CSE

Principles of Programming Languages

UNIT - III

Data Types

  • Topics
    * Introduction
    * Primitive Data Types
    * Character String Types
    * User-Defined Ordinal Types
    * Array Types
    * Associative Arrays
    * Record Types
    * Union Types
    * Pointer and Reference Types
    * Names
    * Variables
    * The Concept of Binding
    * Type Checking
    * Strong Typing
    * Type Compatibility
    * Scope
    * Scope and Lifetime
    * Referencing Environments
    * Named Constants
    Introduction
    * A data type defines a collection of data objects and a set of predefined operations on those objects
    * A descriptor is the collection of the attributes of a variable
    * An object represents an instance of a user-defined (abstract data) type
    * One design issue for all data types: What operations are defined and how are they specified?
    Primitive Data Types
    * Almost all programming languages provide a set of primitive data types
    * Primitive data types: Those not defined in terms of other data types
    * Some primitive data types are merely reflections of the hardware
    * Others require only a little non-hardware support for their implementation
    Primitive Data Types: Integer
    * Almost always an exact reflection of the hardware so the mapping is trivial
    * There may be as many as eight different integer types in a language
    * Java‘s signed integer sizes: byte, short, int, long
    Primitive Data Types: Floating Point
    * Model real numbers, but only as approximations
    * Languages for scientific use support at least two floating-point types (e.g., float and double; sometimes more
    * Usually exactly like the hardware, but not always
    * IEEE Floating-Point
    Standard 754
    Primitive Data Types: Complex
    * Some languages support a complex type, e.g., Fortran and Python
    * Each value consists of two floats, the real part and the imaginary part
    * Literal form (in Python):
    (7 + 3j), where 7 is the real part and 3 is the imaginary part
    Primitive Data Types: Decimal
    * For business applications (money)
    > Essential to COBOL
    > C# offers a decimal data type
    * Store a fixed number of decimal digits, in coded form (BCD)
    * Advantage: accuracy
    * Disadvantages: limited range, wastes memory
    Primitive Data Types: Boolean
    * Simplest of all
    * Range of values: two elements, one for ―true‖ and one for ―false‖
    * Could be implemented as bits, but often as bytes
    > Advantage: readability
    Primitive Data Types: Character
    * Stored as numeric codings
    * Most commonly used coding: ASCII
    * An alternative, 16-bit coding: Unicode
    > Includes characters from most natural languages
    > Originally used in Java
    > C# and JavaScript also support Unicode
    Character String Types
    * Values are sequences of characters
    * Design issues:
    > Is it a primitive type or just a special kind of array?
    > Should the length of strings be static or dynamic?
    Character String Types Operations
    * Typical operations:
    > Assignment and copying
    > Comparison (=, >, etc.)
    > Catenation
    > Substring reference
    > Pattern matching
    Character String Type in Certain Languages
    * C and C++
    > Not primitive
    > Use char arrays and a library of functions that provide operations
    * SNOBOL4 (a string manipulation language)
    > Primitive
    > Many operations, including elaborate pattern matching
    * Fortran and Python
    > Primitive type with assignment and several operations
    * Java
    > Primitive via the String class
    * Perl, JavaScript, Ruby, and PHP
    > Provide built-in pattern matching, using regular expressions
    Character String Length Options
    * Static: COBOL, Java‘s String class
    * Limited Dynamic Length: C and C++
    > In these languages, a special character is used to indicate the end of a string‘s characters, rather than maintaining the length
    * Dynamic (no maximum): SNOBOL4, Perl, JavaScript
    * Ada supports all three string length options
    Character String Type Evaluation
    * Aid to writability
    * As a primitive type with static length, they are inexpensive to provide--why not have them?
    * Dynamic length is nice, but is it worth the expense?
    Character String Implementation
    * Static length: compile-time descriptor
    * Limited dynamic length: may need a run-time descriptor for length (but not in C and C++)
    * Dynamic length: need run-time descriptor; allocation/de-allocation is the biggest implementation problem
    Compile- and Run-Time Descriptors

