Steps of initialisation:

Size: px

Start display at page:

Download "Steps of initialisation:"

Eugene Taylor
5 years ago
Views:

1 1. a. Arrays in C# are objects and derive from System.Array. They are the simplest collection or data structure in C# and may contain any value or reference type. In fact, an array is the only collection that is part of the System namespace. They are declared with a specific type and they cannot change their size once declared. Steps of declaration: At step (1), a reference variable is created a certain type array. At step (2), space is allocated to store values of certain type. The square brackets [] must be placed after the type in both steps (1) and (2). Only at st ep (2), however, is the actual number of elements placed within the brackets. int[] myarray; // (1) One dimensional array myarray = new int[3]; // (2) int[] myarray = new int[3]; Steps of initialisation: So far, the myarray array has been declared, but each array element has not been explicitly initialized. Therefore, each array element is initialized implicitly to its default value, in this case, 0. It is important to note that if the array type was our Id class instead of int in other words, a reference type instead of a value type then myarray would be an array of references initialized by default to null. Elements in the array, however, can be initialized explicitly in a number of ways. The use of an initializer, as in C/C++ and Java, is often preferred to declare and initialize an array at the same time: int[] myarray={1,3,5}; In this case, the compiler determines the number of integers within the initializer and implicitly creates an array of size 3. In fact, the preceding example is equivalent to this more explicit one: int[] myarray = new int[3] { 1, 3, 5 }; Accessing an array: Elements of an array are accessed by following the array name with an index in square brackets. Bearing in mind that indices begin at 0, myarray[2] accesses the third element. C# supports two kinds of multi-dimensional arrays: rectangular and jagged. Rectangular arrays like matrices have more than one index and have a fixed size for each dimension. The comma (,) separates each dimension in the array declaration as illustrated here: int[,] matrix = new int[2,3]; // 2x3 matrix (6 elements). int[,,] cube = new int[2,3,4]; // 2x3x4 cube (24 elements). Accessing the element at the first row and second column of the matrix is done as follows: matrix[0,1]; // matrix [ <row>, <column> ] Jagged arrays are arrays of arrays where each element is a reference pointing to another array. Unlike rectangular arrays, jagged arrays may have a different size for each dimension. Page 1 of 9

2 In the following example, jaggedmatrix allocates space for eight integer elements, three in the first row and five in the second: int[ ][ ] jaggedmatrix = new int[2][ ]; // An array with 2 arrays (rows). jaggedmatrix[0] = new int[3]; // A row of 3 integers. jaggedmatrix[1] = new int[5]; // A row of 5 integers. Accessing the element at the first row and second column of the jaggedmatrix is done as follows: jaggedmatrix[0][1]; // jaggedmatrix [ <row> ] [ <column> ] Of course, attempting to access jaggedmatrix[0][3] throws an IndexOutOfRangeException. Although access to rectangular arrays is more efficient than access to jagged arrays, the latter is more flexible for cases such as sparse matrices. Nonetheless, dimensions for both rectangular and jagged arrays are fixed. When dimensions must grow or when fixed sizes cannot be determined by the application requirements, collections, which are a far more flexible alternative to multi-dimensional arrays. Related Notes on arrays ARRAYS An array is a collection of elements, all of a specific type. And these elements are stored together in memory. Each element is accessed by an integer index. An array is a fixed collection of same-type data that are stored contiguously and that are accessible by an index. ONE-DIMENSIONAL ARRAY: Arrays of Integers Code snippet to allocates and initializes an integer array of three elements. The array is zerobased. The first element is element zero. Element indexes proceed upwards by one. Initialization. This syntax form is shorter. We use curly brackets to assign element values in one line. This is concise. The length of the array is automatically determined in Illustration c and so in Illustration a and b if size is not specified. Illustration a: int[] values = new int[3];//declare int array. 3 is specifying the size of the array (number of elements). Always specify. It s Invalid to put 3 in the first square brackets values[0] = 5;/* initialising first element to be 5. The value zero in square brackets is the index value. Indexing always starts at zero thus index value = postion-1, 1-1 =0*/ values[1] = 10; /* initialising 2nd element to be 1 (value in square brackets)*/ Page 2 of 9

