Recursive pattern matching in C Sharp (C#) - Mini Project: Build & Apply

Recursive Pattern Matching

📖 Scenario: Imagine you have a simple tree structure representing folders and files. Each folder can contain files or other folders. You want to find if a specific file exists anywhere inside this tree.

🎯 Goal: Build a C# program that uses recursive pattern matching to check if a file with a given name exists inside a nested folder structure.

📋 What You'll Learn

Create a record type File with a Name property.

Create a record type Folder with a Name property and a list of Items which can be File or Folder.

Write a recursive method ContainsFile that uses pattern matching to check if a file with a given name exists in the folder or any subfolder.

Print "Found" if the file exists, otherwise "Not Found".

💡 Why This Matters

🌍 Real World

File systems and many applications use nested folder structures. Searching files inside them is a common task.

💼 Career

Understanding recursive pattern matching helps in writing clean, readable code for complex data structures like trees, useful in software development and data processing jobs.

1

Create the data structure

Create a record called File with a string property Name, and a record called Folder with a string property Name and a list of objects called Items.

C Sharp (C#)

// Your code here

Hint

Use C# 9+ record syntax to create simple data containers.

2

Create sample folder data

Create a variable root of type Folder representing a folder named "root" that contains a File named "readme.txt" and a subfolder named "docs" which contains a File named "manual.pdf".

C Sharp (C#)

public record File(string Name);
public record Folder(string Name, List Items);

// Your code here

Hint

Use nested lists to represent folders inside folders.

3

Write the recursive pattern matching method

Write a method bool ContainsFile(Folder folder, string fileName) that uses a foreach loop and recursive pattern matching with switch expressions to check if any File with Name equal to fileName exists in folder or its subfolders.

C Sharp (C#)

public record File(string Name);
public record Folder(string Name, List Items);

var root = new Folder("root", new List
{
    new File("readme.txt"),
    new Folder("docs", new List
    {
        new File("manual.pdf")
    })
});

// Write the ContainsFile method below

Hint

Use switch with pattern matching and recursion to check nested folders.

4

Print the search result

Write a Console.WriteLine statement that prints "Found" if ContainsFile(root, "manual.pdf") returns true, otherwise prints "Not Found".

C Sharp (C#)

public record File(string Name);
public record Folder(string Name, List Items);

var root = new Folder("root", new List
{
    new File("readme.txt"),
    new Folder("docs", new List
    {
        new File("manual.pdf")
    })
});

bool ContainsFile(Folder folder, string fileName)
{
    foreach (var item in folder.Items)
    {
        switch (item)
        {
            case File f when f.Name == fileName:
                return true;
            case Folder subFolder:
                if (ContainsFile(subFolder, fileName))
                    return true;
            break;
        }
    }
    return false;
}

// Print the result of searching for "manual.pdf" in root

Hint

Use the ternary operator inside Console.WriteLine to print the result.

Practice

1. What is the main purpose of recursive pattern matching in C#?

easy

A. To break down complex data structures step-by-step

B. To create loops that run forever

C. To declare variables without types

D. To handle exceptions automatically

Solution

1. Step 1: Understand recursive pattern matching concept

   Recursive pattern matching is used to analyze data by breaking it down into smaller parts recursively.

2. Step 2: Identify the purpose from options

   To break down complex data structures step-by-step correctly states that it breaks down complex data structures step-by-step, which matches the concept.

3. Final Answer:

   To break down complex data structures step-by-step -> Option A

4. Quick Check:

   Recursive pattern matching = step-by-step data breakdown [OK]

Hint: Think: recursive means breaking down data stepwise [OK]

Common Mistakes:

• Confusing recursion with infinite loops
• Thinking it declares variables
• Assuming it handles exceptions

2. Which of the following is the correct syntax to use recursive pattern matching in a C# switch expression?

easy

A. case Node(var value, var next):

B. case Node[value, next]:

C. case Node->(value, next):

D. case Node[value; next]:

Solution

1. Step 1: Recall C# recursive pattern syntax

   In C#, recursive patterns use parentheses with variables inside, like Node(var value, var next).

2. Step 2: Compare options to correct syntax

   case Node(var value, var next): matches the correct syntax. Other options use invalid symbols like brackets or arrows.

