Constants in C Language

Introduction

• Understanding constants is essential before writing a C program.
• Constants have fixed values that do not change during the execution of a program.
• Types of constants:
  • Symbolic constants
  • Numeric constants
  • Character constants

Symbolic Constants

• Defined using #define directive.
• Example: #define PI 3.14
• The value defined cannot be changed.
• Use uppercase for symbolic constant names for better distinction.

Types of Constants

1. **Numeric Constants:
   • Integer Constants:
     • Decimal Constants (Base 10): Valid values from 0-9 (e.g., 10, 5).
     • Octal Constants (Base 8): Starts with 0 (e.g., 05).
     • Hexadecimal Constants (Base 16): Starts with 0x or 0X, uses 0-9 and A-F letters (e.g., 0xA5).
   • Real Constants (Floating Point Constants):
     • With Decimal Point (e.g., 12.56).
     • Exponential Form: Mantissa e Exponent (e.g., 1.23e4).
2. **Character Constants:
   • Single Character Constants: Enclosed within single quotes (e.g., 'A').
   • Backslash Character Constants (Escape Sequences): E.g., \n for newline.
3. **String Constants:
   • Sequence of characters enclosed within double quotes (e.g., "hello").
   • Always ends with a null character (\0).**

Rules for Declaring Integer Constants

• No commas or spaces between digits.
• Decimal integers cannot have leading zeros (unless it's meant to be octal).
• The sign can be either + or - (default is +).

Rules for Declaring Real Constants

• Must have a decimal point.
• Can be signed (+ or -, default is +).
• Only one decimal point allowed.
• Can be expressed in exponential form.

Rules for Declaring Character Constants

• Enclosed in single quotes.
• Can include any single character, digits, or special characters.
• Represented internally as ASCII values.
• Constants like \n, \t, etc., have special meanings.

Rules for Declaring String Constants

• Enclosed in double quotes.
• Can include letters, digits, spaces, and special characters.
• Always ends with a null character (\0).

Declaring Constants in C

• Using const keyword:
  • Example: const int a = 10;
  • Makes a value immutable within the program.
• Using #define directive:
  • Example: #define A 10
  • No equal sign or semicolon used.

Summary

• Constants in C are important for maintaining fixed values throughout the program.
• Proper declaration ensures correct usage and prevents unintended changes.
• Having a clear understanding of types and rules for constants makes programming in C more efficient.