In C programming, the character set refers to the set of characters that can be used to write programs. Identifiers are names used to identify variables, functions, arrays, and other user-defined items in a C program. Here’s a breakdown of the character set and the rules for creating identifiers:

Character Set in C

1. Alphanumeric Characters:
   • Includes letters (both uppercase and lowercase: A-Z, a-z) and digits (0-9).
2. Special Characters:
   • The underscore (_) is used in identifiers. Other special characters (like @, $, #, etc.) are not allowed in identifiers.
3. Whitespace:
   • Spaces, tabs, and newlines are used for separating tokens in code but are not part of identifiers.

Rules for Identifiers

1. Start with a Letter or Underscore:
   • Identifiers must begin with a letter (uppercase or lowercase) or an underscore. They cannot start with a digit.
2. Subsequent Characters:
   • After the first character, identifiers can include letters, digits, and underscores.
3. Case Sensitivity:
   • Identifiers are case-sensitive. For example, Variable, variable, and VARIABLE are considered different identifiers.
4. Length:
   • While the C standard allows identifiers to be quite long, it’s good practice to keep them reasonable (typically fewer than 32 characters) for readability.
5. No Reserved Keywords:
   • Identifiers cannot be the same as C reserved keywords (like int, return, if, etc.).

Examples of Valid and Invalid Identifiers

• Valid Identifiers:
  • myVariable
  • count1
  • _temp
  • MAX_SIZE
• Invalid Identifiers:
  • 1stVariable (cannot start with a digit)
  • my-variable (hyphen is not allowed)
  • int (reserved keyword)
  • my variable (spaces are not allowed)

Summary

• Character Set: Alphanumeric characters and underscores.
• Identifiers: Must start with a letter or underscore, can contain letters, digits, and underscores, are case-sensitive, and cannot be reserved keywords.