CSC204 Chapter 5: Variables and Scope

Introduction to Variables

• Variables have several attributes: type, scope, lifetime, type checking, initialization, and type compatibility.

Names

• Length of names varies across languages:
  • FORTRAN I: maximum 6 characters
  • FORTRAN 95: maximum 31 characters
  • COBOL: maximum 30 characters
  • Ada: no limit, all characters are significant
  • C++ and C#: no limit, but implementors often impose one
  • PHP: names must begin with a $
  • Perl: $, @, or % at the beginning specifies the type
• Case sensitivity:
  • Disadvantage: readability (names that look alike are different)
  • C-based languages are case sensitive
  • Other languages are not case sensitive
• Special words:
  • Keywords: special words in certain contexts, e.g., in Fortran
  • Reserved words: special words that cannot be used as user-defined names
  • Potential problem: many reserved words can cause collisions, e.g., COBOL has 300 reserved words

The Concept of Binding

• Binding: association between an attribute and an entity
• Binding time: when the association takes place
• Possible binding times:
  • Language design time
  • Language implementation time
  • Compile time
  • Load time
  • Runtime
  • Link time

Static and Dynamic Binding

• Static binding: binding occurs before runtime and remains unchanged
  • Advantages: reliability, efficiency
  • Disadvantages: less flexibility
• Dynamic binding: binding occurs during execution or can change during execution
  • Languages that use dynamic binding: Python, Ruby, JavaScript, and PHP
  • Advantages: flexibility
  • Disadvantages:
    • Less reliable
    • Cost at execution time

Storage Bindings and Lifetime

• Allocation: getting a memory cell from a pool
• Deallocation: putting a memory cell back into the pool
• Lifetime: time during which a variable is bound to a memory cell
• Types of storage binding based on lifetimes:
  • Static variables
  • Stack-dynamic variables
  • Explicit heap-dynamic variables
  • Implicit heap-dynamic variables

Categories of Variables by Lifetime

• Static variables:
  • Bound to memory cells before execution
  • Advantages: efficiency, history-sensitive subprogram support
  • Disadvantage: lack of flexibility
• Stack-dynamic variables:
  • Storage bindings created during execution
  • Advantages: allows recursion, conserves storage
  • Disadvantages:
    • Overhead of allocation and deallocation
    • Subprograms cannot be history sensitive
    • Inefficient references
• Explicit heap-dynamic variables:
  • Allocated and deallocated by explicit directives
  • Advantages: provides dynamic storage management
  • Disadvantage: inefficient and unreliable
• Implicit heap-dynamic variables:
  • Allocation and deallocation caused by assignment statements
  • Advantages: flexibility
  • Disadvantages:
    • Inefficient
    • Loss of error detection

Scope

• Scope: range of statements over which a variable is visible
• Local variables: declared in a program unit
• Nonlocal variables: visible within a block, but not declared there
• Global variables: special category of nonlocal variables
• Scope rules: determine how references to names are associated with variables

Static Scope

• Static scoping: binding names to nonlocal variables
• The scope of a variable is determined by the block structure

Blocks

• Blocks create static scopes
• The reference to a variable inside a block is hidden from the same variable outside the block

Declaration Order

• C99, C++, Java, and C# allow variable declarations to appear anywhere in a program unit
• Declarations may create scopes that are not associated with subprograms

This quiz covers the basics of variables, including properties such as type, scope, lifetime, type checking, initialization, and type compatibility. Learn about the concept of binding, named constants, and more.