Dynamic Stack Using Resizable Array in DSA C# - Build from Scratch

Dynamic Stack Using Resizable Array

📖 Scenario: Imagine you are building a simple program to manage a stack of books. The stack can grow or shrink as you add or remove books. You will create a dynamic stack using an array that resizes itself when needed.

🎯 Goal: Build a dynamic stack class in C# that uses an array to store items. The array should double in size when full. You will implement methods to push (add) and pop (remove) items, and print the stack contents.

📋 What You'll Learn

Create a class called DynamicStack with an integer array called items and an integer count to track the number of elements.

Add a constructor to initialize items with size 2 and count to 0.

Add a method Push(int item) that adds an item to the stack and doubles the array size if full.

Add a method Pop() that removes and returns the top item from the stack.

Add a method PrintStack() that prints the stack items from top to bottom.

💡 Why This Matters

🌍 Real World

Dynamic stacks are used in undo features, expression evaluation, and backtracking algorithms where the number of items changes over time.

💼 Career

Understanding dynamic data structures like resizable stacks is important for software developers working on efficient memory management and algorithm implementation.

1

Create the DynamicStack class with initial array and count

Create a class called DynamicStack with a private integer array called items initialized to size 2, and a private integer count set to 0.

DSA C#

public class DynamicStack
{
    // Your code here
}

Hint

Use private int[] items = new int[2]; and private int count = 0; inside the class.

2

Add the Push method with resizing logic

Add a public method Push(int item) that adds item to items. If count equals the length of items, create a new array double the size, copy old items, and assign it to items. Then add the new item and increment count.

DSA C#

public class DynamicStack
{
    private int[] items = new int[2];
    private int count = 0;

    // Add Push method here
}

Hint

Check if count == items.Length to resize. Use a for loop to copy old items.

3

Add the Pop method to remove and return the top item

Add a public method Pop() that returns an integer. It should check if count is 0 and throw an exception if empty. Otherwise, decrement count and return the item at items[count].

DSA C#

public class DynamicStack
{
    private int[] items = new int[2];
    private int count = 0;

    public void Push(int item)
    {
        if (count == items.Length)
        {
            int[] newItems = new int[items.Length * 2];
            for (int i = 0; i < items.Length; i++)
            {
                newItems[i] = items[i];
            }
            items = newItems;
        }
        items[count] = item;
        count++;
    }

    // Add Pop method here
}

Hint

Check if stack is empty before popping. Decrement count and return the top item.

4

Add PrintStack method and test the stack

Add a public method PrintStack() that prints the stack items from top to bottom using a for loop from count - 1 down to 0. Then create a Main method to create a DynamicStack, push 10, 20, 30, pop one item, and print the stack.

DSA C#

using System;

public class DynamicStack
{
    private int[] items = new int[2];
    private int count = 0;

    public void Push(int item)
    {
        if (count == items.Length)
        {
            int[] newItems = new int[items.Length * 2];
            for (int i = 0; i < items.Length; i++)
            {
                newItems[i] = items[i];
            }
            items = newItems;
        }
        items[count] = item;
        count++;
    }

    public int Pop()
    {
        if (count == 0)
        {
            throw new InvalidOperationException("Stack is empty");
        }
        count--;
        return items[count];
    }

    // Add PrintStack method and Main method here
}

Hint

Print items from top (count - 1) down to 0. Test by pushing 10, 20, 30, popping once, then printing.

Practice

1. What is the main advantage of using a dynamic stack with a resizable array over a fixed-size array stack?

easy

A. It can grow or shrink automatically as elements are pushed or popped.

B. It uses less memory than a fixed-size array stack.

C. It always has a fixed capacity to avoid resizing overhead.

D. It does not allow popping elements.

Solution

1. Step 1: Understand fixed-size array stack limitation

   A fixed-size array stack has a set capacity and cannot grow beyond it, causing overflow if exceeded.

2. Step 2: Understand dynamic stack advantage

   A dynamic stack resizes its array automatically when full, allowing more elements without overflow.

3. Final Answer:

   It can grow or shrink automatically as elements are pushed or popped. -> Option A

4. Quick Check:

   Dynamic resizing = automatic growth/shrink [OK]

Hint: Dynamic stacks resize arrays automatically when full [OK]

Common Mistakes:

• Thinking dynamic stack uses less memory always
• Believing dynamic stack has fixed capacity
• Confusing dynamic stack with linked list stack

2. Which of the following is the correct way to double the size of the internal array in a dynamic stack in C#?

easy

A. array = new int[array.Length * 2];

B. array.Length = array.Length * 2;

C. array = array.Concat(array).ToArray();

D. Array.Resize(ref array, array.Length * 2);

Solution

1. Step 1: Understand array resizing in C#

   Arrays in C# have fixed length; to resize, use Array.Resize(ref array, newSize) which copies old elements.