    User-Defined Ordinal Types
    * An ordinal type is one in which the range of possible values can be easily associated with the set of positive integers
    * Examples of primitive ordinal types in Java
    > integer
    > char
    > boolean
    Enumeration Types
    * All possible values, which are named constants, are provided in the definition
    * C# example
    enum days {mon, tue, wed, thu, fri, sat, sun};
    * Design issues
    > Is an enumeration constant allowed to appear in more than one type definition, and if so, how is the type of an occurrence of that constant checked?
    > Are enumeration values coerced to integer?
    > Any other type coerced to an enumeration type?
    Evaluation of Enumerated Type
    * Aid to readability, e.g., no need to code a color as a number
    * Aid to reliability, e.g., compiler can check:
    > Operations (don‘t allow colors to be added)
    > No enumeration variable can be assigned a value outside its defined range
    > Ada, C#, and Java 5.0 provide better support for enumeration than C++ because enumeration type variables in these languages are not coerced into integer types
    Subrange Types
    * An ordered contiguous subsequence of an ordinal type
    > Example: 12..18 is a subrange of integer type
    * Ada‘s design
    type Days is (mon, tue, wed, thu, fri, sat, sun);
    subtype Weekdays is Days range mon..fri;
    subtype Index is Integer range 1..100;
    Day1: Days;
    Day2: Weekday;
    Day2 := Day1;
    Subrange Evaluation
    * Aid to readability
    > Make it clear to the readers that variables of subrange can store only certain range of values
    * Reliability
    > Assigning a value to a subrange variable that is outside the specified range is detected as an error
    Implementation of User-Defined Ordinal Types
    * Enumeration types are implemented as integers
    * Subrange types are implemented like the parent types with code inserted (by the compiler) to restrict assignments to subrange variables
    Array Types
    * An array is an aggregate of homogeneous data elements in which an individual element is identified by its position in the aggregate, relative to the first element.
    Array Design Issues
    * What types are legal for subscripts?
    * Are subscripting expressions in element references range checked?
    * When are subscript ranges bound?
    * When does allocation take place?
    * What is the maximum number of subscripts?
    * Can array objects be initialized?
    * Are any kind of slices supported?
    Array Indexing
    * Indexing (or subscripting) is a mapping from indices to elements
    array_name (index_value_list) an element
    * Index Syntax
    > FORTRAN, PL/I, Ada use parentheses
    * Ada explicitly uses parentheses to show uniformity between array references and function calls because both are mappings
    > Most other languages use brackets
    Arrays Index (Subscript) Types
    * FORTRAN, C: integer only
    * Ada: integer or enumeration (includes Boolean and char)
    * Java: integer types only
    * Index range checking
    > C, C++, Perl, and Fortran do not specify range checking
    > Java, ML, C# specify range checking
    > In Ada, the default is to require range checking, but it can be turned off
    Subscript Binding and Array Categories
    * Static: subscript ranges are statically bound and storage allocation is static (before run-time)
    > Advantage: efficiency (no dynamic allocation)
    * Fixed stack-dynamic: subscript ranges are statically bound, but the allocation is done at declaration time
    > Advantage: space efficiency
    * Stack-dynamic: subscript ranges are dynamically bound and the storage allocation is dynamic (done at run-time)
    > Advantage: flexibility (the size of an array need not be known until the array is to be used)
    * Fixed heap-dynamic: similar to fixed stack-dynamic: storage binding is dynamic but fixed after allocation (i.e., binding is done when requested and storage is allocated from heap, not stack)
    Subscript Binding and Array Categories (continued)
    * Heap-dynamic: binding of subscript ranges and storage allocation is dynamic and can change any number of times
    > Advantage: flexibility (arrays can grow or shrink during program execution)
    * C and C++ arrays that include static modifier are static
    * C and C++ arrays without static modifier are fixed stack-dynamic
    * C and C++ provide fixed heap-dynamic arrays
    * C# includes a second array class ArrayList that provides fixed heap-dynamic
    * Perl, JavaScript, Python, and Ruby support heap-dynamic arrays
    Array Initialization
    * Some language allow initialization at the time of storage allocation
    > C, C++, Java, C# example
    int list [] = {4, 5, 7, 83}
    > Character strings in C and C++
    char name [] = ―freddie;
    > Arrays of strings in C and C++
    char *names [] = {―Bob, ―Jake, ―Joe];
    > Java initialization of String objects
    String[] names = {―Bob, ―Jake, ―Joe};
    Heterogeneous Arrays
    * A heterogeneous array is one in which the elements need not be of the same type
    * Supported by Perl, Python, JavaScript, and Ruby
    Arrays Operations
    * APL provides the most powerful array processing operations for vectors and matrixes as well as unary operators (for example, to reverse column elements)
    * Ada allows array assignment but also catenation
    * Python‘s array assignments, but they are only reference changes. Python also supports array catenation and element membership operations
    * Ruby also provides array catenation
    * Fortran provides elemental operations because they are between pairs of array elements
    > For example, + operator between two arrays results in an array of the sums of the element pairs of the two arrays
    Rectangular and Jagged Arrays
    * A rectangular array is a multi-dimensioned array in which all of the rows have the same number of elements and all columns have the same number of elements
    * A jagged matrix has rows with varying number of elements
    > Possible when multi-dimensioned arrays actually appear as arrays of arrays
    * C, C++, and Java support jagged arrays
    * Fortran, Ada, and C# support rectangular arrays (C# also supports jagged arrays)
    Slices
    * A slice is some substructure of an array; nothing more than a referencing mechanism
    * Slices are only useful in languages that have array operations
    Slice Examples
    * Fortran 95
    Integer, Dimension (10) :: Vector
    Integer, Dimension (3, 3) :: Mat
    Integer, Dimension (3, 3) :: Cube
    Vector (3:6) is a four element array
    Slices Examples in Fortran 95

