Recursive pattern matching in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match a tuple with two integers using recursive pattern matching.

C Sharp (C#)

bool IsPairOfInts(object obj) => obj is ([1], int);

Drag options to blanks, or click blank then click option'

Aint

Bstring

Cdouble

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using string or other types instead of int.

The pattern int matches any integer value in the tuple.

2fill in blank

medium

Complete the code to recursively match a nested tuple with an integer and another tuple.

C Sharp (C#)

bool IsNestedPair(object obj) => obj is (int, (int, [1]));

Drag options to blanks, or click blank then click option'

Adouble

Bstring

Cbool

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect types like string or bool.

The inner tuple's second element should be an integer, so use int.

3fill in blank

hard

Fix the error in the recursive pattern matching to correctly match a tuple with a string and a nested tuple of two integers.

C Sharp (C#)

bool MatchPattern(object obj) => obj is ([1], (int, int));

Drag options to blanks, or click blank then click option'

Aint

Bstring

Cbool

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using int instead of string for the first element.

The first element is a string, so the pattern should be string.

4fill in blank

hard

Fill both blanks to match a tuple with a string and a nested tuple where the first element is an int and the second is a bool.

C Sharp (C#)

bool CheckTuple(object obj) => obj is ([1], ([2], bool));

Drag options to blanks, or click blank then click option'

Astring

Bint

Cbool

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the types or using incorrect types like double.

The outer tuple's first element is a string, and the nested tuple's first element is an int.

5fill in blank

hard

Fill all three blanks to match a tuple with a string, a nested tuple with an int and a nested tuple with a bool and a double.

C Sharp (C#)

bool ComplexMatch(object obj) => obj is ([1], ([2], ([3], double)));

Drag options to blanks, or click blank then click option'

Astring

Bint

Cbool

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of types or using wrong types like double in the wrong place.

The outer tuple's first element is a string, the middle nested tuple's first element is an int, and the innermost nested tuple's first element is a bool.

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