Operator

An operator is a symbol that performs some operation on one or more operands. Operators are used to assign values, compare values or perform calculations with values.

• Arithmetic Operators

• Relational Operators

• Logical Operators

Keywords (or) Reserved Words

Keywords or reserved words are words that have a special meaning in a programming language. They are reserved by the compiler and cannot be used as variable names, function names, class names, etc.

Keywords can be classified as:

1. Pre-defined data types

   • char

   • int

   • float

   • double

   • void

2. Modifiers

   • short

   • long

   • signed

   • unsigned

3. Qualifiers

   • const

   • volatile

4. Storage classes

   • auto

   • static

   • register

   • extern

5. Control Structure

   • if

   • else

   • while

   • do

   • for

   • switch

   • goto

   • case

   • default

   • break

   • continue

   • return

6. User-defined data type

   • struct

   • union

   • enum

   • typedef

7. sizeof

   • sizeof()

Words used in C programs are either keywords or identifiers.

Identifiers

Identifiers are names given to entities in a program like variables, functions, constants, etc. They are used to identify these entities.

Some rules for identifiers in C are:

1. An identifier must start with the letter A-Z, a-z or underscore ( _ ).

2. After the first character, identifiers can contain letters, digits and underscore.

3. Identifiers are case-sensitive. age and Age are two different identifiers.

4. Keywords cannot be used as identifiers.

5. Identifiers cannot start with a digit.

Some valid identifiers:

age name salary1 firstname

Some invalid identifiers:

1age - starts with a digit

class - is a keyword

func# - contains special character #

C-preprocessor

The C preprocessor is a text substitution tool that performs macro definition, macro expansion, file inclusion, and conditional compilation. It is not part of the compiler itself but rather a separate step in the compilation process.

The preprocessor uses directives that start with # to instruct the compiler to perform required preprocessing before actual compilation. Using the preprocessor has the following advantages:

• It makes programs easier to develop by:

  • Allowing macro definitions to reduce redundant code

  • Allowing file inclusion to split large programs into multiple files

• It makes programs easier to read and maintain by:

  • Using macros and #defines for constants

  • Giving variables and functions meaningful names

• It makes programs easier to modify by:

  • Allowing conditional compilation to compile different versions

  • Allowing macro parameters to change behaviour

#define

Defines a macro substitution. It performs simple textual substitution of the macro name before actual compilation.

Syntax:

#define macro_name replacement_text

Example:

#define PI 3.14

#define square(x) x*x

#include

Used to insert the contents of a header file into the source code at the location of the #include statement.

Syntax:

#include <filename> // For system header files

#include "filename" // For user-defined header files

Example:

#include <stdio.h>

#include "functions.h"

Advantages:

• Allows splitting large programs into multiple files.

• Header files contain function prototypes and macro definitions that are shared across multiple source files.