1. C Basics

1. Character Set in C

A character set is a set of alphabets, letters and some special characters that are valid in C language.

• Alphabets: Uppercase (A-Z) and Lowercase (a-z)
• Digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
• Special Characters: ~ @ # $ % ^ & * ( ) _ - + = { } [ ] ; : ' " < > , . / ? | \
• White Space Characters: space, newline (\n), horizontal tab (\t), carriage return (\r), form feed (\f)

2. Tokens

Tokens are the smallest individual units in a C program. When a compiler compiles a program, it breaks the code into tokens.

1. Keywords: Reserved words that have special meaning to the compiler (e.g., int, if, while, return). C has 32 standard keywords.
2. Identifiers: User-defined names for variables, functions, arrays, etc.
3. Constants: Fixed values that do not change during program execution.
4. Strings: Sequence of characters enclosed in double quotes (e.g., "Hello").
5. Special Symbols: Characters having special meaning (e.g., [ ] ( ) { } , ;).
6. Operators: Symbols that perform operations on variables and values (e.g., +, -, *).

3. Variables

A variable is a name given to a memory location to store data. Its value can be changed during execution.

Rules for naming variables (Identifiers):

• Must begin with a letter or an underscore (_).
• Can consist of letters, digits, and underscores.
• Cannot be a reserved keyword.
• Case-sensitive (e.g., num and Num are different).

Syntax:

datatype variable_name;

Example: int age = 20;

4. Escape Characters (Escape Sequences)

Escape sequences are used in C to represent characters that cannot be typed directly (like newline) or have special meaning (like quotation marks).

• \n: Newline
• \t: Horizontal Tab
• \\: Backslash
• \': Single quote
• \": Double quote
• \a: Alert / Bell
• \b: Backspace
• \0: Null character (used to terminate strings)

5. Constants

Constants are values that remain fixed during the execution of a program.

1. Integer Constants: Whole numbers without a decimal point.
   • Base 10 (Decimal): Allowed digits 0-9. E.g., 15, -20, 0
   • Base 8 (Octal): Must start with 0. Allowed digits 0-7. E.g., 014 (equals 12 in decimal)
   • Base 16 (Hexadecimal): Must start with 0x or 0X. Allowed digits 0-9, A-F. E.g., 0x1A
2. Floating-point Constants: Numbers with a fractional part or decimal point. E.g., 3.14, -0.5, 2.0e5 (exponential notation)
3. Character Constants: A single character enclosed in single quotes. E.g., 'A', '1', '\n'
4. String Constants: A sequence of characters enclosed in double quotes. E.g., "Hello World"
5. Symbolic Constants: Defined using the #define preprocessor directive. They do not occupy memory like variables.

   #define PI 3.14159
   #define MAX 100