    Implementation of Arrays
    * Access function maps subscript expressions to an address in the array
    * Access function for single-dimensioned arrays:
    address(list[k]) = address (list[lower_bound])
    + ((k-lower_bound) * element_size)
    Accessing Multi-dimensioned Arrays
    * Two common ways:
    > Row major order (by rows) – used in most languages
    > Column major order (by columns) – used in Fortran
    Locating an Element in a Multi-dimensioned Array

    Compile-Time Descriptors

    Associative Arrays
    * An associative array is an unordered collection of data elements that are indexed by an equal number of values called keys
    > User-defined keys must be stored
    * Design issues:
    > What is the form of references to elements?
    > Is the size static or dynamic?
    Associative Arrays in Perl
    * Names begin with %; literals are delimited by parentheses
    %hi_temps = ("Mon" => 77, "Tue" => 79, ―Wed => 65, …);
    * Subscripting is done using braces and keys
    $hi_temps{"Wed"} = 83;
    > Elements can be removed with delete
    delete $hi_temps{"Tue"};
    Record Types
    * A record is a possibly heterogeneous aggregate of data elements in which the individual elements are identified by names
    * Design issues:
    > What is the syntactic form of references to the field?
    > Are elliptical references allowed
    Definition of Records in COBOL
    * COBOL uses level numbers to show nested records; others use recursive definition
    01 EMP-REC.
    02 EMP-NAME.
    05 FIRST PIC X(20).
    05 MID PIC X(10).
    05 LAST PIC X(20).
    02 HOURLY-RATE PIC 99V99.
    Definition of Records in Ada
    * Record structures are indicated in an orthogonal way
    type Emp_Rec_Type is record
    First: String (1..20);
    Mid: String (1..10);
    Last: String (1..20);
    Hourly_Rate: Float;
    end record;
    Emp_Rec: Emp_Rec_Type;
    References to Records
    * Record field references
    1. COBOL
    field_name OF record_name_1 OF ... OF record_name_n
    2. Others (dot notation)
    record_name_1.record_name_2. ... record_name_n.field_name
    * Fully qualified references must include all record names
    * Elliptical references allow leaving out record names as long as the reference is unambiguous, for example in COBOL
    FIRST, FIRST OF EMP-NAME, and FIRST of EMP-REC are elliptical references to the employee‘s first name
    Operations on Records
    * Assignment is very common if the types are identical
    * Ada allows record comparison
    * Ada records can be initialized with aggregate literals
    * COBOL provides MOVE CORRESPONDING
    > Copies a field of the source record to the corresponding field in the target record
    Evaluation and Comparison to Arrays
    * Records are used when collection of data values is heterogeneous
    * Access to array elements is much slower than access to record fields, because subscripts are dynamic (field names are static)
    * Dynamic subscripts could be used with record field access, but it would disallow type checking and it would be much slower
    Implementation of Record Type

