Peek Top Element of Stack in DSA C - Build from Scratch

Peek Top Element of Stack

📖 Scenario: You are building a simple program to manage a stack of books. You want to be able to see the top book on the stack without removing it.

🎯 Goal: Create a stack using an array, set up a variable to track the top, write code to peek at the top element, and print it.

📋 What You'll Learn

Create an integer array called stack with exactly these values: 10, 20, 30, 40, 50

Create an integer variable called top and set it to 4

Create an integer variable called peek and set it to the top element of the stack using stack[top]

Print the value of peek using printf

💡 Why This Matters

🌍 Real World

Stacks are used in many places like undo features in apps, browser history, and managing function calls.

💼 Career

Understanding stack operations like peek is important for software development, debugging, and working with low-level programming.

1

Create the stack array

Create an integer array called stack with these exact values: 10, 20, 30, 40, 50

DSA C

# Create the stack array with values 10, 20, 30, 40, 50
// Your code here

Hint

Use int stack[5] = {10, 20, 30, 40, 50}; to create the array.

2

Set the top index

Create an integer variable called top and set it to 4 to represent the top index of the stack

DSA C

int stack[5] = {10, 20, 30, 40, 50};
// Create the top index variable and set it to 4
// Your code here

Hint

Use int top = 4; to set the top index.

3

Peek the top element

Create an integer variable called peek and set it to the top element of the stack using stack[top]

DSA C

int stack[5] = {10, 20, 30, 40, 50};
int top = 4;
// Create peek variable and assign stack[top]
// Your code here

Hint

Use int peek = stack[top]; to get the top element.

4

Print the peeked element

Print the value of peek using printf with the format specifier %d

DSA C

int stack[5] = {10, 20, 30, 40, 50};
int top = 4;
int peek = stack[top];
// Print the peeked value
// Your code here

Hint

Use printf("%d\n", peek); to print the top element.

Practice

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

easy

A. Removes the top element from the stack

B. Returns the top element without removing it

C. Adds a new element to the top of the stack

D. Checks if the stack is empty

Solution

1. Step 1: Understand the peek operation

   The peek operation looks at the top element of the stack without changing the stack.

2. Step 2: Compare with other stack operations

   Pop removes the top element, push adds an element, and isEmpty checks if stack is empty.

3. Final Answer:

   Returns the top element without removing it -> Option B

4. Quick Check:

   Peek = top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing peek with pop operation
• Thinking peek removes the element
• Mixing peek with push or isEmpty

2. Which of the following is the correct syntax to peek the top element of a stack named stack in C if top is the index of the top element and arr is the array storing stack elements?

easy

A. int x = arr[top - 1];

B. int x = arr[top + 1];

C. int x = arr[0];

D. int x = arr[top];

Solution

1. Step 1: Understand stack top indexing

   Top points to the current top element index in the array.

2. Step 2: Access the top element

   Accessing arr[top] gives the top element. Using top+1 or top-1 is incorrect as it points to wrong positions.

3. Final Answer:

   int x = arr[top]; -> Option D

4. Quick Check:

   Top index points to top element [OK]

Hint: Use arr[top] to peek top element [OK]

Common Mistakes:

• Using top+1 or top-1 instead of top
• Accessing arr[0] assuming stack bottom is top
• Confusing top index with element value

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

int stack[3] = {10, 20, 30};
int top = 2;
printf("%d", stack[top]);

medium

A. 10

B. 20

C. 30

D. Runtime error

Solution

1. Step 1: Identify the top element index and stack contents

   Stack array has elements 10 at index 0, 20 at 1, and 30 at 2. Top is 2.

2. Step 2: Access and print the top element

   stack[top] = stack[2] = 30, so printf prints 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   stack[top] = 30 [OK]

Hint: Print stack[top] to get top element [OK]

Common Mistakes:

• Confusing index 0 as top
• Assuming top is out of range
• Expecting pop removes element but code only prints

4. Find the error in this C code snippet that tries to peek the top element of a stack:

int stack[5] = {1, 2, 3, 4, 5};
int top = 4;
int peek() {
  return stack[top + 1];
}

medium

A. Accessing stack[top + 1] causes out-of-bounds error

B. top should be initialized to -1

C. Function should return void, not int

D. Stack array size is too small

Solution

1. Step 1: Analyze the peek function access

   Peek returns stack[top + 1], which is stack[5] here (top=4), exceeding array size 5 (0-4).

2. Step 2: Understand correct top indexing

   Top points to current top element, so peek should return stack[top], not stack[top + 1].

3. Final Answer:

   Accessing stack[top + 1] causes out-of-bounds error -> Option A

4. Quick Check:

   Index out of bounds error [OK]

Hint: Peek uses stack[top], not stack[top+1] [OK]

Common Mistakes:

• Using top+1 instead of top for peek
• Ignoring array bounds causing runtime error
• Incorrect top initialization assumptions

5. Given a stack implemented using an array stack[5] and a variable top initialized to -1, which sequence of operations correctly peeks the top element after pushing 3 elements (10, 20, 30)?

hard

A. stack[++top] = 10; stack[++top] = 20; stack[++top] = 30; int x = stack[top];

B. stack[top] = 10; top = top + 1; stack[top] = 20; top = top + 1; stack[top] = 30; top = top + 1; int x = stack[top];

C. stack[top++] = 10; stack[top++] = 20; stack[top++] = 30; int x = stack[top];

D. top = 0; stack[top] = 10; top = 1; stack[top] = 20; top = 2; stack[top] = 30; int x = stack[top + 1];

Solution

1. Step 1: Understand push operation with top

   Using stack[++top] = value increments top first, then assigns value, correctly pushing elements.

2. Step 2: Peek top element after pushes

   After pushing 3 elements, top points to last pushed element. Accessing stack[top] gives top element.

3. Final Answer:

   stack[++top] = 10; stack[++top] = 20; stack[++top] = 30; int x = stack[top]; -> Option A

4. Quick Check:

   Push increments top, peek uses stack[top] [OK]

Hint: Use stack[++top] to push, stack[top] to peek [OK]

Common Mistakes:

• Using post-increment top++ causing wrong indexing
• Accessing stack[top + 1] for peek
• Not initializing top to -1 before push