2. Step 2: Analyze options

   Array.Resize(ref array, array.Length * 2); uses Array.Resize correctly. array = new int[array.Length * 2]; creates a new array but loses old data. array.Length = array.Length * 2; is invalid syntax. array = array.Concat(array).ToArray(); uses LINQ but is inefficient and not typical for stack resizing.

3. Final Answer:

   Array.Resize(ref array, array.Length * 2); -> Option D

4. Quick Check:

   Array.Resize method resizes arrays properly [OK]

Hint: Use Array.Resize(ref array, newSize) to resize arrays in C# [OK]

Common Mistakes:

• Trying to set array.Length directly
• Creating new array without copying old data
• Using LINQ Concat for resizing inefficiently

3. Consider the following C# code for a dynamic stack using a resizable array:

DynamicStack stack = new DynamicStack();
stack.Push(10);
stack.Push(20);
stack.Push(30);
stack.Pop();
stack.Push(40);
stack.PrintStack();

What will be the printed output?

medium

A. 30 -> 20 -> 10 -> null

B. 40 -> 20 -> 10 -> null

C. 20 -> 10 -> null

D. 40 -> 30 -> 20 -> 10 -> null

Solution

1. Step 1: Trace stack operations

   Push 10, stack: 10
   Push 20, stack: 20 -> 10
   Push 30, stack: 30 -> 20 -> 10
   Pop removes 30, stack: 20 -> 10
   Push 40, stack: 40 -> 20 -> 10

2. Step 2: Determine printed output

   PrintStack prints from top to bottom ending with null: 40 -> 20 -> 10 -> null

3. Final Answer:

   40 -> 20 -> 10 -> null -> Option B

4. Quick Check:

   Stack top after operations = 40 [OK]

Hint: Track push/pop order carefully to find top element [OK]

Common Mistakes:

• Forgetting pop removes top element
• Printing stack in wrong order
• Including popped elements in output

4. The following code snippet for a dynamic stack has a bug. What is the error?

public void Push(int item) {
  if (top == array.Length) {
    Array.Resize(ref array, array.Length * 2);
  }
  array[top] = item;
  top++;
}

medium

A. No bug; the code resizes and pushes correctly.

B. The top variable should be incremented before assigning the item.

C. The condition should be top == array.Length to resize, but top starts at 1 instead of 0.

D. The condition should be top == array.Length - 1 to resize.

Solution

1. Step 1: Analyze resizing condition

   top equals array.Length means array is full, so resizing is needed before adding new item.

2. Step 2: Check push logic

   Assign item at array[top], then increment top. This correctly places item and moves top pointer.

3. Final Answer:

   No bug; the code resizes and pushes correctly. -> Option A

4. Quick Check:

   Resize when full, assign then increment top [OK]

Hint: Resize when top equals length; assign then increment top [OK]

Common Mistakes:

• Using wrong resize condition (off by one)
• Incrementing top before assignment
• Assuming top starts at 1 instead of 0

5. You want to implement a dynamic stack that shrinks its internal array size by half when the number of elements falls below one-quarter of the array capacity. Which approach correctly handles this resizing to optimize memory without losing data?

hard

A. Resize array to array.Length * 2 after popping elements to avoid frequent resizing.

B. Resize array to array.Length / 4 whenever an element is popped.

C. After popping, check if top <= array.Length / 4, then resize array to array.Length / 2 using Array.Resize(ref array, newSize).

D. Never shrink the array to avoid complexity; only grow it when full.

Solution

1. Step 1: Understand shrinking condition

   To save memory, shrink when usage is low: top ≤ 1/4 capacity triggers halving the array size.

2. Step 2: Apply resizing correctly

   Use Array.Resize(ref array, array.Length / 2) to shrink safely after popping, preserving existing elements.

3. Final Answer:

   After popping, check if top <= array.Length / 4, then resize array to array.Length / 2 using Array.Resize(ref array, newSize). -> Option C

4. Quick Check:

   Shrink when quarter full using Array.Resize [OK]

Hint: Shrink array when usage ≤ 25% by halving size [OK]

Common Mistakes:

• Shrinking too often causing overhead
• Shrinking to quarter size instead of half
• Not preserving data during resize