CSC204 Chapter 5: Variables and Scope

Variables Characterized

• A variable is characterized by a collection of properties or attributes:
  • Type
  • Scope
  • Lifetime
  • Type checking
  • Initialization
  • Type compatibility

Names

• Length of names varies by language:
  • FORTRAN I: maximum 6 characters
  • FORTRAN 95: maximum 31 characters
  • COBOL: maximum 30 characters
  • Ada: no limit, and all are significant
  • C++: no limit, but implementors often impose one
  • C# and Java: no limit, and all are significant
  • PHP: must begin with a $
  • Perl: $, @, or % at the beginning means it specifies its type
• Case sensitivity:
  • Disadvantage: readability (names that look alike are different)
  • Names in C-based languages are case sensitive
  • Names in other languages are not
  • Worse in C++, Java, and C# because predefined names are mixed case

Special Words

• Keywords:
  • A keyword is a word that is special only in certain contexts
  • Example: Real in Fortran (data type followed by a name)
• Reserved words:
  • A reserved word is a special word that cannot be used as a user-defined name
  • Potential problem: If there are too many, many collisions occur (e.g., COBOL has 300 reserved words!)

Variables

• A variable is an abstraction of a memory cell that can be characterized as follows:
  • Name
  • Address
  • Value
  • Type
  • Lifetime
  • Scope

Variables Attributes

• Name:
  • Most variables have names as previously discussed
• Address:
  • The memory address with which it is associated
  • A variable may have different addresses at different times during execution
  • A variable may have different addresses at different places in a program
  • If two variable names can be used to access the same memory location, they are called aliases
• Type:
  • Determines the range of values of variables and the set of operations that are defined for values of that type
  • In the case of floating point, type also determines the precision
• Value:
  • The contents of the location with which the variable is associated
  • The l-value of a variable is its address
  • The r-value of a variable is its value
• Abstract memory cell:
  • It is the associated size required by the variable
  • Example: Floating-point value may occupy four physical bytes while it is thought to occupy one abstract memory cell

The Concept of Binding

• A binding is an association between an attribute and an entity:
  • Between a variable and its type or value
  • Between an operation and a symbol
• Binding time:
  • The time at which a binding takes place
• Possible binding times:
  • Language design time
  • Language implementation time
  • Compile time
  • Load time
  • Runtime
  • Link time

Static and Dynamic Binding

• A binding is static if it first occurs before run time and remains unchanged throughout program execution
• A binding is dynamic if it first occurs during execution or can change during execution of the program
• Languages that use dynamic binding:
  • Python
  • Ruby
  • JavaScript
  • PHP

Categories of Variables by Lifetime

• Explicit heap-dynamic variables:
  • Allocated and deallocated by explicit directives
  • Specified by the programmer which take effect during execution
  • Referenced only through pointers or references
  • Example: dynamic objects in C++ (via new and delete), all objects in Java
• Implicit heap-dynamic variables:
  • Allocation and de-allocation caused by assignment statements
  • Example: all variables in APL; all strings and arrays in Perl, JavaScript, and PHP

Scope

• The scope of a variable is the range of statements over which it is visible
• A variable is local in a program if it is declared there
• The nonlocal variables of a program unit or block are those that are visible within the block, but are not declared there
• Global variables:
  • A special category of nonlocal variables
  • The scope rules of a language determine how references to names are associated with variables

Global Scope

• C, C++, PHP, JavaScript, and Python allow variable definitions to appear outside the functions
• Declarations specify types and other attributes, but do not cause allocation of storage
• Definitions specify attributes and cause storage allocation
• C and C++ have both declarations and definitions of global data

Evaluation of Static Scoping

• Advantages: provides a method of nonlocal access
• Disadvantages:
  • Subprograms end up being nested at the same level
  • Use more global variables than required

Dynamic Scope

• Dynamic scoping:
  • Is based on the calling sequence of subprograms
  • The reference of a variable in a subprogram is the variable declared in its caller subprogram
• Example: JavaScript code snippet demonstrating dynamic scoping

Evaluation of Dynamic Scoping

• Advantages: parameters passed from one subprogram to another are implicitly visible in the called subprogram
• Disadvantages:
  • Difficult to read and inability to type checking references
  • Less reliable
  • Execute slower than equivalent static-scoped programs