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C Keywords and Identifiers

Sep 17, 2025

Overview

This lecture explains identifiers and keywords in C, discusses their differences, and outlines important rules for each, as well as their roles in C programming.

Keywords in C

  • Keywords are reserved words in C with predefined meanings (e.g., int, float, break, continue, for, if, while).
  • There are 32 keywords in ANSI C, standardized in 1989.
  • Keywords cannot be used as identifiers (e.g., variable or function names).
  • All keywords must be written in lowercase letters; C is case-sensitive.
  • The meaning of keywords cannot be changed during program execution.
  • Keywords are basic building blocks for writing instructions in C programs.

Identifiers in C

  • Identifiers are names given by users to variables, functions, arrays, structures, or unions.
  • Identifiers help uniquely identify elements in code.
  • Identifiers can consist of letters, digits, and underscores, but must begin with a letter or an underscore.
  • No special characters (e.g., $, %, @, -, space, dot) are allowed in identifiers.
  • Identifiers are case-sensitive: sum, Sum, and SUM are different identifiers.
  • Keywords cannot be used as identifiers.
  • According to ANSI C, the first 31 characters of an identifier are significant.
  • Both uppercase and lowercase letters are allowed in identifiers.

Differences Between Keywords and Identifiers

  • Keywords are predefined and reserved; identifiers are user-defined names.
  • Keywords must be lowercase; identifiers can use both cases.
  • Keywords cannot be used as variable or function names; identifiers are used for that purpose.
  • The meaning of keywords is fixed; identifiers' meanings are assigned by the user.

Key Terms & Definitions

  • Keyword — a reserved, predefined word in C with a specific meaning (e.g., int, for, float).
  • Identifier — a user-defined name for a variable, function, array, etc.

Action Items / Next Steps

  • Review the rules for forming valid identifiers in C.
  • Prepare for the next lesson on data types in C.
  • (Optional) Comment examples of valid/invalid identifiers as discussed in the lecture.

Full transcript