Evaluate Postfix Expression Using Stack in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push a number onto the stack.

DSA Javascript

stack.push([1]);

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

Astack

Boperator

Cresult

Dtoken

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing the operator instead of the number.

Using the stack variable incorrectly.

In postfix evaluation, when the token is a number, we push it onto the stack.

2fill in blank

medium

Complete the code to pop the top two numbers from the stack for operation.

DSA Javascript

const b = stack.pop(); const a = stack.[1]();

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

Apeek

Bpush

Cpop

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() instead of pop().

Using peek() which does not remove the element.

We pop the top two numbers from the stack to apply the operator.

3fill in blank

hard

Fix the error in the switch case to correctly perform addition.

DSA Javascript

switch(operator) { case '+': result = a [1] b; break; }

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

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+'.

Using '*' or '/' which perform different operations.

The '+' operator adds two numbers in JavaScript.

4fill in blank

hard

Fill both blanks to check if the token is an operator.

DSA Javascript

if (token [1] operators) { const operator = token; const b = stack.pop(); const a = stack.pop(); const result = applyOperator(a, b, operator); stack.[2](result); }

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

Ain

Bpush

C==

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'in' for membership check.

Using 'pop' instead of 'push' to add result.

The 'in' keyword checks if token is in operators; 'push' adds result back to stack.

5fill in blank

hard

Fill all three blanks to complete the applyOperator function.

DSA Javascript

function applyOperator(a, b, operator) { switch(operator) { case '+': return a [1] b; case '-': return a [2] b; case '*': return a [3] b; default: throw new Error('Invalid operator'); } }

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up operators in cases.

Using division '/' instead of multiplication '*'.

The function returns the result of applying the operator to a and b.

Practice

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

easy

A. Linked List

B. Queue

C. Stack

D. Array

Solution

1. Step 1: Understand postfix evaluation process

   Postfix expressions are evaluated using a stack to hold operands until an operator is encountered.

2. Step 2: Identify the correct data structure

   The stack allows pushing operands and popping them for operations, which fits postfix evaluation perfectly.

3. Final Answer:

   Stack -> Option C

4. Quick Check:

   Postfix evaluation uses stack = Stack [OK]

Hint: Postfix uses stack to hold operands until operators appear [OK]

Common Mistakes:

• Choosing queue instead of stack
• Using array without stack operations
• Confusing with infix evaluation

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

easy

A. stack.push(element);

B. stack.add(element);

C. stack.insert(element);

D. stack.enqueue(element);

Solution

1. Step 1: Recall JavaScript stack operations

   JavaScript arrays use push() to add elements to the end, simulating stack push.

2. Step 2: Match correct method

   push() is the standard method; add(), insert(), enqueue() are incorrect for stack push.

3. Final Answer:

   stack.push(element); -> Option A

4. Quick Check:

   Push uses push() method = stack.push(element); [OK]

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

Common Mistakes:

• Using add() or insert() which don't exist
• Confusing enqueue() with queue operation
• Using pop() instead of push()

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

function evaluatePostfix(expr) {
  const stack = [];
  for (const ch of expr) {
    if (!isNaN(ch)) {
      stack.push(Number(ch));
    } else {
      const b = stack.pop();
      const a = stack.pop();
      if (ch === '+') stack.push(a + b);
      else if (ch === '*') stack.push(a * b);
    }
  }
  return stack.pop();
}
console.log(evaluatePostfix("23+5*"));

medium

A. 25

B. 15

C. 10

D. Error

Solution

1. Step 1: Evaluate postfix expression "23+5*" step-by-step

   Push 2, push 3, then '+' operator: pop 3 and 2, add = 5, push 5. Push 5, then '*' operator: pop 5 and 5, multiply = 25, push 25.

2. Step 2: Final stack value

   Only one value 25 remains, which is returned and printed.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   2+3=5, 5*5=25 = 25 [OK]

Hint: Calculate stepwise: operands pushed, operators pop two and push result [OK]

Common Mistakes:

• Mixing operand order in subtraction/division
• Forgetting to convert char to number
• Confusing operator precedence

4. Identify the error in the following postfix evaluation code snippet:

function evalPostfix(expr) {
  const stack = [];
  for (const ch of expr) {
    if (ch >= '0' && ch <= '9') {
      stack.push(ch);
    } else {
      const b = stack.pop();
      const a = stack.pop();
      if (ch === '+') stack.push(a + b);
      else if (ch === '-') stack.push(a - b);
    }
  }
  return stack.pop();
}

medium

A. Stack pop order is reversed

B. Operands are pushed as strings, not numbers

C. Missing handling for multiplication operator

D. Loop does not iterate over expression

Solution

1. Step 1: Check how operands are pushed

   The code pushes characters directly without converting to numbers, so operands are strings.

2. Step 2: Effect of pushing strings on arithmetic

   Adding strings with '+' concatenates instead of numeric addition, causing wrong results.

3. Final Answer:

   Operands are pushed as strings, not numbers -> Option B

4. Quick Check:

   Push operands as numbers, not strings = Operands are pushed as strings, not numbers [OK]

Hint: Convert chars to numbers before pushing to stack [OK]

Common Mistakes:

• Pushing chars without Number()
• Ignoring operator cases
• Assuming pop order is wrong

5. Given the postfix expression "82/3-4*", what is the correct evaluation result using a stack-based approach?

hard

A. 6

B. 0

C. 8

D. 4

Solution

1. Step 1: Evaluate "82/3-4*" step-by-step

   Push 8, push 2, '/' operator: 8 / 2 = 4, push 4. Push 3, '-' operator: 4 - 3 = 1, push 1. Push 4, '*' operator: 1 * 4 = 4, push 4.

2. Step 2: Final stack value

   Only one value 4 remains, but check carefully: The last '*' uses 1 and 4, result 4.

3. Step 3: Re-check calculation

   Actually, after '-' operation, stack has 1. Then push 4, then '*': 1 * 4 = 4.

4. Final Answer:

   4 -> Option D

5. Quick Check:

   8/2=4, 4-3=1, 1*4=4 = 4 [OK]

Hint: Evaluate stepwise: divide, subtract, then multiply [OK]

Common Mistakes:

• Mixing operand order in division
• Skipping operator precedence
• Incorrect stack pop sequence