2. Variables

Properties of Variables

• Name: string of characters with some constraints (e.g. max length)
• Type: indicates range of values that are allowed as well as operations that are available.
• Address: memory address to which the variable is associated (also known as l-value)
• Value: contents of memory associated with variable (also known as r-value)
• Lifetime: time during which the variable is allocated a specific memory location (between allocation / deallocation)

Lifetime Classes

There are 4 classes of variables according to their lifetime:

• Static Lifetime

  Static: bound to a memory location before program execution begins and until it ends. This is efficient as there is no allocation / deallocation at run time.

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• Stack Dynamic Lifetime

  Stack dynamic: allocated when declaration of variable is processed at run time.

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• Explicit Heap-Dynamic Lifetime

  Explicit heap-dynamic: nameless memory cells which are allocated / deallocated by program instructions, using pointers:

  // C++ memory explicit heap-dynamic allocation
  int * v = new int; // allocation
  delete v; // deallocation

  In Java, objects are explicit heap-dynamic and can be accessed through reference variables but there is no way to destroy them explicitly, implicit garbage collection is used.

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• Implicit Heap-Dynamic Lifetime

  Implicit heap-dynamic variables: bound to storage only when they are assigned values. For example, strings / arrays in JavaScript, this is flexible but can be difficult to detect errors.

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Scope

Variable Scope

Scope: the range of instructions in which the variable is visible

• Local variables are those declared in the block of the program that is executed.
• Global variables are those that are visible but not local.

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Static Scope

Static Scope is when scope is determined before the execution of a program (at compile time). When a variable is referenced, the compiler looks for its declaration in the same block, then ascending parents, etc. If no declaration is found, we throw an error.

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Dynamic Scope

Dynamic Scope is when scope is determined during the run time of a program. It depends on the call sequence of sub routines. This prevents us from doing static type-checking and may make programs very difficult to understand. Some shell scripting languages use dynamic scoping.

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Variable assignment generally uses the syntax variable-name = expression, but this may vary between languages. The value of the expression is stored in the variable. In typed languages, the expression and variable must have compatible types.