3 values[2] = 15; /* initialising first element to be index 2 (value in square brackets)*/ Console.WriteLine(values[0]);/*accessing the value at index 0 which is 5 and print it to console*/ Console.WriteLine(values[1]); /*accessing the value at index 1 which is 10 and print Console.WriteLine(values[2]); /*accessing the value at index 2 which is 15 and print Illustration b: int[] values = new int[3] { 5, 10, 15 }; /*declaration and initialisation and specifying the size in square brackets after new int. You can also not specify the size in this case since you have initialised i.e leave the square brackets empty*/ Console.WriteLine(values[0]);/*accessing the value at index 0 which is 5 and print it to console*/ Console.WriteLine(values[1]); /*accessing the value at index 1 which is 10 and print Console.WriteLine(values[2]); /*accessing the value at index 2 which is 15 and print Illustration c: int[] values = { 5, 10, 15 }; /*declaration and initialisation, simplest way Console.WriteLine(values[0]);/*accessing the value at index 0 which is 5 and print it to console*/ Console.WriteLine(values[1]); /*accessing the value at index 1 which is 10 and print Console.WriteLine(values[2]); /*accessing the value at index 2 which is 15 and print Arrays of strings string[] ferns = new string[4];// declare and specify the size of the array (number of elements) i.e 4 ferns[0] = "Psilotopsida";//initialising the value at index 0 to Psilotopsida ferns[1] = "Equisetopsida";//initialising the value at index 1 to Equisetopsida ferns[2] = "Marattiopsida";//initialising the value at index 2 to Marattiopsida ferns[3] = "Polypodiopsida";//initialising the value at index 3 to Polypodiopsida Console.WriteLine(ferns[0]);//accessing the value at index 0 and printing it Console.WriteLine(ferns[1]);//accessing the value at index 1 and printing it Console.WriteLine(ferns[2]);//accessing the value at index 2 and printing it Console.WriteLine(ferns[3]);//accessing the value at index 3 and printing it Page 3 of 9

4 string[] ferns = new string[4] { "Psilotopsida", "Equisetopsida", "Marattiopsida", "Polypodiopsida" };/*declaration and initialistion, four elements. Specify or not specify the size*/ Console.WriteLine(ferns[0]);//accessing the value at index 0 and printing it Console.WriteLine(ferns[1]);//accessing the value at index 1 and printing it Console.WriteLine(ferns[2]);//accessing the value at index 2 and printing it Console.WriteLine(ferns[3]);//accessing the value at index 3 and printing it string[] ferns = { "Psilotopsida", "Equisetopsida", "Marattiopsida", "Polypodiopsida" };/*declaration and initialistion, four elements. Always leave the square brackets empty. Console.WriteLine(ferns[0]);//accessing the value at index 0 and printing it Console.WriteLine(ferns[1]);//accessing the value at index 1 and printing it Console.WriteLine(ferns[2]);//accessing the value at index 2 and printing it Console.WriteLine(ferns[3]);//accessing the value at index 3 and printing it MULTI-DIMENSIONAL ARRAY (SIZE 2,3 N) C# Jagged Arrays Data comes in various shapes. Sometimes the shape is uneven. With jagged arrays, we can store efficiently many rows of varying lengths. Important:Jagged arrays have different performance characteristics. Indexing jagged arrays is fast. Allocating them is somewhat slow. Illustration a: // Declare local jagged array with 3 rows(always specify# of rows). The number of columns are not specified since they might vary or specify to make them fixed. int[][] jagged = new int[3][]; // Set first row: Create a new array in the jagged array, and assign it. jagged[0] = new int[2];//first row consists of two elements(first column and second column i.e index 0 and 1 respectively) jagged[0][0] = 1;//first row first column store the value 1 i.e at index 0 for row and index 0 jagged[0][1] = 2;//first row second column store the value 2 i.e at index 0 for row and index 1 // Set second row jagged[1] = new int[3];// second row consists of 3 elements Page 4 of 9