    Unions Types
    * A union is a type whose variables are allowed to store different type values at different times during execution
    * Design issues
    > Should type checking be required?
    > Should unions be embedded in records?
    Discriminated vs. Free Unions
    * Fortran, C, and C++ provide union constructs in which there is no language support for type checking; the union in these languages is called free union
    * Type checking of unions require that each union include a type indicator called a discriminant
    > Supported by Ada
    Ada Union Types
    type Shape is (Circle, Triangle, Rectangle);
    type Colors is (Red, Green, Blue);
    type Figure (Form: Shape) is record
    Filled: Boolean;
    Color: Colors;
    case Form is
    when Circle => Diameter: Float;
    when Triangle =>
    Leftside, Rightside: Integer;
    Angle: Float;
    when Rectangle => Side1, Side2: Integer;
    end case;
    end record;
    Ada Union Type Illustrated

    Evaluation of Unions
    * Free unions are unsafe
    > Do not allow type checking
    * Java and C# do not support unions
    > Reflective of growing concerns for safety in programming language
    * Ada‘s descriminated unions are safe
    Pointer and Reference Types
    * A pointer type variable has a range of values that consists of memory addresses and a special value, nil
    * Provide the power of indirect addressing
    * Provide a way to manage dynamic memory
    * A pointer can be used to access a location in the area where storage is dynamically created (usually called a heap)
    Design Issues of Pointers
    * What are the scope of and lifetime of a pointer variable?
    * What is the lifetime of a heap-dynamic variable?
    * Are pointers restricted as to the type of value to which they can point?
    * Are pointers used for dynamic storage management, indirect addressing, or both?
    * Should the language support pointer types, reference types, or both?
    Pointer Operations
    * Two fundamental operations: assignment and dereferencing
    * Assignment is used to set a pointer variable‘s value to some useful address
    * Dereferencing yields the value stored at the location represented by the pointer‘s value
    > Dereferencing can be explicit or implicit
    > C++ uses an explicit operation via *
    j = *ptr
    sets j to the value located at ptr
    Pointer Assignment Illustrated

