Variable

→ Thing that changes. See Constant and Change for cool idea.

CS241E: “Abstraction of a storage location that can hold a value”.

Kinds of Variables

• Global variables
  • Defined outside of any enclosing function/class/struct
  • Come into existence when declared and die at the end of the program
• Local variables
  • Come into existence when the function is called
• Member / instance variables

Variable Lookup Order

Variable Instances

This is only important at low-level development, considered in the context of CS241E.

Consider the function factorial(x) = x if (x < 2) else x * factorial(x-1).

When we run factorial(x), there is a single variable x. However, when we actually compute the value, say factorial(3), we actually get three variable instances during computation: x=3, x=2, x=1.

Depending on the type of variable, we have different implementations:

Variable Kind	Extent*	Implementation
Global Variable	Entire Execution of program	Fixed memory address (label)
Local Variable	Execution of the procedure (LIFO)	[[notes/Stack
Field of an object/record	From creation time to last use	[[notes/Heap

Extent (of a variable) = the time interval during program execution when the variable can be accessed