Variables and Data types in C

 

1. Keywords / Reserve Words

Keywords are predefined, reserved words used in programming that have special meanings to the compiler. • As C is a case-sensitive language, all keywords must be written in lowercase.

2.Identifiers

An identifier in C consists of a sequence of letters, digits, or underscore characters.

 • Identifiers are set when you declare variables, arrays, enumerations, structures, union, typedef and functions.

• Identifier must be unique.

• They are created to give unique name to a entity to identify it during the execution of the program.

 • You can choose any name for an identifier (excluding keywords). 

• However, if you give meaningful name to an identifier, it will be easy to understand and work on for you and your fellow programmers.  

• Rules for writing an identifier

▫ A valid identifier can have letters (both uppercase and lowercase letters), digits and underscores. 

▫ The first letter of an identifier should be either a letter or an underscore. 

▫ However, it is discouraged to start an identifier name with an underscore. 

▫ There is no rule on length of an identifier. 

▫ However, the first 31 characters of identifiers are discriminated by the compiler.

     Variable

• Programming language enables you to assign symbolic names, known as variable names, for storing program computations and results.  

• A variable name can be chosen by you in a meaningful way to reflect the type of value that is to be stored in that variable. 

• Variables can be used to store integers, floating-point numbers, characters, strings and even pointers to locations inside the computer’s memory.

  

                                        

▫ Examples for invalid Variable Names 

 sum$value - $ is not a valid character. 

 piece flag - Embedded spaces are not permitted. 

 3Spencer - Variable names cannot start with a number. 

 int - int is a reserved word. ▫ You should always remember that upper- and lowercase letters are distinct in C. 

 Therefore, the variable names sum, Sum, and SUM each refer to a different variable.

Data Types

The C programming language provides int, float and char as basic data types. 

• Additionally there are double and _Bool types available. 

▫ int – for storing integral numbers. 

▫ float – for storing floating-point numbers. 

 double – for storing double precision floating point numbers.

 ▫ char – for storing a single character. 

▫ _Bool - for indicating an on/off, yes/no, or true/false situation. For storing value 1 or 0.

Data Types & Constants 

• In C, any number, single character, or character string is known as a constant. 

▫ E.g. the number 58 represents a constant integer value. 

▫ The character string "Programming in C is fun.\n" is an example of a constant character string. 

• There are five different types of constants available in C programming language; 

▫ Integer constants 

▫ Floating point constants 

▫ Character constants

 ▫ Character String Constants 

▫ Enumeration Constants 

Constant Expression 

• Expressions consisting entirely of constant values are called constant expressions. 

▫ E.g. Expression 128 + 7 – 17 is a constant expression because each of the terms of the expression is a constant value. 

▫ 128 + 7 – i would not represent a constant expression beause i was represented as a variable.

Format Modifiers