Stack Implementation Using Array in DSA C# - Build from Scratch

Stack Implementation Using Array

📖 Scenario: Imagine you are building a simple undo feature for a text editor. Each action is stored in a stack, so the last action can be undone first.

🎯 Goal: You will create a stack using an array, add a configuration for the stack size, implement push and pop operations, and finally display the stack's content.

📋 What You'll Learn

Create an integer array called stack with size 5

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

Write a method Push that adds an integer to the stack if there is space

Write a method Pop that removes and returns the top element if the stack is not empty

Print the current elements in the stack from bottom to top

💡 Why This Matters

🌍 Real World

Stacks are used in undo features, expression evaluation, and backtracking algorithms.

💼 Career

Understanding stack implementation helps in software development, debugging, and system design.

1

Create the stack array and top pointer

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

DSA C#

// Create the stack array and top pointer
// Your code here

Hint

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

2

Add a method to push elements onto the stack

Add a method called Push that takes an integer parameter value. Inside the method, check if top is less than 4, then increment top and assign value to stack[top].

DSA C#

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

// Add Push method below
// Your code here

Hint

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

3

Add a method to pop elements from the stack

Add a method called Pop that returns an integer. Inside the method, check if top is greater than or equal to 0, then return stack[top] and decrement top. If empty, return -1.

DSA C#

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

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

// Add Pop method below
// Your code here

Hint

Return -1 if the stack is empty to indicate no elements to pop.

4

Print the current stack elements

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

DSA C#

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

void Push(int value)
{
    if (top = 0)
    {
        int value = stack[top];
        top--;
        return value;
    }
    return -1;
}

Push(10);
Push(20);
Push(30);

// Print stack elements from bottom to top
// Your code here

Hint

Use a for loop from 0 to top and print each element with a space.

Practice

1. What is the main purpose of using an array in a stack implementation?

easy

A. To allow random access to elements anywhere in the stack

B. To store elements in a fixed-size, ordered collection

C. To automatically resize the stack when full

D. To sort elements as they are added

Solution

1. Step 1: Understand stack storage

   A stack stores elements in order, last-in first-out, so it needs a structure to hold elements sequentially.

2. Step 2: Role of array in stack

   An array provides a fixed-size, ordered collection to hold stack elements efficiently.

3. Final Answer:

   To store elements in a fixed-size, ordered collection -> Option B

4. Quick Check:

   Stack uses array for fixed order storage [OK]

Hint: Stack uses array for fixed order storage, not random access [OK]

Common Mistakes:

• Thinking array resizes automatically
• Confusing stack with queue behavior
• Assuming array sorts elements

2. Which of the following is the correct way to declare an integer stack array of size 5 in C#?

easy

A. int[] stack = new int[5];

B. int stack = new int[5];

C. int[] stack = int[5];

D. stack int[] = new int(5);

Solution

1. Step 1: Recall C# array declaration syntax

   In C#, arrays are declared with type[], variable name, and new type[size].

2. Step 2: Match correct syntax

   int[] stack = new int[5]; matches this pattern: int[] stack = new int[5];

3. Final Answer:

   int[] stack = new int[5]; -> Option A

4. Quick Check:

   C# array declaration = int[] name = new int[size]; [OK]

Hint: Use type[] name = new type[size]; for arrays in C# [OK]

Common Mistakes:

• Missing [] after type
• Using parentheses instead of brackets
• Incorrect variable declaration order

3. What will be the output after the following operations on a stack implemented using an array of size 3?

Push(10); Push(20); Pop(); Push(30); Pop(); Pop();

Assume the stack is initially empty and prints the popped values.

medium

A. 20 30 10

B. 20 30 20

C. 20 30 30

D. 10 20 30

Solution

1. Step 1: Trace each operation step-by-step

   Push(10) stack=[10]
   Push(20) stack=[10,20]
   Pop() removes 20, output=20, stack=[10]
   Push(30) stack=[10,30]
   Pop() removes 30, output=30, stack=[10]
   Pop() removes 10, output=10, stack=[]

2. Step 2: Collect popped outputs in order

   The popped values printed are 20, then 30, then 10.

3. Final Answer:

   20 30 10 -> Option A

4. Quick Check:

   Stack LIFO pop order = 20 30 10 [OK]

Hint: Pop prints last pushed element, follow LIFO order [OK]

Common Mistakes:

• Confusing push and pop order
• Assuming FIFO instead of LIFO
• Missing last pop output

4. Identify the error in this stack push method code snippet in C#:

public void Push(int item) {
  if (top == stack.Length) {
    Console.WriteLine("Stack Overflow");
  }
  stack[top] = item;
  top++;
}

medium

A. top should be compared with stack.Length - 1

B. top++ should be before assigning stack[top]

C. No return after overflow check causes overwrite

D. stack array should be resized when full

Solution

1. Step 1: Analyze overflow check logic

   The code checks if top == stack.Length and prints overflow, but continues to assign and increment top anyway.

2. Step 2: Identify missing control flow after overflow

   Without a return or exit after overflow message, the code overwrites stack[top] causing error.

3. Final Answer:

   No return after overflow check causes overwrite -> Option C

4. Quick Check:

   Overflow check must stop push to avoid overwrite [OK]

Hint: Stop push after overflow check to prevent overwrite [OK]

Common Mistakes:

• Assuming top comparison off by one
• Moving top++ before assignment causes wrong index
• Ignoring need to stop after overflow

5. You want to implement a stack using an array that doubles its size automatically when full. Which approach correctly achieves this in C#?

hard

A. Increase the size of the existing array using stack.Resize() method

B. Overwrite oldest elements when stack is full to make space

C. Use a List<int> instead of array to avoid resizing manually

D. Create a new array double the size, copy elements, then assign it to the stack variable

Solution

1. Step 1: Understand array resizing in C#

   Arrays have fixed size; to resize, create a new larger array and copy elements over.

2. Step 2: Evaluate options for resizing stack array

   Create a new array double the size, copy elements, then assign it to the stack variable correctly describes creating a new array double the size, copying elements, and reassigning stack variable.

3. Final Answer:

   Create a new array double the size, copy elements, then assign it to the stack variable -> Option D

4. Quick Check:

   Resize array by new array + copy + assign [OK]

Hint: Arrays fixed size; resize by new array and copy [OK]

Common Mistakes:

• Assuming arrays can resize in place
• Using non-existent Resize method on arrays
• Overwriting data instead of resizing