List patterns in C Sharp (C#) - Step-by-Step Execution

Concept Flow - List patterns

Create or have a list

↓

Start loop or access elements

↓

Check condition or pattern

Yes No↓

Process element

↓

Update index or move next

↩Back to loop start or end

This flow shows how we work with lists by looping through elements, checking conditions, processing items, and moving to the next until done.

Execution Sample

C Sharp (C#)

var numbers = new List<int> {1, 2, 3, 4, 5};
foreach (var n in numbers)
{
    if (n % 2 == 0)
        Console.WriteLine(n);
}

This code loops through a list of numbers and prints only the even ones.

Execution Table

Step	Current Element (n)	Condition (n % 2 == 0)	Action	Output
1	1	False	Skip
2	2	True	Print 2	2
3	3	False	Skip
4	4	True	Print 4	4
5	5	False	Skip
6	End of list	N/A	Stop loop

💡 Reached end of list, no more elements to process.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
n	undefined	1	2	3	4	5	End

Key Moments - 3 Insights

Why does the loop skip printing some numbers?

Because the condition 'n % 2 == 0' is false for those numbers, so the action is to skip printing. See execution_table rows 1, 3, and 5.

What happens when the loop reaches the last element?

After processing the last element (5), the loop ends because there are no more elements. See execution_table row 6.

Why do we use 'foreach' instead of a 'for' loop here?

'foreach' automatically goes through each element in the list without needing an index, making the code simpler and less error-prone.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'n' at step 4?

A 2

B 3

C 4

D 5

💡 Hint

Check the 'Current Element (n)' column at step 4 in the execution_table.

At which step does the condition 'n % 2 == 0' become false for the first time?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Condition' column in execution_table rows 1 and 2.

If we add number 6 to the list, what will be the output at step 7?

A No output

B 6

C Error

D 5

💡 Hint

Even numbers print output; 6 is even. See pattern in execution_table outputs.

Concept Snapshot

List patterns in C#:
- Use 'foreach' to loop through list elements.
- Check conditions inside loop to filter or process items.
- Use 'if' to decide actions per element.
- Loop ends when all elements are processed.
- Simple and safe way to handle lists.

Full Transcript

This example shows how to use list patterns in C#. We start with a list of numbers. We loop through each number using 'foreach'. For each number, we check if it is even by using 'n % 2 == 0'. If true, we print the number. Otherwise, we skip it. The loop continues until all numbers are checked. This pattern helps process lists easily and clearly.

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