5 jagged[1][0] = 3;//second row first column store the value 3 i.e at index 1 for row and index 0 jagged[1][1] = 4;//second row second column store the value 4 i.e at index 1 for row and index 1 jagged[1][2] = 5;//second row third column store the value 3 i.e at index 1 for row and index 2 // Set third row jagged[2] = new int[4];//third row consists of 4 elements jagged[2][0] = 6;//third row first column stores the value 6 i.e at index 2 for row and index 0 jagged[2][1] = 7;//third row second column stores the value 7 i.e at index 2 for row and index 1 jagged[2][2] = 8;//third row third column stores the value 8 i.e at index 2 for row and index 2 jagged[2][3] = 9;//third row fourth column stores the value 9 i.e at index 2 for row and index 3 Console.WriteLine(jagged[0][0]);//accessing the value at index 0 for row and index 0 for Console.WriteLine(jagged[0][1]);//accessing the value at index 0 for row and index 1 for Console.WriteLine(jagged[1][0]);//accessing the value at index 1 for row and index 0 for Console.WriteLine(jagged[1][1]);//accessing the value at index 1 for row and index 1 for Console.WriteLine(jagged[1][2]);//accessing the value at index 1 for row and index 2 for Console.WriteLine(jagged[2][0]);//accessing the value at index 2 for row and index 0 for Console.WriteLine(jagged[2][1]);//accessing the value at index 2 for row and index 1 for Console.WriteLine(jagged[2][2]);//accessing the value at index 2 for row and index 2 for Console.WriteLine(jagged[2][3]);//accessing the value at index 2 for row and index 3 for Console.ReadLine(); Illustration b: // Declare local jagged array with 3 rows(always specify# of rows). The number of columns are not specified since they might vary or specify to make them fixed. int[][] jagged = new int[3][]; jagged[0] = new int[2] {1,2};//first row initialised with two elements;using array initializer jagged[1] = new int[3] {3,4,5};//second row initialised with 3 elements jagged[2] = new int[4] {6,7,8,9};//third row initialised with 4 elements Console.WriteLine(jagged[0][0]);//accessing the value at index 0 for row and index 0 in that row and printing it Page 5 of 9

6 Console.WriteLine(jagged[0][1]);//accessing the value at index 0 for row and index 1 in that row and printing it Console.WriteLine(jagged[1][0]);//accessing the value at index 1 for row and index 0 in that row and printing it Console.WriteLine(jagged[1][1]);//accessing the value at index 1 for row and index 1 in that row and printing it Console.WriteLine(jagged[1][2]);//accessing the value at index 1 for row and index 2 in that row and printing it Console.WriteLine(jagged[2][0]);//accessing the value at index 2 for row and index 0 in that row and printing it Console.WriteLine(jagged[2][1]);//accessing the value at index 2 for row and index 1 in that row and printing it Console.WriteLine(jagged[2][2]);//accessing the value at index 2 for row and index 2 in that row and printing it Console.WriteLine(jagged[2][3]);//accessing the value at index 2 for row and index 3 in that row and printing it Console.ReadLine(); // Create a new array in the jagged array, and assign it. int[,,] threedimensional = new int[3, 5, 4]; threedimensional[0, 0, 0] = 1; threedimensional[0, 1, 0] = 2; threedimensional[0, 2, 0] = 3; threedimensional[0, 3, 0] = 4; threedimensional[0, 4, 0] = 5; threedimensional[1, 1, 1] = 2; threedimensional[2, 2, 2] = 3; threedimensional[2, 2, 3] = 4; int[,] numbers = new int[3, 2] { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = new string[2, 2] { {"Mike","Amy"}, {"Mary","Albert"} }; You can omit the size of the array, like this: int[,] numbers = new int[,] { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = new string[,] { {"Mike","Amy"}, {"Mary","Albert"} }; You can also omit the new operator if an initializer is provided, like this: int[,] numbers = { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = { {"Mike", "Amy"}, {"Mary", "Albert"} }; Page 6 of 9

7 int[,] numbers = new int[3, 2] { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = new string[2, 2] { {"Mike","Amy"}, {"Mary","Albert"} }; You can omit the size of the array, like this: int[,] numbers = new int[,] { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = new string[,] { {"Mike","Amy"}, {"Mary","Albert"} }; You can also omit the new operator if an initializer is provided, like this: int[,] numbers = { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = { {"Mike", "Amy"}, {"Mary", "Albert"} }; b. Boxing is the process of implicitly casting a value-type variable or literal into a reference type. In other words, it allows value types to be treated as objects. This is done by creating an optimized temporary reference type that refers to the value type. Boxing a value via explicit casting is legal but unnecessary. int i = 2; object o=i; //Implicit casting (or boxing).// Implicit conversion object p = (object)i; // Explicit casting (unnecessary). On the other hand, it is not possible to unbox a reference type into a value type without an explicit cast. The intent must be clear from the compiler s point of view. object o;... short s = (short)o;// Unboxing, Explicit conversion The ability to treat value types as objects bridges the gap that exists in most programming languages. For example, a Stack class can provide push and pop methods that take and return value and reference objects: Page 7 of 9

8 c. Page 8 of 9

9 Page 9 of 9

High Institute of Computer Science & Information Technology Term : 1 st. El-Shorouk Academy Acad. Year : 2013 / Year : 2 nd

El-Shorouk Academy Acad. Year : 2013 / 2014 High Institute of Computer Science & Information Technology Term : 1 st Year : 2 nd Computer Science Department Object Oriented Programming Section (1) Arrays