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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push operands onto the stack.

DSA C++

if (isdigit(exp[i])) {
    int operand = exp[i] - '0';
    s.[1](operand);
}

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

Apop

Bpush

Ctop

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of push()

Trying to access top() to add elements

We use push() to add operands to the stack.

2fill in blank

medium

Complete the code to get the value of the top element from the stack.

DSA C++

int val2 = s.[1]();

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

Apush

Bpop

Cempty

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which returns void

Using empty() which checks if stack is empty

top() returns the top element without removing it; pop() removes it but returns void.

3fill in blank

hard

Fix the error in the code to correctly pop the element after reading it.

DSA C++

int val1 = s.top();
s.[1]();

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

Aempty

Bpush

Cpop

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Calling top() again instead of pop()

Using push() mistakenly

After reading the top element, pop() removes it from the stack.

4fill in blank

hard

Fill all three blanks to perform addition on two popped values and push the result.

DSA C++

int val2 = s.top();
s.[1]();
int val1 = s.top();
s.[2]();
int res = val1 [3] val2;
s.push(res);

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 remove elements

Using '-' instead of '+' for addition

We pop both values, add them with '+', then push the result.

5fill in blank

hard

Fill all three blanks to evaluate the postfix expression correctly for multiplication.

DSA C++

int val2 = s.top();
s.[1]();
int val1 = s.top();
s.[2]();
int res = val1 [3] val2;
s.push(res);

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

Apop

Bpush

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*'

Using push() instead of pop() to remove elements

Pop both operands, multiply with '*', then push the result.

Practice

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

easy

A. Queue

B. Stack

C. Array

D. Linked List

Solution

1. Step 1: Understand postfix evaluation process

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

2. Step 2: Identify the data structure used

   Operands are pushed onto the stack, and when an operator is found, operands are popped for calculation, then the result is pushed back.

3. Final Answer:

   Stack -> Option B

4. Quick Check:

   Postfix evaluation uses Stack [OK]

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

Common Mistakes:

• Using queue instead of stack
• Trying to evaluate without any data structure
• Confusing postfix with infix evaluation

2. Which of the following is the correct syntax to push an integer value '5' onto a stack named 's' in C++?

easy

A. push(s, 5);

B. s.insert(5);

C. s.add(5);

D. s.push(5);

Solution

1. Step 1: Recall C++ stack push syntax

   The standard C++ stack uses the member function push() to add elements.

2. Step 2: Match syntax with options

   Only s.push(5); uses s.push(5); which is correct syntax.

3. Final Answer:

   s.push(5); -> Option D

4. Quick Check:

   Stack push syntax = s.push(value) [OK]

Hint: Use s.push(value) to add elements to stack [OK]

Common Mistakes:

• Using non-existent methods like add() or insert()
• Calling push as a free function instead of member function
• Confusing push with pop

3. What is the output of the following C++ code that evaluates the postfix expression "231*+9-"?

#include <iostream>
#include <stack>
#include <string>
using namespace std;

int evaluatePostfix(string exp) {
    stack<int> s;
    for (char c : exp) {
        if (isdigit(c)) {
            s.push(c - '0');
        } else {
            int val2 = s.top(); s.pop();
            int val1 = s.top(); s.pop();
            switch(c) {
                case '+': s.push(val1 + val2); break;
                case '-': s.push(val1 - val2); break;
                case '*': s.push(val1 * val2); break;
                case '/': s.push(val1 / val2); break;
            }
        }
    }
    return s.top();
}

int main() {
    cout << evaluatePostfix("231*+9-");
    return 0;
}

medium

A. -4

B. -2

C. 4

D. 2

Solution

1. Step 1: Evaluate postfix expression "231*+9-" step-by-step

   Push 2, push 3, push 1. '*' operator: pop 1 and 3, multiply 3*1=3, push 3. '+' operator: pop 3 and 2, add 2+3=5, push 5. Push 9. '-' operator: pop 9 and 5, subtract 5-9=-4, push -4.

2. Step 2: Final value on stack is -4

   After all operations, the stack top is -4, which is the result printed.

3. Final Answer:

   -4 -> Option A

4. Quick Check:

   Postfix "231*+9-" = -4 [OK]

Hint: Evaluate stepwise: push numbers, apply operators on top two [OK]

Common Mistakes:

• Mixing operand order in subtraction
• Forgetting to convert char digits to int
• Popping operands in wrong order

4. The following code snippet is intended to evaluate a postfix expression but produces incorrect results. What is the bug?

int evaluatePostfix(string exp) {
    stack<int> s;
    for (char c : exp) {
        if (isdigit(c)) {
            s.push(c - '0');
        } else {
            int val1 = s.top(); s.pop();
            int val2 = s.top(); s.pop();
            switch(c) {
                case '+': s.push(val1 + val2); break;
                case '-': s.push(val1 - val2); break;
                case '*': s.push(val1 * val2); break;
                case '/': s.push(val1 / val2); break;
            }
        }
    }
    return s.top();
}

medium

A. Missing return statement

B. isdigit() check is incorrect

C. Operands val1 and val2 are popped in wrong order

D. Stack is not initialized properly

Solution

1. Step 1: Analyze operand popping order

   The code pops val1 first, then val2. In postfix evaluation, val2 is the top operand, val1 is below it.

2. Step 2: Correct operand order for operations

   Operations should be val1 (second popped) operator val2 (first popped). The code reverses this, causing wrong results.

3. Final Answer:

   Operands val1 and val2 are popped in wrong order -> Option C

4. Quick Check:

   Operand order matters: val2 = top, val1 = next [OK]

Hint: Pop second operand first, then first operand [OK]

Common Mistakes:

• Swapping operand order in operations
• Not popping operands before using them
• Confusing isdigit() usage

5. Given the postfix expression "56+82/-", what is the final result after evaluation using a stack?

hard

A. 7

B. 5

C. 1

D. 3

Solution

1. Step 1: Evaluate postfix "56+82/-" stepwise

   Push 5, push 6. '+' operator: pop 6 and 5, add 5+6=11, push 11. Push 8, push 2. '/' operator: pop 2 and 8, divide 8/2=4, push 4. '-' operator: pop 4 and 11, subtract 11-4=7, push 7.

2. Step 2: Final value on stack is 7

   After all operations, the stack top is 7, which is the result.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   Postfix "56+82/-" = 7 [OK]

Hint: Process operators immediately on top two stack values [OK]

Common Mistakes:

• Incorrect operand order in division and subtraction
• Misreading the expression characters
• Forgetting to convert char digits to int