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C# Initialize ArrayInitialize arrays with different syntax forms. Use array initializer expressions.
Array, initialize. In .NET, an array can be initialized in several ways. With array initializer syntax, we can specify individual elements directly.
Array
Initialize methods. We can assign each element in an array to one value. This is often needed in programs, and the syntax is important to review.
Array initializers. An array initializer uses curly brackets with elements in comma-separated lists. The length (and number of dimensions) is inferred from the expression.
Part 1 We see array initializers for one-dimensional int arrays. The first 2 array initializers are equivalent.
Part 2 We create string arrays with initializers. For array 5, we create a 2D array with 2 columns and 2 rows.
2D Array
Part 3 We print the arrays to the screen. We display ranks (the number of dimensions) of the arrays.
Important Array initializers compile to the same instructions as direct assignments to populate arrays.
C# program that uses array initializers
using System; class Program { static void Main() { // Part 1: declare arrays with curly brackets. // ... It is allowed to omit the type. int[] array1 = { 1, 2, 3 }; int[] array2 = new int[] { 1, 2, 3 }; // Part 2: use array initializations with strings. // ... We can specify two-dimensional arrays. // ... We can use empty arrays. string[] array3 = { "dot", "net", "perls" }; string[] array4 = new string[] { "DOT", "NET", "PERLS", null }; string[,] array5 = { { "dot", "perls" }, { "framework", "4.0" } }; string[] array6 = { }; // Part 3: print the length and ranks. Console.WriteLine("array1: {0} length", array1.Length); Console.WriteLine("array2: {0} length", array2.Length); Console.WriteLine("array3: {0} length", array3.Length); Console.WriteLine("array4: {0} length", array4.Length); Console.WriteLine("array5: {0} length, {1} rank", array5.Length, array5.Rank); Console.WriteLine("array6: {0} length, {1} rank", array6.Length, array6.Rank); } }
array1: 3 length array2: 3 length array3: 3 length array4: 4 length array5: 4 length, 2 rank array6: 0 length, 1 rank
Example, for-loop. We can initialize arrays with for-loops, which (overall) may be best for a team—it uses the more standard style. We create a helper method for this purpose.
Static We use 2 static methods, which save no state, and which receive strongly-typed arrays. The values they initialize are hard-coded.
Static
Tip We can modify the methods to receive a second parameter, the value we want to initialize to. Extension methods could be used.
Extension
C# program that initializes arrays
using System; class Program { static void Main() { // Initialize an array of -1 integers. int[] arr1 = new int[10]; InitIntArray(arr1); foreach (int i in arr1) { Console.Write(i); } Console.WriteLine(); // Initialize an array of space chars. char[] arr2 = new char[5]; InitCharArray(arr2); foreach (char c in arr2) { Console.Write(c); } Console.WriteLine(); } /// <summary> /// Initialize array to -1 /// </summary> static void InitIntArray(int[] arr) { for (int i = 0; i < arr.Length; i++) { arr[i] = -1; } } /// <summary> /// Initialize array to ' ' /// </summary> static void InitCharArray(char[] arr) { for (int i = 0; i < arr.Length; i++) { arr[i] = ' '; } } }
-1-1-1-1-1-1-1-1-1-1
Enumerable.Repeat. Here we use Enumerable.Repeat to assign a new array to a single value series. We ensure the System.Linq namespace is included.
Tip This style of code is often called list comprehension. We specify an entire array in a single declaration.
Style Each style of code suits different developers and teams. I tend to use the more standard loop style with for.
C# program that uses Enumerable
using System; using System.Linq; class Program { static void Main() { // Initialize an array of -1 integers. int[] arr1 = Enumerable.Repeat(-1, 10).ToArray(); foreach (int i in arr1) { Console.Write(i); } Console.WriteLine(); // Initialize an array of space chars. char[] arr2 = Enumerable.Repeat(' ', 5).ToArray(); foreach (char c in arr2) { Console.Write(c); } Console.WriteLine(); } }
-1-1-1-1-1-1-1-1-1-1
Enumerable.Range. Here we use Enumerable.Range to initialize an array to an entire range of numbers or other values. This can be replaced with a loop. We include the System.Linq namespace.
C# program that uses Range
using System; using System.Linq; class Program { static void Main() { // Initialize array of 5 sequential integers. int[] arr1 = Enumerable.Range(5, 5).ToArray(); foreach (int i in arr1) { Console.WriteLine(i); } } }
5 6 7 8 9
Initialize, benchmark. Here we benchmark Enumerable.Repeat. This method is elegant, but if it causes a performance loss, it may be necessary to avoid it in some programs.
Version 1 This code creates a new int array with 100 elements, and then initializes each element with a for-loop.
Int Array
Version 2 Here we call Enumerable.Repeat and then convert to an array with the ToArray extension method.
ToArray
Result Tested in 2021 with .NET 5 for Linux, Enumerable is slower than a for-loop and direct allocation.
C# program that benchmarks Enumerable.Repeat, ToArray
using System; using System.Diagnostics; using System.Linq; class Program { const int _max = 1000000; static void Main() { // Version 1: initialize with for-loop. var s1 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { int[] array = new int[100]; InitArray(array); } s1.Stop(); // Version 2: use Enumerable.Repeat. var s2 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { int[] array = Enumerable.Repeat(-1, 100).ToArray(); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); } static void InitArray(int[] array) { // Use for-loop. for (int i = 0; i < array.Length; i++) { array[i] = -1; } } }
71.91 ns for-loop 101.78 ns Enumerable.Repeat, ToArray
Discussion. The C# language specification describes array initializers. We see that an array initializer is converted to a sequence of assignments into the newly-allocated arrays.
Array.CreateInstance. With this method, we can create an array based on runtime parameters. So a method can create a string or int array (for example) based on its arguments.
Note CreateInstance does not initialize the array to any special value (the default value is used). It only allocates (creates) the array.
Array.CreateInstance
A summary. The C# compiler can understand and infer array creations. The dimensions and the length of the arrays is inferred from the initializers.
For repeat values, we used methods and loops. We can fill an array with a single value, such as -1, or with a range of values. For smaller arrays, it is faster to use a for-loop.
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