remember: postfix needs LIFO structure, so use stack [OK]

Common Mistakes:

• Confusing stack with queue
• Thinking array alone can manage operations
• Using linked list without stack behavior

2. Which of the following is the correct way to push a number onto a stack in TypeScript?

easy

A. stack.push(number);

B. stack.add(number);

C. stack.insert(number);

D. stack.enqueue(number);

Solution

1. Step 1: Recall stack methods in TypeScript

   Stacks in TypeScript are usually arrays, and pushing an element uses the push() method.

2. Step 2: Identify correct method

   push() adds an element to the end of the array, simulating stack push operation.

3. Final Answer:

   stack.push(number); -> Option A

4. Quick Check:

   Use push() to add to stack [OK]

Hint: Use push() to add elements to stack in TypeScript [OK]

Common Mistakes:

• Using add() or insert() which are not array methods
• Confusing enqueue() with queue operation
• Trying to use non-existent stack methods

3. What is the output of evaluating the postfix expression "6 2 3 + - 3 8 2 / + *" using a stack?

medium

A. 7

B. 36

C. 24

D. 30

Solution

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

   Expression: 6 2 3 + - 3 8 2 / + *
   Push 6 -> [6]
   Push 2 -> [6, 2]
   Push 3 -> [6, 2, 3]
   + : pop 3 (right), pop 2 (left) -> 2 + 3 = 5 -> [6, 5]
   - : pop 5 (r), pop 6 (l) -> 6 - 5 = 1 -> [1]
   Push 3 -> [1, 3]
   Push 8 -> [1, 3, 8]
   Push 2 -> [1, 3, 8, 2]
   / : pop 2 (r), 8 (l) -> 8 / 2 = 4 -> [1, 3, 4]
   + : pop 4 (r), 3 (l) -> 3 + 4 = 7 -> [1, 7]
   * : pop 7 (r), 1 (l) -> 1 * 7 = 7 -> [7]

2. Step 2: Confirm final stack value

   Final result: 7

3. Final Answer:

   7 -> Option A

4. Quick Check:

   Postfix evaluation result = 7 [OK]

Hint: Evaluate stepwise, push numbers, pop two for operators [OK]

Common Mistakes:

• Mixing operand order in subtraction/division
• Not popping two operands for each operator
• Assuming division is integer without checking

4. Identify the error in this TypeScript code snippet for postfix evaluation:

function evalPostfix(expr: string): number {
  const stack: number[] = [];
  for (const ch of expr.split(' ')) {
    if (!isNaN(Number(ch))) {
      stack.push(Number(ch));
    } else {
      const a = stack.pop();
      const b = stack.pop();
      switch (ch) {
        case '+': stack.push(a + b); break;
        case '-': stack.push(b - a); break;
        case '*': stack.push(a * b); break;
        case '/': stack.push(b / a); break;
      }
    }
  }
  return stack.pop()!;
}

medium

A. Operands are popped in wrong order for addition and multiplication

B. Missing check for empty stack before popping

C. Operands are popped in wrong order for subtraction and division

D. Using Number() instead of parseInt() causes errors

Solution

1. Step 1: Check for potential runtime errors

   The code pops from the stack without verifying if there are enough elements. For operators, it needs at least 2 elements; at the end, at least 1.

2. Step 2: Identify the issue

   Missing check for empty stack (or insufficient elements) before popping can cause runtime crashes.

3. Final Answer:

   Missing check for empty stack before popping -> Option B

4. Quick Check:

   Always validate stack size before pop [OK]

Hint: Check stack.length before every pop() to avoid errors [OK]

Common Mistakes:

• Swapping operand order for subtraction/division
• Not checking stack empty before pop
• Confusing Number() and parseInt() usage

5. Given the postfix expression "5 1 2 + 4 * + 3 -", what is the final result after evaluation using a stack?

hard

A. 17

B. 13

C. 14

D. 10

Solution

1. Step 1: Evaluate inner expressions stepwise

   Push 5, 1, 2
   '+' pops 2 and 1, sums 3; stack: [5,3]
   Push 4
   '*' pops 4 and 3, multiplies 12; stack: [5,12]
   '+' pops 12 and 5, sums 17; stack: [17]
   Push 3
   '-' pops 3 and 17, computes 17-3=14; stack: [14]

2. Step 2: Confirm final stack value

   Only one value remains: 14, which is the final result.

3. Final Answer:

   14 -> Option C

4. Quick Check:

   Postfix evaluation result = 14 [OK]

Hint: Evaluate operators stepwise, watch operand order carefully [OK]

Common Mistakes:

• Adding instead of subtracting last step
• Mixing operand order in subtraction
• Skipping multiplication step