C# Identifiers and Keywords PDF

IT1808 Data Types Identifiers and Keywords An identifier is a name of a program component programmers use to uniquely identify namespaces, classes, methods, variables, constants, etc. Identifiers are user-defined words. For example, in the...

IT1808 Data Types Identifiers and Keywords An identifier is a name of a program component programmers use to uniquely identify namespaces, classes, methods, variables, constants, etc. Identifiers are user-defined words. For example, in the program shown in Code Listing 1, the identifiers are ConsoleApp, ComputeRectangleArea, length, width, area, WriteLine, and ReadKey. Code Listing 1. Sample class with identifiers using System; namespace ConsoleApp { class ComputeRectangleArea { static void Main() { int length, width, area; length = 50; width = 8; area = length * width; Console.WriteLine("The area of the rectangle is " + area); Console.ReadKey(); } } } The following are the syntax rules when naming an identifier in C#: It must start with a letter of the English alphabet or an underscore character. The identifier's name can only have any combination of letters, digits, and underscores. White spaces are not allowed. Identifiers are case sensitive. For example, the identifier Area is not the same with identifiers area or AREA. It cannot be a reserved keyword. The classes and methods in C# must always begin with a capital letter. The following are the examples of valid and invalid identifiers: Valid Identifiers Invalid Identifiers _score score_ 1score 1score is invalid because it begins with number. first_Name ScoreClass first Name first Name is invalid because it contains white space. grade1 ComputeScore class class is an invalid identifier because it is a reserved keyword. Table 1. Example valid identifiers Table 2. Example invalid identifiers Keywords are reserved words a programming language uses for its own use, and they have a special predefined meaning to the compiler. These cannot be used as identifiers. If you do use a keyword in a program, the compiler will throw an error message. Table 3 shows the list of keywords in C#. abstract as base bool break byte case catch char checked class const continue decimal default delegate do double else enum event explicit extern false finally fixed float for foreach goto if implicit in int interface internal is lock long namespace new null object operator out override params private protected public readonly ref return sbyte sealed short sizeof stackalloc static string struct switch this throw true try typeof uint ulong unchecked unsafe ushort using virtual void volatile while Table 3. C# keywords (Harwani, 2015) 02 Handout 1 *Property of STI  [email protected] Page 1 of 5  IT1808 Variables A variable is an identifier and a memory location that stores a specific value. Variables hold a value that can be changed during program execution. For example, a variable named score assigned an initial value of 25. When the program starts, the value of variable score will change to 85. The basic syntax of declaring a variable is as follows: data_type identifier; The data_type is one (1) of C#’s data types, and the identifier is the name of the variable. For example, int score; Variables are initialized, or assigned a value, with an equal sign followed by the value. The following are some valid examples of declaring and initializing variables: You can initialize a variable at the time of declaration. For example: int score = 25; You can declare and initialize more than one (1) variable of the same time data type using a comma in a single statement: int score, age; score = 85; age = 24; int length = 8, width = 5; When creating a program, you must define a variable with a meaningful name that is easy to understand what value must store on it. For example, the variables gameScore and age must store integer type values. Use camelCase notation that starts with a lowercase letter for naming local variables. For example, numberOfStudents, age, totalPrice, etc. Constants A constant is an identifier and a memory location whose value cannot be changed during program execution. Constants must initialize a value at the time of declaration. The basic syntax of initializing a constant in C# is as follows: const data_type IDENTIFIER = value; Constants in C# are defined using the const keyword. For example: const double PI = 3.14159;. In the example, the constant PI with the value of 3.14159 cannot be changed during program execution. The name of the constants must be in all capital to make it easy to identify that its value must not change. Data Types Data types are used to specify a set of values and their operations associated with variables or constants. The values have a particular data type because they are stored in memory in the same format and have the same operations defined for them. A variable or constant stores data of a specific type. When declaring a variable or constant to store a data, the appropriate data type for that data must be identified. The data type will instruct the compiler what kind of value a variable or constant can hold and its operations. There are two (2) types of primitive data types in C#: Value types. These data types store the value directly. The data type int is a value type that stores its value in a memory location directly. Reference types. These data types do not store the actual value in a variable or constant. Instead, they store the reference (or address) where the value is stored. The class objects, strings, and arrays are reference types because they hold references to blocks of memory and are managed on the heap. Value Types A data type is a value type if it holds the data within its own memory allocation. Value types directly store the values within the variable. For example, consider the following figure: In Figure 1, the variable length of int type is assigned a value of 50. When this statement is executed, the compiler will instruct the computer system to directly store 50 in the memory space allocated for the variable length. Figure 1. Memory allocation for value type 02 Handout 1 *Property of STI  [email protected] Page 2 of 5  IT1808 The table below lists the available value types in C#. Data Type Description Range of Values Default Value Example sbyte 8-bit signed integer -128 to 127 0 sbyte size = 100; short 16-bit signed integer -32,768 to 32,767 0 short score = 1400 int 32-bit signed integer -2,147,483,648 to 0 int num = 12400; 2,147,483,647 long 64-bit signed integer -263 to 263-1 0L long length = 124000L; byte 8-bit unsigned integer 0 to 255 0 byte age = 100; ushort 16-bit unsigned integer 0 to 65,535 0 ushort uScore = 1400; uint 32-bit unsigned integer 0 