CSC204 Chapter 5: Variables and Scope
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Questions and Answers

What are the characteristics of variables in programming languages?

  • Type, scope, lifetime and initialization
  • Scope, lifetime, and type compatibility
  • Type, scope, lifetime, type checking, initialization, and type compatibility (correct)
  • Type, scope, and lifetime

    • What is the maximum length of a name in FORTRAN I?

  • 31
  • 6 (correct)
  • 5
  • 30

    • Which of the following languages does not impose a limit on the length of a name?

  • Ada
  • C
  • C++
  • All of the above (correct)

    • What is the disadvantage of case sensitivity in names?

    <p>Readability</p> Signup and view all the answers

    Which of the following languages has mixed case predefined names?

    <p>All of the above</p> Signup and view all the answers

    What is a keyword in programming languages?

    <p>A word that is special only in certain contexts</p> Signup and view all the answers

    What is a reserved word in programming languages?

    <p>A special word that cannot be used as a user-defined name</p> Signup and view all the answers

    What is a potential problem with reserved words?

    <p>Too many collisions occur</p> Signup and view all the answers

    What is a variable characterized by?

    <p>Name, Address, Value, Type, Lifetime, and Scope</p> Signup and view all the answers

    What is the memory address associated with a variable?

    <p>Address</p> Signup and view all the answers

    What is the purpose of a variable's type?

    <p>Both A and B</p> Signup and view all the answers

    What is the r-value of a variable?

    <p>The value of the variable</p> Signup and view all the answers

    What is an alias in programming?

    <p>Two variable names that access the same memory location</p> Signup and view all the answers

    What is the abstract memory cell of a variable?

    <p>The size required by the variable in memory</p> Signup and view all the answers

    Why are aliases harmful to readability?

    <p>Because readers must remember all of them</p> Signup and view all the answers

    What is created via pointers, reference variables, or the union of C and C++?

    <p>Aliases</p> Signup and view all the answers

    What is a binding in the context of programming languages?

    <p>A binding is an association between an attribute and an entity</p> Signup and view all the answers

    What happens during language design time in terms of binding?

    <p>Binding operator symbols to operations</p> Signup and view all the answers

    Which of the following languages provides implicit declarations?

    <p>Perl</p> Signup and view all the answers

    What is the advantage of implicit declarations?

    <p>Writability</p> Signup and view all the answers

    What type of binding occurs before run time and remains unchanged throughout program execution?

    <p>Static binding</p> Signup and view all the answers

    Which of the following languages use dynamic binding?

    <p>Python, Ruby, JavaScript, and PHP</p> Signup and view all the answers

    What is the disadvantage of implicit declarations?

    <p>Reliability</p> Signup and view all the answers

    What occurs during load time in terms of binding?

    <p>Binding a C or C++ static variable to a memory cell</p> Signup and view all the answers

    What is the main characteristic of explicit heap-dynamic variables?

    <p>They are allocated and deallocated by explicit directives specified by the programmer.</p> Signup and view all the answers

    What is the advantage of implicit heap-dynamic variables?

    <p>They allow for flexible and generic code.</p> Signup and view all the answers

    What is the scope of a variable?

    <p>The range of statements over which it is visible.</p> Signup and view all the answers

    What is a local variable?

    <p>A variable that is declared within a program unit or block.</p> Signup and view all the answers

    What are the disadvantages of implicit heap-dynamic variables?

    <p>They are inefficient and lose error detection.</p> Signup and view all the answers

    What determines how references to names are associated with variables?

    <p>The scope rules of a language.</p> Signup and view all the answers

    In C, C++, PHP, JavaScript, and Python, where can variable definitions appear?

    <p>Anywhere in the program</p> Signup and view all the answers

    What is the purpose of declarations in programming languages?

    <p>To specify types and attributes, but not allocate storage</p> Signup and view all the answers

    What is an advantage of static scoping?

    <p>It provides a method of nonlocal access</p> Signup and view all the answers

    What is a disadvantage of dynamic scoping?

    <p>It is difficult to read and inability to type checking references</p> Signup and view all the answers

    In dynamic scoping, what is the scope of a variable?

    <p>Based on the calling sequence of subprograms</p> Signup and view all the answers

    What is an alternative to static scoping?

    <p>Encapsulation</p> Signup and view all the answers

    What is true about definitions in programming languages?

    <p>They specify attributes and allocate storage</p> Signup and view all the answers

    What is a characteristic of dynamic scoping?

    <p>Variables are scoped based on the calling sequence of subprograms</p> Signup and view all the answers

    Study Notes

    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

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    Description

    This quiz covers the concepts of variables, scope, and lifetime in programming languages, including properties, attributes, and type checking.

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