    The assignment operation j = *ptr
    Problems with Pointers
    * Dangling pointers (dangerous)
    > A pointer points to a heap-dynamic variable that has been deallocated
    * Lost heap-dynamic variable
    > An allocated heap-dynamic variable that is no longer accessible to the user
    program (often called garbage)
    * Pointer p1 is set to point to a newly created heap-dynamic variable
    * Pointer p1 is later set to point to another newly created heap-dynamic variable
    * The process of losing heap-dynamic variables is called memory leakage
    Pointers in Ada
    * Some dangling pointers are disallowed because dynamic objects can be automatically deallocated at the end of pointer's type scope
    * The lost heap-dynamic variable problem is not eliminated by Ada (possible with UNCHECKED_DEALLOCATION)
    Pointers in C and C++
    * Extremely flexible but must be used with care
    * Pointers can point at any variable regardless of when or where it was allocated
    * Used for dynamic storage management and addressing
    * Pointer arithmetic is possible
    * Explicit dereferencing and address-of operators
    * Domain type need not be fixed (void *)
    void * can point to any type and can be type
    checked (cannot be de-referenced)
    Pointer Arithmetic in C and C++
    float stuff[100];
    float *p;
    p = stuff;
    *(p+5) is equivalent to stuff[5] and p[5]
    *(p+i) is equivalent to stuff[i] and p[i]
    Reference Types
    * C++ includes a special kind of pointer type called a reference type that is used primarily for formal parameters
    > Advantages of both pass-by-reference and pass-by-value
    * Java extends C++‘s reference variables and allows them to replace pointers entirely
    > References are references to objects, rather than being addresses
    * C# includes both the references of Java and the pointers of C++
    Evaluation of Pointers
    * Dangling pointers and dangling objects are problems as is heap management
    * Pointers are like goto's--they widen the range of cells that can be accessed by a variable
    * Pointers or references are necessary for dynamic data structures--so we can't design a language without them
    Representations of Pointers
    * Large computers use single values
    * Intel microprocessors use segment and offset
    Dangling Pointer Problem
    * Tombstone: extra heap cell that is a pointer to the heap-dynamic variable
    > The actual pointer variable points only at tombstones
    > When heap-dynamic variable de-allocated, tombstone remains but set to nil
    > Costly in time and space
    > Locks-and-keys: Pointer values are represented as (key, address) pairs
    > Heap-dynamic variables are represented as variable plus cell for integer lock value
    > When heap-dynamic variable allocated, lock value is created and placed in lock cell and key cell of pointer
    Heap Management
    * A very complex run-time process
    * Single-size cells vs. variable-size cells
    * Two approaches to reclaim garbage
    > Reference counters (eager approach): reclamation is gradual
    > Mark-sweep (lazy approach): reclamation occurs when the list of variable space becomes empty
    Reference Counter
    * Reference counters: maintain a counter in every cell that store the number of pointers currently pointing at the cell
    > Disadvantages: space required, execution time required, complications for cells connected circularly
    > Advantage: it is intrinsically incremental, so significant delays in the application execution are avoided
    Mark-Sweep
    * The run-time system allocates storage cells as requested and disconnects pointers from cells as necessary; mark-sweep then begins
    > Every heap cell has an extra bit used by collection algorithm
    > All cells initially set to garbage
    > All pointers traced into heap, and reachable cells marked as not garbage
    > All garbage cells returned to list of available cells
    > Disadvantages: in its original form, it was done too infrequently. When done, it caused significant delays in application execution. Contemporary mark-sweep algorithms avoid this by doing it more often—called incremental mark-sweep
    Marking Algorithm

