Evaluate Postfix Expression Using Stack in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push a value onto the stack.

DSA C

void push(int stack[], int *top, int value) {
    stack[++(*top)] = [1];
}

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

Avalue

Bstack

Ctop

D*top

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' instead of 'value' to push.

Not incrementing the top before assignment.

We push the given value onto the stack by incrementing top and assigning value.

2fill in blank

medium

Complete the code to pop a value from the stack.

DSA C

int pop(int stack[], int *top) {
    return stack[[1]--];
}

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

A*top

Btop

Cstack

D(*top)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' instead of '*top' causing wrong memory access.

Decrementing top before accessing the value.

We return the value at the current top and then decrement top.

3fill in blank

hard

Fix the error in the switch case to perform addition.

DSA C

switch (exp[i]) {
    case '+':
        res = val1 [1] val2;
        break;
    // other cases
}

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

A-

B/

C+

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causing subtraction.

Using '*' or '/' causing wrong operation.

The '+' operator performs addition between val1 and val2.

4fill in blank

hard

Fill both blanks to correctly pop two values for operation.

DSA C

int val2 = pop(stack, [1]);
int val1 = pop(stack, [2]);

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

Atop

B*top

C&top

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Passing top instead of &top causing incorrect stack behavior.

Passing *top causing type mismatch.

We pass the address of top (&top) to pop to update the top index correctly.

5fill in blank

hard

Fill all three blanks to correctly evaluate the postfix expression.

DSA C

for (int i = 0; exp[i] != '\0'; i++) {
    if (isdigit(exp[i])) {
        push(stack, [1], exp[i] - '0');
    } else {
        int val2 = pop(stack, [2]);
        int val1 = pop(stack, [3]);
        // perform operation
    }
}

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

A&top

Btop

C*top

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Passing top or *top causing incorrect stack updates.

Passing stack instead of &top causing errors.

We pass &top to push and pop so that the top index is updated correctly.

Practice

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

easy

A. Stack

B. Queue

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 data structure to store operands temporarily.

2. Step 2: Identify the suitable data structure

   A stack is used because it allows pushing operands and popping them when an operator is encountered, following Last-In-First-Out order.

3. Final Answer:

   Stack -> Option A

4. Quick Check:

   Postfix evaluation uses stack = Stack [OK]

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

Common Mistakes:

• Confusing stack with queue
• Using array without stack operations
• Thinking linked list is primary for postfix

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

easy

A. stack_push(x);

B. s.push(x);

C. push(x, s);

D. push(s, x);

Solution

1. Step 1: Understand typical stack push function signature

   In C, stack push usually takes the stack and the value to push as arguments, e.g., push(stack, value).

2. Step 2: Match the correct syntax

   push(s, x); matches the common pattern push(s, x); where 's' is the stack and 'x' is the value.

3. Final Answer:

   push(s, x); -> Option D

4. Quick Check:

   push(stack, value) = push(s, x) [OK]

Hint: Remember: push(stack, value) in C [OK]

Common Mistakes:

• Using object.method() syntax in C
• Swapping argument order
• Using undefined function names

3. What is the result of evaluating the postfix expression "6 2 3 + -" using a stack?

medium

A. 3

B. 1

C. 5

D. -1

Solution

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

   Push 6, push 2, push 3. Encounter '+', pop 3 and 2, add: 2+3=5, push 5. Encounter '-', pop 5 and 6, subtract: 6-5=1.

2. Step 2: Verify final stack value

   After all operations, the stack has one value: 1.

3. Final Answer:

   1 -> Option B

4. Quick Check:

   6 - (2 + 3) = 1 [OK]

Hint: Process operators by popping two operands [OK]

Common Mistakes:

• Subtracting in wrong order (operand2 - operand1)
• Adding instead of subtracting
• Not pushing intermediate results

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

char* expr = "5 9 8 + 4 6 * * 7 + *";
int i = 0;
while(expr[i] != '\0') {
  if(isdigit(expr[i])) push(stack, expr[i]);
  else { /* operator handling */ }
  i++;
}

medium

A. Pushing character digit instead of integer value

B. Using while loop instead of for loop

C. Missing stack initialization

D. Incorrect operator handling

Solution

1. Step 1: Check what is pushed onto stack

   The code pushes expr[i], which is a character like '5', not the integer 5.

2. Step 2: Explain why this is wrong

   Characters have ASCII codes; pushing '5' pushes 53, not integer 5, causing wrong calculations.

3. Final Answer:

   Pushing character digit instead of integer value -> Option A

4. Quick Check:

   Push integer, not char digit [OK]

Hint: Convert char to int before push: expr[i] - '0' [OK]

Common Mistakes:

• Pushing char instead of int
• Ignoring spaces in expression
• Not handling operators properly

5. Given the postfix expression "4 5 7 2 + - *", what is the final evaluated result using stack evaluation?

hard

A. -24

B. 16

C. -16

D. 24

Solution

1. Step 1: Evaluate inner expression 7 2 +

   Push 4, 5, 7, 2. '+' operator: pop 2 and 7, add 7+2=9, push 9.

2. Step 2: Evaluate next operator '-'

   Next operator '-', pop 9 and 5, subtract 5-9 = -4, push -4.

3. Step 3: Evaluate final operator '*'

   Pop -4 and 4, multiply 4 * -4 = -16, push -16.

4. Final Answer:

   -16 -> Option C

5. Quick Check:

   4 * (5 - (7 + 2)) = -16 [OK]

Hint: Evaluate inner expressions first, then combine [OK]

Common Mistakes:

• Wrong operator order in subtraction
• Not evaluating nested operations correctly
• Mixing up operand order in multiplication