List patterns in C Sharp (C#) - Mini Project: Build & Apply

List Patterns in C#

📖 Scenario: You are working with a list of daily temperatures recorded over a week. You want to check if the list matches certain patterns to quickly understand the weather trend.

🎯 Goal: Build a C# program that uses list patterns to check if the temperature list starts with a cold day, ends with a hot day, or has a specific pattern in the middle.

📋 What You'll Learn

Create a list of integers called temperatures with exact values

Create a variable called patternToCheck to hold a pattern description

Use list patterns in a switch expression to match temperatures

Print the result of the pattern matching

💡 Why This Matters

🌍 Real World

Checking patterns in lists is useful in many real-world tasks like analyzing sensor data, processing user inputs, or validating sequences.

💼 Career

Understanding list patterns helps in writing clean, readable code for data processing, which is valuable in software development and data analysis jobs.

1

Create the temperature list

Create a list of integers called temperatures with these exact values: 12, 15, 20, 25, 30, 35, 40.

C Sharp (C#)

using System.Collections.Generic;

// Create the list of temperatures
// Your code here

Hint

Use List<int> and initialize it with the exact numbers inside curly braces.

2

Add a pattern description variable

Create a string variable called patternToCheck and set it to an empty string "".

C Sharp (C#)

using System.Collections.Generic;

List temperatures = new() { 12, 15, 20, 25, 30, 35, 40 };

// Create the patternToCheck variable
// Your code here

Hint

Declare a string variable and assign it an empty string.

3

Use list patterns to check the temperatures

Use a switch expression on temperatures to set patternToCheck to:
- "Starts cold" if the first temperature is less than 15,
- "Ends hot" if the last temperature is greater than 35,
- "Warm middle" if the middle three temperatures are between 20 and 30 inclusive,
- Otherwise, set it to "No pattern".
Use list patterns with temperatures in the switch expression.

C Sharp (C#)

using System.Collections.Generic;

List temperatures = new() { 12, 15, 20, 25, 30, 35, 40 };

string patternToCheck = "";

// Use a switch expression with list patterns to set patternToCheck
// Your code here

Hint

Use list patterns with switch and when guards to check the first and last elements, and a pattern for the middle three.

4

Print the pattern result

Write a Console.WriteLine statement to print the value of patternToCheck.

C Sharp (C#)

using System.Collections.Generic;

List temperatures = new() { 12, 15, 20, 25, 30, 35, 40 };

string patternToCheck = temperatures switch
{
    [var first, ..] when first  "Starts cold",
    [.., var last] when last > 35 => "Ends hot",
    [_, _, >= 20 and = 20 and = 20 and  "Warm middle",
    _ => "No pattern"
};

// Print the patternToCheck variable
// Your code here

Hint

Use Console.WriteLine(patternToCheck); to show the result.

Practice

1. What does the list pattern [] match in C#?

easy

A. An empty list with no elements

B. A list with exactly one element

C. A list with two or more elements

D. Any list regardless of elements

Solution

1. Step 1: Understand the empty list pattern

   The pattern [] is used to match lists that have no elements at all.

2. Step 2: Compare with other list patterns

   Patterns like [var x] match one element, and [var x, .. var rest] match lists with multiple elements, so they don't match empty lists.

3. Final Answer:

   An empty list with no elements -> Option A

4. Quick Check:

   Empty list = [] pattern [OK]

Hint: Empty list pattern is just empty brackets [] [OK]

Common Mistakes:

• Confusing [] with single element pattern
• Thinking [] matches any list
• Assuming [] matches lists with elements

2. Which of the following is the correct syntax to match a list with exactly one element using list patterns in C#?

easy

A. [var x, .. var rest]

B. [var x]

C. [.. var rest]

D. []

Solution

1. Step 1: Identify the pattern for one element

   The pattern [var x] matches a list with exactly one element, binding it to x.