    Variable-Size Cells
    * All the difficulties of single-size cells plus more
    * Required by most programming languages
    * If mark-sweep is used, additional problems occur
    > The initial setting of the indicators of all cells in the heap is difficult
    > The marking process in nontrivial
    > Maintaining the list of available space is another source of overhead
    Names
    * Design issues for names:
    * Maximum length?
    * Are connector characters allowed?
    * Are names case sensitive?
    * Are special words reserved words or keywords?
    * Length
    * If too short, they cannot be connotative
    * Language examples:
    * FORTRAN I: maximum 6
    * COBOL: maximum 30
    * FORTRAN 90 and ANSI C: maximum 31
    * Ada and Java: no limit, and all are significant
    * C++: no limit, but implementors often impose one;
    * Connectors;
    * Pascal, Modula-2, and FORTRAN 77 don't allow
    * Others do
    * Case sensitivity
    * Disadvantage: readability (names that look alike are different)
    * Worse in C++ and Java because predefined names are mixed case
    (e.g. IndexOutOfBoundsException)
    * C, C++, and Java names are case sensitive
    * The names in other languages are not
    * Special words
    * An aid to readability; used to delimit or separate statement clauses
    * Def: A keyword is a word that is special only in certain contexts i.e. in Fortran: Real * VarName (Real is data type followed with a name, therefore Real is a keyword)
    * Real = 3.4 (Real is a variable)
    * Disadvantage: poor readability
    * Def: A reserved word is a special word that cannot be used as a user-defined name
    Variables
    * A variable is an abstraction of a memory cell
    * Variables can be characterized as a sextuple of attributes: (name, address, value, type, lifetime, and scope)
    * Name - not all variables have them (anonymous)
    * Address - the memory address with which it is associated (also called l-value)
    * 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
    * Aliases are harmful to readability (program readers must remember all of them)
    * How aliases can be created:
    * Pointers, reference variables, C and C++ unions, (and through parameters - discussed in Chapter 9)
    * Some of the original justifications for aliases are no longer valid; e.g. memory reuse in FORTRAN
    * Replace them with dynamic allocation
    * 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
    * Abstract memory cell - the physical cell or collection of cells associated with a variable
    The Concept of Binding
    * The l-value of a variable is its address The r-value of a variable is its value
    * Def: A binding is an association, such as between an attribute and an entity, or between an operation and a symbol
    * Def: Binding time is the time at which a binding takes place.
    * Possible binding times:
    * Language design time--e.g., bind operator symbols to operations
    * Language implementation time--e.g., bind floating point type to a representation
    * Compile time--e.g., bind a variable to a type in C or Java
    * Load time--e.g., bind a FORTRAN 77 variable to a memory cell (or a C static variable)
    * Runtime--e.g., bind a nonstatic local variable to a memory cell
    * Def: A binding is static if it first occurs before run time and remains unchanged throughout program execution.
    * Def: A binding is dynamic if it first occurs during execution or can change during execution of the program.
    * Type Bindings
    * How is a type specified?
    * When does the binding take place?
    * If static, the type may be specified by either an explicit or an implicit declaration
    * Def: An explicit declaration is a program statement used for declaring the types of variables
    * Def: An implicit declaration is a default mechanism for specifying types of variables (the first appearance of the variable in the program)
    * FORTRAN, PL/I, BASIC, and Perl provide implicit declarations
    * Advantage: writability
    * Disadvantage: reliability (less trouble with Perl)
    * Dynamic Type Binding (JavaScript and PHP)
    * Specified through an assignment statement e.g., JavaScript
    list = [2, 4.33, 6, 8];
    list = 17.3;

