Push Operation on Stack in DSA C - Build from Scratch

Push Operation on Stack

📖 Scenario: Imagine you have a stack of plates. You can only add a new plate on top. This is called a push operation in a stack data structure.

🎯 Goal: You will create a simple stack using an array and write code to push (add) an element onto the stack.

📋 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 called push that takes an integer parameter value and adds it to the stack if there is space

Print the stack elements after pushing values

💡 Why This Matters

🌍 Real World

Stacks are used in undo features in apps, browser history, and expression evaluation.

💼 Career

Understanding stack operations is essential for software development and technical 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

int stack[5];
int top = -1;
// Your code here

Hint

Use int stack[5]; to create the array and int top = -1; to track the top position.

2

Create the push function

Write a function called push that takes an integer parameter value. Inside the function, check if top is less than 4. If yes, increase top by 1 and assign value to stack[top].

DSA C

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

// Write push function below
// Your code here

Hint

Remember to check if the stack is full before pushing.

3

Push values onto the stack

Call the push function three times with values 10, 20, and 30.

DSA C

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

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

// Call push function with 10, 20, 30
// Your code here

Hint

Call push three times with the given values.

4

Print the stack elements

Use a for loop with variable i from 0 to top to print each element of stack[i] separated by spaces.

DSA C

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

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

int main() {
    push(10);
    push(20);
    push(30);
    
    // Print stack elements
    // Your code here
    return 0;
}

Hint

Use printf inside the loop to print each element followed by a space.

Practice

1. What does the push operation do in a stack?

easy

A. Checks if the stack is empty

B. Adds an element to the top of the stack

C. Removes the top element from the stack

D. Returns the bottom element of the stack

Solution

1. Step 1: Understand stack operations

   A stack is a data structure where elements are added or removed from the top only.

2. Step 2: Define push operation

   The push operation adds a new element on top of the existing elements in the stack.

3. Final Answer:

   Adds an element to the top of the stack -> Option B

4. Quick Check:

   Push means add on top = C [OK]

Hint: Push always adds on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Mixing stack with queue operations

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

easy

A. s.push = 5;

B. s.push(5);

C. push(5, s);

D. push(s, 5);

Solution

1. Step 1: Understand typical C stack push syntax

   In C, stack operations are usually functions where the stack is passed as the first argument.

2. Step 2: Match syntax with parameters

   The correct function call to push 5 onto stack s is push(s, 5);.

3. Final Answer:

   push(s, 5); -> Option D

4. Quick Check:

   Function push(stack, value) = A [OK]

Hint: In C, function push(stack, value) is standard [OK]

Common Mistakes:

• Using object.method syntax (not C style)
• Swapping argument order
• Assigning value instead of calling function

3. What will be the content of the stack after executing the following code?

int stack[5];
int top = -1;
// Push function
void push(int val) {
  if(top < 4) {
    top++;
    stack[top] = val;
  }
}

push(10);
push(20);
push(30);

medium

A. [10, 20, 30]

B. [30, 20, 10]

C. [10, 20]

D. Empty stack

Solution

1. Step 1: Trace each push call

   Initially, top = -1. After push(10), top=0, stack[0]=10. After push(20), top=1, stack[1]=20. After push(30), top=2, stack[2]=30.

2. Step 2: List stack content from bottom to top

   Stack elements from index 0 to top (2) are [10, 20, 30].

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   Push adds at top increasing index = B [OK]

Hint: Stack stores pushed elements in order at increasing top index [OK]

Common Mistakes:

• Reversing order of elements
• Confusing top index with element values
• Assuming stack is empty after pushes

4. Identify the error in the following push function for a stack of size 3:

int stack[3];
int top = 0;
void push(int val) {
  if(top <= 3) {
    stack[top] = val;
    top++;
  }
}

medium

A. Condition should be top < 3, not top <= 3

B. top should be incremented before assignment

C. stack array size should be 4

D. top should start at -1, not 0

Solution

1. Step 1: Analyze top and condition

   top starts at 0, which is valid for empty stack at index 0. The condition allows top to be 3, which is out of bounds for stack[3] (indices 0 to 2).

2. Step 2: Correct the condition

   Change condition to top < 3 to prevent overflow and invalid access.

3. Final Answer:

   Condition should be top < 3, not top <= 3 -> Option A

4. Quick Check:

   Stack max index 2 means top < 3 [OK]

Hint: Check array bounds: top must be less than size [OK]

Common Mistakes:

• Allowing top to equal array size causing overflow
• Starting top at -1 but not adjusting code
• Incrementing top before assignment causing skip

5. Given a stack implemented with an array of size 4 and top = 2 currently, what will happen if we try to push two more elements without popping?
Choose the correct behavior.

hard

A. Both pushes fail immediately due to full stack

B. Both pushes succeed, stack size automatically increases

C. First push succeeds, second push causes stack overflow

D. First push causes overflow, second push is not attempted

Solution

1. Step 1: Understand current stack state

   Array size is 4, so valid indices are 0 to 3. top=2 means top element is at index 2, so one slot remains at index 3.

2. Step 2: Push two elements

   First push increments top to 3 and stores element at index 3 (valid). Second push tries to increment top to 4, which is out of bounds, causing overflow.

3. Final Answer:

   First push succeeds, second push causes stack overflow -> Option C

4. Quick Check:

   Array size 4, top max index 3, overflow after that [OK]

Hint: Stack size fixed; overflow after last index [OK]

Common Mistakes:

• Assuming stack auto-expands
• Thinking both pushes fail immediately
• Confusing top index with count