to 4,294,967,295 0 uint uNum = 12400; ulong 64-bit unsigned integer 0 to 264-1 0 ulong uLength = 124000L; float Single-precision 32-bit 1.5E-45 to 3.4E+38 0.0F float rate = 235.25F; floating-point number double Double-precision 64-bit 5E-324 to 1.7E+308 0.0D double price = 235.25; floating-point number bool 8-bit Logical Boolean type true or false false bool isCorrect = true; char Unicode 16-bit character '\u0000' to '\uffff' '\0' char firstLetter = 'C'; Table 4. Value types in C# (Harwani, 2015) Reference Types A reference type does not store an actual value, but it stores the address where the value is stored. It means that the reference types contain the memory locations of where the value is stored. For example, consider the figure below: Figure 2. Memory allocation for reference type In Figure 2, the compiler will instruct the computer system to select a different memory location for the variable strName. The content of the variable strName is "0x7600", which is the address or memory location of the actual string value "Jess Diaz". The reference types in C# are Strings, Objects, Arrays, and Delegates. Type Conversion Type conversion or type casting is the process of converting a value of one (1) type of data to another data type. This is used to ensure that a function correctly processes the variables. Converting a string to an integer is an example of type conversion. The following are the two (2) forms of type casting in C#: Implicit conversion. This is the conversion of a lower precision data type to a value of higher precision data type. A compiler automatically performs implicit conversion if the precision of data type to convert is lower than precision of preferred data type. For example, a variable of short data type is capable of storing values up to 32,767, and the maximum value allowed into a byte is 255; so, there will be no problem when you convert the value of byte type into a value of short data type. An example of implicit conversion includes converting the value of int data type to a value of double data type, because int data type has a lower precision than double data type. Table 5 shows the list of all the valid implicit numeric conversions in C#. From To Example sbyte short, int, long, float, or double sbyte a = 25; short b = a; byte short, ushort, int, uint, long, ulong, float, or double byte a = 25; int b = a; short int, long, float, or double short a = 25; int b = a; ushort int, uint, long, ulong, float, or double ushort a = 25; long b = a; 02 Handout 1 *Property of STI  [email protected] Page 3 of 5  IT1808 From To Example int long, float, or double int a = 25; double b = a; //the value of b is 25.0 uint long, ulong, float, or double uint a = 25; long b = a; long float or double long a = 25; double b = a; ulong float or double ulong a = 25; double b = a; float double float a = 25.0F; double b = a; char ushort, int, uint, long, ulong, float, or double char a = 'a'; int b = a; //the value of b is 97 because the decimal value of character 'a' in ASCII table is 97 Table 5. Implicit numeric conversions (Deitel, P. and Deitel, H., 2015) Note: There is no implicit conversion of any data type to char type, so the values of the other integral types do not automatically convert to the char type. The bool and double data types have no possible implicit conversions to the other data types. Explicit conversion. Converting a higher precision data type to a lower precision data type is invalid, and the compiler will return an error. For example, the statements double num1 = 25.0; int num2 = a; will return an error because the precision of variable num1 of double data type is higher than the variable num2 of int type. However, explicit conversions allow the users to convert a value of higher precision data type into a value of lower precision data type by using a cast operator. The following is the general syntax of performing explicit conversion in C#: (data_type) data_to_convert; The (data_type) is the cast operator that will create a converted copy of the value in data_to_convert. For example, the following statements uses a cast operator to explicitly convert data types: //first example int num = 25; byte b = (byte) num; //second example double price = 75.5; int varA = (int) price; //the value of variable a will be 75 In the first example, the value 25 is assigned to the variable num of int type, then the converted copy of value from variable num is assigned to the variable b. The value stored in variable num is still an integer type because the cast operator created a converted copy of its value as byte data type. Explicit conversion involves the risk of losing information, i.e., from double to int data type. For example, in the second example, the value of the variable price of double data type is copied and converted into int data type then stored in the variable varA of int data type. The cast operator dropped the decimal part of the floating-point number 75.5 to convert it to 75 as int data type. The following statements shows some of the ways on how to use the cast operator: int a = (int) 7.9; //the return value of a is 7 double b = (double) (5 + 3); //the value of b is 8.0 Console.WriteLine((int) 2.5); //the output is 2 int c = 64; char d = (char) c; //the return value of d is the character '@' The following table shows the list of all valid explicit conversions in C#. From To sbyte byte, ushort, uint, ulong, or char byte sbyte or char short sbyte, byte, ushort, uint, ulong, or char ushort sbyte, byte, short, or char int sbyte, byte, short, ushort, uint, ulong, or char uint sbyte, byte, short, ushort, int, uint, long, ulong, or char long sbyte, byte, short, ushort, int, uint, ulong, or char 02 Handout 1 *Property of STI  [email protected] Page 4 of 5  IT1808 From To ulong sbyte, byte, short, ushort, int, uint, long, or char char sybte, byte, or short float sbyte, byte, short, ushort, int, uint, long, ulong, or char double sbyte, byte, short, ushort, int, uint, long, ulong, char, or double Table 6. Explicit conversions (Deitel, P. and Deitel, H., 2015) The explicit conversion of a data type to char data type will return the corresponding character of the value from the ASCII table. int a = 64; char b = (char) a; //the value of b is the character '@' because it is the corresponding character of decimal value 64 in ASCII table REFERENCES: Deitel, P. and Deitel, H. (2015). Visual c# 2012 how to program, 5th edition. USA: Pearson Education, Inc. Gaddis, T. (2016). Starting out with visual c#, 4th edition. USA: Pearson Education, Inc. Harwani, B. (2015). Learning object-oriented programming in c# 5.0. USA: Cengage Learning PTR. 02 Handout 1 *Property of STI  [email protected] Page 5 of 5