    * Advantage: flexibility (generic program units)
    * Disadvantages:
    * High cost (dynamic type checking and interpretation)
    * Type error detection by the compiler is difficult
    * Type Inferencing (ML, Miranda, and Haskell)
    * Rather than by assignment statement, types are determined from the context of the reference
    * Storage Bindings & Lifetime
    * Allocation - getting a cell from some pool of available cells
    * Deallocation - putting a cell back into the pool
    * Def: The lifetime of a variable is the time during which it is bound to a particular memory cell
    * Categories of variables by lifetimes
    * Static--bound to memory cells before execution begins and remains bound to the same memory cell throughout execution.
    e.g. all FORTRAN 77 variables, C static variables
    * Advantages: efficiency (direct addressing), history-sensitive subprogram support
    * Disadvantage: lack of flexibility (no recursion)
    * Categories of variables by lifetimes
    * Stack-dynamic--Storage bindings are created for variables when their declaration statements are elaborated.
    * If scalar, all attributes except address are statically bound e.g. local variables in C subprograms and Java methods
    * Advantage: allows recursion; conserves storage
    * Disadvantages:
    * Overhead of allocation and deallocation
    * Subprograms cannot be history sensitive
    * Inefficient references (indirect addressing)
    * Categories of variables by lifetimes
    * Explicit heap-dynamic--Allocated and deallocated by explicit directives, specified by the programmer, which take effect during execution
    * Referenced only through pointers or references e.g. dynamic objects in C++ (via new and delete) all objects in Java
    * Advantage: provides for dynamic storage management
    * Disadvantage: inefficient and unreliable
    * Categories of variables by lifetimes
    * Implicit heap-dynamic--Allocation and deallocation caused by assignment statements e.g. all variables in APL; all strings and arrays in Perl and JavaScript
    * Advantage: flexibility
    * Disadvantages:
    * Inefficient, because all attributes are dynamic
    * Loss of error detection
    Type Checking
    * Generalize the concept of operands and operators to include subprograms and assignments
    * Type checking is the activity of ensuring that the operands of an operator are of compatible types
    * A compatible type is one that is either legal for the operator, or is allowed under language rules to be implicitly converted, by compiler- generated code, to a legal type. This automatic conversion is called a coercion.
    * A type error is the application of an operator to an operand of an inappropriate
    type
    * If all type bindings are static, nearly all type checking can be static
    * If type bindings are dynamic, type checking must be dynamic
    * Def: A programming language is strongly typed if type errors are always detected
    Strong Typing
    * Advantage of strong typing: allows the detection of the misuses of variables that result in type errors
    * Language examples:
    * FORTRAN 77 is not: parameters, EQUIVALENCE
    * Pascal is not: variant records
    * C and C++ are not: parameter type checking can be avoided; unions are not type checked
    * Ada is, almost (UNCHECKED CONVERSION is loophole)
    (Java is similar)
    * Coercion rules strongly affect strong typing--they can weaken it considerably (C++ versus Ada)
    * Although Java has just half the assignment coercions of C++, its strong typing is still far less effective than that of Ada
    Type Compatibility
    * Our concern is primarily for structured types
    * Def: Name type compatibility means the two variables have compatible types if they are in either the same declaration or in declarations that use the same type name
    * Easy to implement but highly restrictive:
    * Subranges of integer types are not compatible with integer types
    * Formal parameters must be the same type as their corresponding actual parameters (Pascal)
    * Structure type compatibility means that two variables have compatible types if their types have identical structures
    * More flexible, but harder to implement
    * Consider the problem of two structured types:
    * Are two record types compatible if they are structurally the same but use different field names?
    * Are two array types compatible if they are the same except that the subscripts are different? (e.g. [1..10] and [0..9])
    * Are two enumeration types compatible if their components are spelled differently?
    * With structural type compatibility, you cannot differentiate between types of the same structure (e.g. different units of speed, both float)
    * Language examples:
    * Pascal: usually structure, but in some cases name is used (formal parameters)
    * C: structure, except for records
    * Ada: restricted form of name
    * Derived types allow types with the same structure to be different
    * Anonymous types are all unique, even in:
    A, B : array (1..10) of INTEGER:
    * Scope
    * The scope of a variable is the range of statements over which it is visible
    * The nonlocal variables of a program unit are those that are visible but not declared there
    * The scope rules of a language determine how references to names are associated with variables
    * Static scope
    * Based on program text
    * To connect a name reference to a variable, you (or the compiler) must find the declaration
    * Search process: search declarations, first locally, then in increasingly larger enclosing scopes, until one is found for the given name
    * Enclosing static scopes (to a specific scope) are called its static ancestors; the nearest static ancestor is called a static parent
    * Variables can be hidden from a unit by having a "closer" variable with the same name
    * C++ and Ada allow access to these "hidden" variables
    * In Ada: unit.name
    * In C++: class_name::name
    * Blocks
    * A method of creating static scopes inside program units--from ALGOL 60
    * Examples:
    C and C++: for (...)
    {
    int index;