2. Step 2: Understand other patterns

   [var x, .. var rest] matches lists with at least one element plus the rest, [.. var rest] matches any list, and [] matches empty lists.

3. Final Answer:

   [var x] -> Option B

4. Quick Check:

   One element list = [var x] [OK]

Hint: One element list uses [var x] syntax [OK]

Common Mistakes:

• Using [var x, .. var rest] for single element
• Using [] for single element list
• Confusing .. var rest with single element

3. What is the output of this C# code snippet using list patterns?

var list = new List<int> { 5, 10, 15 };
switch (list)
{
    case []:
        Console.WriteLine("Empty");
        break;
    case [var x]:
        Console.WriteLine($"One element: {x}");
        break;
    case [var x, .. var rest]:
        Console.WriteLine($"First: {x}, Rest count: {rest.Count}");
        break;
}

medium

A. Empty

B. One element: 5

C. No match

D. First: 5, Rest count: 2

Solution

1. Step 1: Analyze the list contents

   The list has three elements: 5, 10, and 15.

2. Step 2: Match the list pattern cases

   The first case matches empty list, which is false. The second case matches exactly one element, which is false. The third case matches a list with at least one element: [var x, .. var rest]. Here, x is 5, and rest contains 10 and 15, so count is 2.

3. Final Answer:

   First: 5, Rest count: 2 -> Option D

4. Quick Check:

   List with 3 elements matches [x, ..rest] pattern [OK]

Hint: More than one element matches [x, ..rest] pattern [OK]

Common Mistakes:

• Choosing empty or one element case incorrectly
• Miscounting rest elements
• Ignoring pattern order in switch

4. Identify the error in this C# code using list patterns:

var numbers = new List<int> { 1, 2, 3 };
if (numbers is [var first, var second])
{
    Console.WriteLine($"First: {first}, Second: {second}");
}

medium

A. The variable names first and second are invalid

B. List patterns cannot be used with List<int>

C. The pattern [var first, var second] requires exactly two elements, but list has three

D. Missing else block after if

Solution

1. Step 1: Understand the pattern [var first, var second]

   This pattern matches lists with exactly two elements, binding them to first and second.

2. Step 2: Check the list size

   The list has three elements: 1, 2, and 3, so it does not match the pattern requiring exactly two elements.

3. Final Answer:

   The pattern [var first, var second] requires exactly two elements, but list has three -> Option C

4. Quick Check:

   Pattern length must match list length exactly [OK]

Hint: Pattern length must match list length exactly [OK]

Common Mistakes:

• Assuming pattern matches partial list
• Thinking variable names cause error
• Ignoring list size in pattern matching

5. Given a list of integers, how can you use list patterns in C# to check if the list starts with 1 and has at least two elements? Choose the correct pattern.

hard

A. [1, var second, .. var rest]

B. [var first, 1, .. var rest]

C. [1, .. var rest]

D. [.. var rest, 1]

Solution

1. Step 1: Understand the requirement

   The list must start with 1, then have a second element, and then zero or more elements after that (at least two elements total).

2. Step 2: Analyze each pattern

   [1, var second, .. var rest]: [1, var second, .. var rest] matches a list starting with 1, then a second element, then the rest (zero or more). This fits the requirement.
   [var first, 1, .. var rest]: [var first, 1, .. var rest] matches lists where the second element is 1, not the first.
   [1, .. var rest]: [1, .. var rest] matches lists starting with 1 but allows zero or more elements after, so it matches lists with only one element too.
   [.. var rest, 1]: [.. var rest, 1] matches lists ending with 1, not starting.

3. Final Answer:

   [1, var second, .. var rest] -> Option A

4. Quick Check:

   Start with 1 and at least two elements = [1, var second, .. rest] [OK]

Hint: Start with 1 and two elements uses [1, var second, .. rest] [OK]

Common Mistakes:

• Confusing element positions in pattern
• Using patterns that allow only one element
• Matching end of list instead of start