3. Final Answer:

   case Node(var value, var next): -> Option A

4. Quick Check:

   Recursive pattern syntax uses parentheses ( ) [OK]

Hint: Look for parentheses with variables inside in case patterns [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Using arrows or semicolons incorrectly
• Missing 'var' keyword before variables

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

record Node(int Value, Node? Next);
Node list = new(1, new(2, new(3, null)));

string PrintList(Node? node) => node switch
{
    null => "End",
    Node(var v, var n) => $"{v} -> " + PrintList(n)
};

Console.WriteLine(PrintList(list));

medium

A. 1 2 3 End

B. End

C. 1 -> 2 -> 3 -> End

D. 1 -> 2 -> 3

Solution

1. Step 1: Understand the recursive function PrintList

   PrintList prints the current node's value, then calls itself on the next node, until it reaches null and returns "End".

2. Step 2: Trace the output for list nodes 1, 2, 3

   It prints "1 -> " + PrintList(2), then "2 -> " + PrintList(3), then "3 -> " + PrintList(null), which returns "End".

3. Final Answer:

   1 -> 2 -> 3 -> End -> Option C

4. Quick Check:

   Recursive print with null base case = 1 -> 2 -> 3 -> End [OK]

Hint: Follow recursion calls and base case carefully [OK]

Common Mistakes:

• Ignoring the base case output
• Missing arrows in output
• Confusing spaces with arrows

4. Identify the error in this recursive pattern matching code:

record Node(int Value, Node? Next);
string PrintList(Node? node) => node switch
{
    Node(var v, var n) => $"{v} -> " + PrintList(n),
    null => "End"
};

medium

A. Missing semicolon after switch expression

B. No error, code works correctly

C. The pattern 'Node(var v, var n)' is invalid syntax

D. The order of cases should have 'null' before 'Node' to avoid errors

Solution

1. Step 1: Check switch expression order and syntax

   In C#, switch expressions check patterns top to bottom. Here, 'Node' pattern matches non-null nodes, and 'null' matches null nodes. The order is valid.

2. Step 2: Verify syntax correctness

   The pattern syntax and semicolons are correct. No syntax errors exist.

3. Final Answer:

   No error, code works correctly -> Option B

4. Quick Check:

   Pattern order and syntax are valid [OK]

Hint: Null pattern can be last in switch expressions [OK]

Common Mistakes:

• Thinking null must come first
• Assuming missing semicolon error
• Misreading pattern syntax

5. Given a binary tree defined as:

record TreeNode(int Value, TreeNode? Left, TreeNode? Right);

Write a recursive pattern matching expression to calculate the sum of all values in the tree. Which of the following switch expressions correctly implements this?

hard

A. node switch { null => 0, TreeNode(v, l, r) => v + Sum(l) + Sum(r) }

B. node switch { TreeNode(v, l, r) => v + Sum(l) + Sum(r), null => 0 }

C. node switch { null => 0, TreeNode(var v, var l) => v + Sum(l) }

D. node switch { null => 0, TreeNode(var v, var l, var r) => v + Sum(l) + Sum(r) }

Solution

1. Step 1: Understand the tree structure and sum logic

   The tree node has three parts: Value, Left, and Right. The sum should add the current value plus sums of left and right subtrees.

2. Step 2: Check each option for correct pattern and order

   node switch { null => 0, TreeNode(var v, var l, var r) => v + Sum(l) + Sum(r) } correctly matches null first (base case) and then TreeNode with three variables (value, left, right). It sums value + Sum(left) + Sum(right). node switch { TreeNode(v, l, r) => v + Sum(l) + Sum(r), null => 0 } misses 'var' keywords, which is required. node switch { null => 0, TreeNode(v, l, r) => v + Sum(l) + Sum(r) } misses 'var' keywords, which is required. node switch { null => 0, TreeNode(var v, var l) => v + Sum(l) } misses the right subtree.

3. Final Answer:

   node switch { null => 0, TreeNode(var v, var l, var r) => v + Sum(l) + Sum(r) } -> Option D

4. Quick Check:

   Correct pattern with all parts and base case first [OK]

Hint: Include all node parts and base case null first [OK]

Common Mistakes:

• Omitting 'var' in pattern variables
• Missing one subtree in sum
• Placing null case after non-null without care