    ...
    }
    Ada: declare LCL : FLOAT;
    begin
    ...
    end

    * Evaluation of Static Scoping
    * Consider the example:
    Assume MAIN calls A and B
    A calls C and D
    B calls A and E
    Static Scope Example
    Static Scope Example
    Static Scope
    * Suppose the spec is changed so that D must now access some data in B
    * Solutions:
    * Put D in B (but then C can no longer call it and D cannot access A's variables)
    * Move the data from B that D needs to MAIN (but then all procedures can access them)
    * Same problem for procedure access
    * Overall: static scoping often encourages many globals
    * Dynamic Scope
    * Based on calling sequences of program units, not their textual layout (temporal versus spatial)
    * References to variables are connected to declarations by searching back through the chain of subprogram calls that forced execution to this point
    Scope Example
    MAIN
    - declaration of x
    SUB1
    - declaration of x -
    ...
    call SUB2
    ...
    SUB2
    ...
    - reference to x -
    ...
    ...
    call SUB1

    Scope Example
    * Static scoping
    * Reference to x is to MAIN's x
    * Dynamic scoping
    * Reference to x is to SUB1's x
    * Evaluation of Dynamic Scoping:
    * Advantage: convenience
    * Disadvantage: poor readability
    Scope and Lifetime
    * Scope and lifetime are sometimes closely related, but are different concepts
    * Consider a static variable in a C or C++ function
    Referencing Environments
    * Def: The referencing environment of a statement is the collection of all names that are visible in the statement
    * In a static-scoped language, it is the local variables plus all of the visible variables in all of the enclosing scopes
    * A subprogram is active if its execution has begun but has not yet terminated
    * In a dynamic-scoped language, the referencing environment is the local variables plus all visible variables in all active subprograms
    Named Constants
    * Def: A named constant is a variable that is bound to a value only when it is bound to storage
    * Advantages: readability and modifiability
    * Used to parameterize programs
    * The binding of values to named constants can be either static (called manifest constants) or dynamic
    * Languages:
    * Pascal: literals only
    * FORTRAN 90: constant-valued expressions
    * Ada, C++, and Java: expressions of any kind
    * Variable Initialization
    * Def: The binding of a variable to a value at the time it is bound to storage is called initialization
    * Initialization is often done on the declaration statement
    e.g., Java
    int sum = 0;
    Summary
    * The data types of a language are a large part of what determines that language‘s style and usefulness
    * The primitive data types of most imperative languages include numeric, character, and Boolean types
    * The user-defined enumeration and subrange types are convenient and add to the readability and reliability of programs
    * Arrays and records are included in most languages
    * Pointers are used for addressing flexibility and to control dynamic storage management
    * Case sensitivity and the relationship of names to special words represent design
    issues of names
    * Variables are characterized by the sextuples: name, address, value, type, lifetime, scope
    * Binding is the association of attributes with program entities
    * Scalar variables are categorized as: static, stack dynamic, explicit heap dynamic, implicit heap dynamic
    * Strong typing means detecting all type errors
    SUBJECTIVE
    1) Define data type. Why every programming language supports different data types? (What happens if no data types are supported)
    2) What are the design issues of character string types?
    3) Evaluate the two approaches for supporting dynamic allocation and de allocation for dynamic length strings.
    4) What is user-defined data type? Why are they supported?
    5) Assume a programming language is used to extensively manipulate arrays. What are the different array operations which you permit as a language designer? Justify your choices.
    6) Define union. What is the difference between record and union? Explain how union is supported by different programming languages.
    7) Define record. How do you access different fields of a record? What are the operations that can be performed on the record?
    8) What are the advantages and disadvantages of implicit declaration.
    9) Explain in detail dynamic type binding.
    10) Dynamic binding is closely related to implicit heap-dynamic variables. Explain this relationship
    11) While doing type conversion a narrower type is converted to a wider type. What is the advantage of it. What happens if wider type is converted into a narrower type.
    12) Write short notes on coercion, type error, type checking and strong typing.
    13) Define name type compatibility and structure type compatibility. What are its merits and demerits?

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