Lesson 9 — Fixed Collections

Arrays

C# arrays are zero-indexed and fixed-size like Java's, with LINQ extension methods providing powerful one-line operations for sorting, filtering, and aggregating.

CSHARP — Declarations and ranges 
int[] marks  = { 65, 72, 80, 58, 91 };
string[] names = new string[4];
names[0] = "Thandi";
names[1] = "Sipho";

Console.WriteLine(marks[0]);       // 65
Console.WriteLine(marks[^1]);      // 91 — C# index from end
Console.WriteLine(marks.Length);   // 5

// Slice with range (C# 8+):
int[] first3 = marks[0..3];        // { 65, 72, 80 }
int[] last2  = marks[^2..];        // { 58, 91 }
Console.WriteLine(string.Join(", ", first3));

LINQ on Arrays

LINQ extension methods (from System.Linq) provide one-line aggregation, filtering, and sorting on any array or collection.

CSHARP — LINQ aggregation 
using System.Linq;

int[] marks = { 65, 72, 80, 58, 91 };

Console.WriteLine($"Max:     {marks.Max()}");
Console.WriteLine($"Min:     {marks.Min()}");
Console.WriteLine($"Average: {marks.Average():F1}");
Console.WriteLine($"Sum:     {marks.Sum()}");
Console.WriteLine($"Count >=70: {marks.Count(m => m >= 70)}");

// Sort (returns new array, does not modify original):
var sorted = marks.OrderBy(m => m).ToArray();
Console.WriteLine(string.Join(", ", sorted)); // 58, 65, 72, 80, 91

// Array.Sort modifies in place:
Array.Sort(marks);
Console.WriteLine(string.Join(", ", marks));

2D Arrays

CSHARP — 2D arrays 
int[,] grades = {
    { 75, 82, 68 },  // Learner 1
    { 60, 55, 72 },  // Learner 2
    { 90, 88, 95 }   // Learner 3
};

// Access: [row, column] — both zero-based
Console.WriteLine(grades[1, 2]); // 72

// Iterate:
for (int row = 0; row < grades.GetLength(0); row++) {
    for (int col = 0; col < grades.GetLength(1); col++)
        Console.Write($"{grades[row, col],4}");
    Console.WriteLine();
}

Practice Task

Your Turn

Create an int array of 8 marks. Use LINQ to find max, min, average, and count of passing marks (>= 50). Sort the array and print it. Create a second array of only the passing marks using LINQ's Where method.

Common Mistakes

  • Last valid index is Length - 1.
  • 2D arrays use comma syntax: arr[row, col] not arr[row][col].
  • Forgetting using System.Linq; for LINQ methods.
  • Array.Sort modifies in place; LINQ OrderBy returns a new sequence.

Professional Tip

C# range syntax (arr[1..4]) and index-from-end (arr[^1]) are elegant — learn them for working with arrays and spans.

Mini Quiz

How do you access row 2, column 1 of a C# 2D array?