Why Stack Exists and What Problems It Solves in DSA C - See It Work

Why Stack Exists and What Problems It Solves

📖 Scenario: Imagine you are organizing a stack of plates in a kitchen. You can only add or remove the top plate. This is how a stack data structure works in programming.Stacks help solve problems where the last thing added is the first thing you need to remove, like undo actions in text editors or checking if parentheses in math expressions are balanced.

🎯 Goal: You will create a simple stack using an array in C, push some numbers onto it, then pop them off to see how the last number added is the first one removed.

📋 What You'll Learn

Create an integer array called stack with size 5

Create an integer variable called top and set it to -1

Write a function push that adds a number to the stack and updates top

Write a function pop that removes and returns the top number from the stack and updates top

Push the numbers 10, 20, and 30 onto the stack

Pop all numbers from the stack and print them in order

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and expression evaluation.

💼 Career

Understanding stacks is important for software developers to solve problems involving order and reversals, and is often asked in coding interviews.

1

Create the stack array and top variable

Create an integer array called stack with size 5 and an integer variable called top set to -1.

DSA C

# Create the stack array and top variable
// Your code here

Hint

Think of top as the position of the last added item. Starting at -1 means the stack is empty.

2

Write the push function

Write a function called push that takes an integer value, adds it to the stack at position top + 1, and updates top accordingly.

DSA C

int stack[5];
int top = -1;

// Write the push function below
// Your code here

Hint

Check if top is less than 4 before adding to avoid overflow.

3

Write the pop function

Write a function called pop that removes and returns the top value from the stack and updates top. If the stack is empty (top == -1), return -1.

DSA C

int stack[5];
int top = -1;

void push(int value) {
    if (top < 4) {
        top++;
        stack[top] = value;
    }
}

// Write the pop function below
// Your code here

Hint

Remember to decrease top after removing the top item.

4

Push and pop values, then print the popped values

Push the numbers 10, 20, and 30 onto the stack using push. Then pop all values from the stack using pop and print each popped value on a new line.

DSA C

int stack[5];
int top = -1;

void push(int value) {
    if (top < 4) {
        top++;
        stack[top] = value;
    }
}

int pop() {
    if (top == -1) {
        return -1;
    } else {
        int value = stack[top];
        top--;
        return value;
    }
}

int main() {
    // Push 10, 20, 30 onto the stack
    // Pop all values and print them
    // Your code here
    return 0;
}

Hint

Use a loop to pop until pop() returns -1, printing each value.

Practice

1. Why do we use a stack data structure in programming?

easy

A. To store data in random order

B. To keep track of function calls and return addresses

C. To sort numbers quickly

D. To store large amounts of data permanently

Solution

1. Step 1: Understand the purpose of a stack

   A stack stores data in a last-in, first-out order, which is perfect for tracking function calls and returns.

2. Step 2: Identify common uses of stack

   Stacks are mainly used to keep track of function calls, especially in recursion, where the last called function must finish first.

3. Final Answer:

   To keep track of function calls and return addresses -> Option B

4. Quick Check:

   Stack = function call tracking [OK]

Hint: Stack = last-in, first-out for function calls [OK]

Common Mistakes:

• Thinking stack stores data randomly
• Confusing stack with queue
• Assuming stack sorts data

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

easy

A. stack[++top] = value;

B. stack[top] = value++;

C. stack[top++] = value;

D. stack[top--] = value;

Solution

1. Step 1: Understand stack push operation

   To push, we first increase the top index, then assign the value at that position.

2. Step 2: Check each option

   stack[++top] = value; increments top before assignment, which is correct. stack[top++] = value; assigns then increments, which may overwrite. stack[top] = value++; increments value, not top. stack[top--] = value; decrements top, which is wrong for push.

3. Final Answer:

   stack[++top] = value; -> Option A

4. Quick Check:

   Push = increment top then assign [OK]

Hint: Push means increment top first, then assign [OK]

Common Mistakes:

• Incrementing after assignment causes overwrite
• Decrementing top on push
• Incrementing value instead of top

3. What will be the output of the following C code snippet?

int stack[3];
int top = -1;
stack[++top] = 10;
stack[++top] = 20;
stack[++top] = 30;
printf("%d", stack[top]);

medium

A. 10

B. 20

C. 30

D. Runtime error

Solution

1. Step 1: Trace stack push operations

   Initially, top = -1. After first push, top=0, stack[0]=10. Second push: top=1, stack[1]=20. Third push: top=2, stack[2]=30.

2. Step 2: Check printed value

   printf prints stack[top], which is stack[2] = 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Last pushed value = 30 [OK]

Hint: Last pushed value is at stack[top] [OK]

Common Mistakes:

• Printing stack[0] instead of stack[top]
• Confusing top index after pushes
• Assuming runtime error without overflow

4. Identify the error in this stack pop function in C:

int pop(int stack[], int *top) {
    if (*top == -1) {
        printf("Stack Underflow\n");
        return -1;
    }
    return stack[*top--];
}

medium

A. Incorrect decrement of top pointer

B. Missing check for stack overflow

C. Returning wrong stack element

D. No error, function is correct

Solution

1. Step 1: Understand pointer decrement in return statement

   The expression stack[*top--] uses post-decrement on *top, but *top-- decrements the pointer, not the value it points to.

2. Step 2: Correct way to decrement top value

   It should be stack[(*top)--] to decrement the value pointed by top after accessing stack.

3. Final Answer:

   Incorrect decrement of top pointer -> Option A

4. Quick Check:

   Use (*top)-- to decrement value, not pointer [OK]

Hint: Use (*top)-- to decrement value, not pointer [OK]

Common Mistakes:

• Using *top-- instead of (*top)--
• Ignoring pointer vs value decrement
• Not checking stack underflow

5. You want to reverse a string using a stack in C. Which approach correctly uses the stack to solve this problem?

hard

A. Use a queue instead of a stack to reverse the string

B. Push characters only if they are vowels, then pop to reverse

C. Pop characters first, then push them back to reverse

D. Push each character of the string onto the stack, then pop all characters to form the reversed string

Solution

1. Step 1: Understand stack reversal logic

   Pushing all characters onto the stack stores them in order. Popping them reverses the order due to last-in, first-out behavior.

2. Step 2: Evaluate options

   Push each character of the string onto the stack, then pop all characters to form the reversed string correctly pushes all characters then pops to reverse. Push characters only if they are vowels, then pop to reverse only pushes vowels, losing other characters. Pop characters first, then push them back to reverse pops before pushing, which is invalid. Use a queue instead of a stack to reverse the string suggests queue, which reverses order differently.

3. Final Answer:

   Push each character of the string onto the stack, then pop all characters to form the reversed string -> Option D

4. Quick Check:

   Stack reversal = push all, then pop all [OK]

Hint: Push all chars, then pop all to reverse [OK]

Common Mistakes:

• Pushing only some characters
• Popping before pushing
• Using queue instead of stack for reversal