Identifier names include names for variables, constants and subroutine…

The idea

An identifier is the name a programmer gives to an item in a program, and the specification groups three kinds: variables, constants and subroutines. A variable identifier names a stored value that can change while the program runs, such as score or userName. A constant identifier names a value fixed when the program is written and not changed afterwards, such as PI or MAX_ATTEMPTS; by convention constants are often written in capitals to signal that they must not be reassigned. A subroutine identifier names a named block of code that performs a task, such as calculateArea or displayMenu, and is used when the subroutine is called. Recognising which kind of item a name refers to helps a programmer read code correctly: assigning to a constant is an error, and calling a subroutine uses a different syntax from reading a variable.

What earns the marks

• A variable identifier names a value that may change during execution, for example score.
• A constant identifier names a value fixed at design time that should not change, for example MAX_ATTEMPTS.
• A subroutine identifier names a named block of code that carries out a task and is invoked when called, for example calculateArea.
• Classifying an identifier correctly depends on how the item is used in the program, not merely on its spelling.
• Suitable names should be descriptive and obey the language's identifier rules, such as no spaces and a letter at the start.

Common mistakes and how to put them right

• Treating every identifier as a variable.

  Put it right: Is that constants and subroutine names are also identifiers and are distinguished by their role.

• Assuming a name written in capitals is automatically a constant in every language.

  Put it right: Is that capitals are a convention signalling intent, and the actual role depends on how the item is declared and used.

• Confusing a subroutine name with a variable holding its result.

  Put it right: Is that the subroutine name identifies the block of code, while any returned value may be stored in a separate variable.

Exam tips

• Read how each identifier is used in the code before classifying it as a variable, constant or subroutine.
• When suggesting a name, match the role: a fixed value suits a constant-style name, an action suits a verb-like subroutine name.
• Check suggested names against identifier rules as well as describing the purpose clearly.