Programming concepts: variable declaration

The idea

A variable is a named store whose value can change while a program runs. Declaring one tells the translator the name you will use and, in a language that requires it, the data type it will hold, so memory can be reserved and type checking applied. For example, in a strongly typed language you might write a declaration giving a name and an integer type before assigning a starting value; in Python you simply assign a first value and the name is created. The declaration itself does not have to give a value, but the variable must be given one before it is read. Choosing a meaningful name and the correct type prevents errors such as storing text where a number is expected.

What earns the marks

• A variable is a named location in memory whose stored value can be changed during execution.
• A declaration introduces the identifier and, where the language requires it, the data type, so the translator can reserve storage and check types.
• A declaration may be separate from assignment; the variable must hold a value before it is used.
• Choosing a meaningful identifier and an appropriate data type reduces errors such as type mismatches.
• In some languages (for example Python) a first assignment both creates and initialises the variable.

Common mistakes and how to put them right

• Thinking a variable cannot change value.

  Put it right by stressing that a variable's value may be reassigned during the program.

• Confusing declaration with assignment.

  Put it right: Explain that declaring introduces the name and type, while assignment stores a value.

• Assuming every language needs an explicit type in the declaration.

  Put it right: Note that dynamically typed languages infer the type from the assigned value.

Exam tips

• Read the whole option before choosing; eliminate options that describe a constant rather than a variable.
• Link the answer to the idea of a changeable value and, where relevant, a declared data type.
• If two options look similar, decide which one correctly separates declaration from assignment.