Evaluate Postfix Expression Using Stack in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push operands onto the stack.

DSA Java

if (Character.isDigit(ch)) {
    stack.[1](ch - '0');
}

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

Apop

Bempty

Cpeek

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push removes elements instead of adding.

Using peek only views the top element without adding.

We use stack.push() to add operands to the stack for later operations.

2fill in blank

medium

Complete the code to pop two operands from the stack for an operation.

DSA Java

int val2 = stack.[1]();
int val1 = stack.pop();

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

Aempty

Bpop

Cpeek

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop adds elements instead of removing.

Using peek does not remove the element.

We pop operands from the stack to perform the operation in correct order.

3fill in blank

hard

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

DSA Java

switch(ch) {
    case '*':
        result = val1 [1] val2;
        break;
}

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' or '-' instead of '*' changes the operation.

Using '/' divides instead of multiplying.

The multiplication operator is '*', so val1 * val2 computes the product.

4fill in blank

hard

Fill both blanks to correctly apply division and push the result back to the stack.

DSA Java

case '/':
    result = val1 [1] val2;
    stack.[2](result);
    break;

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

A/

Bpush

Cpop

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of '/' changes the operation.

Using pop instead of push removes elements instead of adding.

Division uses '/' operator and the result is pushed back using stack.push().

5fill in blank

hard

Fill all three blanks to complete the postfix evaluation loop with correct operator checks and stack operations.

DSA Java

for (int i = 0; i < expression.length(); i++) {
    char ch = expression.charAt(i);
    if (Character.isDigit(ch)) {
        stack.[1](ch - '0');
    } else {
        int val2 = stack.pop();
        int val1 = stack.pop();
        int result = 0;
        switch(ch) {
            case '+':
                result = val1 [2] val2;
                break;
            case '-':
                result = val1 - val2;
                break;
            case '*':
                result = val1 * val2;
                break;
            case '/':
                result = val1 / val2;
                break;
        }
        stack.[3](result);
    }
}

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

Apush

B+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push to add elements.

Using wrong operator for addition.

Operands are pushed with push(), '+' operator is used for addition, and results are pushed back with push().

Practice

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

easy

A. Queue

B. Array

C. Stack

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 result is pushed back.

3. Final Answer:

   Stack -> Option C

4. Quick Check:

   Postfix evaluation uses stack [OK]

Hint: Remember: postfix uses stack to hold operands [OK]

Common Mistakes:

• Confusing stack with queue
• Thinking array directly evaluates postfix
• Using linked list instead of stack

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

easy

A. stack.add(5);

B. stack.push(5);

C. stack.insert(5);

D. stack.enqueue(5);

Solution

1. Step 1: Recall Java Stack methods

   The Stack class in Java uses the method push() to add elements to the top.

2. Step 2: Identify correct method usage

   stack.push(5); correctly pushes integer 5 onto the stack.

3. Final Answer:

   stack.push(5); -> Option B

4. Quick Check:

   Use push() to add to stack [OK]

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

Common Mistakes:

• Using add() which is for lists
• Using insert() which is invalid for stack
• Using enqueue() which is for queues

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

import java.util.Stack;

public class PostfixEval {
    public static int evaluate(String expr) {
        Stack<Integer> stack = new Stack<>();
        for (char ch : expr.toCharArray()) {
            if (Character.isDigit(ch)) {
                stack.push(ch - '0');
            } else {
                int b = stack.pop();
                int a = stack.pop();
                switch (ch) {
                    case '+': stack.push(a + b); break;
                    case '*': stack.push(a * b); break;
                }
            }
        }
        return stack.pop();
    }

    public static void main(String[] args) {
        System.out.println(evaluate("23+5*"));
    }
}

medium

A. 10

B. 15

C. Error

D. 25

Solution

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

   Push 2, push 3, then '+' pops 3 and 2, adds to 5, push 5. Then push 5, '*' pops 5 and 5, multiplies to 25, push 25.

2. Step 2: Final stack pop

   Pop final result 25 from stack and print.

3. Final Answer:

   25 -> Option D

4. Quick Check:

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

Hint: Calculate stepwise: push digits, pop two for operator [OK]

Common Mistakes:

• Mixing operand order in subtraction/division
• Forgetting to convert char to int
• Not popping operands in correct order

4. Identify the error in the following Java code snippet that evaluates postfix expressions:

Stack<Integer> stack = new Stack<>();
for (char ch : expr.toCharArray()) {
    if (Character.isDigit(ch)) {
        stack.push(ch);
    } else {
        int b = stack.pop();
        int a = stack.pop();
        if (ch == '+') {
            stack.push(a + b);
        }
    }
}

medium

A. Pushing char directly instead of integer value

B. Using == to compare characters

C. Not handling other operators

D. Stack not initialized

Solution

1. Step 1: Check how digits are pushed

   The code pushes ch directly, which is a char, not its integer value.

2. Step 2: Correct way to push integer value

   Must convert char digit to int by ch - '0' before pushing.

3. Final Answer:

   Pushing char directly instead of integer value -> Option A

4. Quick Check:

   Push integer, not char [OK]

Hint: Convert char digit to int before pushing [OK]

Common Mistakes:

• Pushing char instead of int
• Ignoring other operators is not error here
• Using == for char comparison is valid

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

hard

A. 4

B. 0

C. -4

D. 2

Solution

1. Step 1: Evaluate postfix "82/3-4*" stepwise

   Push 8, push 2, '/' pops 2 and 8, 8/2=4 push 4. Push 3, '-' pops 3 and 4, 4-3=1 push 1. Push 4, '*' pops 4 and 1, 1*4=4 push 4.

2. Step 2: Re-check operator order and final pop

   Wait, subtraction order is a - b, so 4 - 3 = 1, then multiply by 4 = 4. Final result is 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   (8/2 - 3) * 4 = 4 [OK]

Hint: Remember operator order: a op b, pop b then a [OK]

Common Mistakes:

• Reversing operand order in division or subtraction
• Forgetting to pop operands in correct order
• Misinterpreting operators precedence