Datatype

Chapter 6: Data Types
Principles of Programming Languages

Contents
•Primitive Data Types
•Character String Types
•User-Defined Ordinal Types
•Array Types
•Associative Arrays
•Record Types
•Union Types
•Pointer and Reference Types
•Type Checking

Introduction
•A data type defines
–a collection of data objects
–a set of predefined operations
•Abstract Data Type: Interface (visible) are separated from the representation and operation (hidden)
•Uses of type system:
–Error detection
–Program modularization assistant
–Documentation

The Next Part
•Primitive Data Types
•Character String Types
•User-Defined Ordinal Types
•Array Types
•Associative Arrays
•Record Types
•Union Types
•Pointer and Reference Types
•Type Checking

Primitive Data Types
•Data types that are not defined in terms of other data types
•Some primitive data types are merely reflections of the hardware
•Others require little non-hardware support

Primitive Data Types: Integer
•Languages may support several sizes of integer
–Java’s signed integer sizes: byte, short, int, long
•Some languages include unsigned integers
•Supported directly by hardware: a string of bits
•To represent negative numbers: twos complement

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)
•Precision and range
•IEEE Floating-Point Standard 754

IEEE 754
Sign bit
Exponent
Fraction
23 bits
8 bits
Single-pricision Sign bit
Exponent
Fraction
52 bits
11 bits
Double-pricision

Primitive Data Types: Decimal
•For business applications (money)
–Essential to COBOL
–C# offers a decimal data type
•Store a fixed number of decimal digits
•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
–Comparison (=, >, etc.)
–Catenation
–Substring reference
–Pattern matching
•Regular Expression

•C and C++
–Not primitive
–Use char arrays and a library of functions that provide operations
•Java
–Primitive via the String class
–Immutable
Character String in Some Languages

Character String Length Options
•Static: Python, Java’s String class
•Limited Dynamic Length: C
–In C, a special character is used to indicate the end of a string’s characters, rather than maintaining the length
•Dynamic (no maximum): Perl, JavaScript, standard C++ library
•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-time
descriptor for
static strings
Run-time descriptor
for limited dynamic
strings
Descriptor

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};
days myDay = Mon, yourDay = Tue;
•Design issues:
–Is an enumeration constant allowed to appear in more than one type definition?
–Are enumeration values coerced to integer?
–Are any other types coerced to an enumeration type?

Evaluation of Enumerated Type
•Readability
–no need to code a color as a number
•Reliability
–Ada, C# and Java 5.0
•operations (don’t allow colors to be added)
•No enumeration variable can be assigned a value outside its defined range
•Better support for enumeration than C++: enumeration type variables are not coerced into integer types

•Enumeration types are implemented as integers
Implementation of User-Defined Ordinal Types

Array Types
•Collection of homogeneous data elements
•Each element is identified by its position relative to the first element
•Homogeneous: data elements are of same type
•Referenced using subscript expression

Array Indexing
•Indexing (or subscripting) is a mapping from indices to elements
array_name (index_value_list)  an element
•Index Syntax
–Fortran, Ada use parentheses
–Most other languages use brackets

Arrays Index (Subscript) Types
•What type are legal for subscripts?
•Pascal, Ada: any ordinal type (integer, boolean, char, enumeration)
•Others: subrange of integers
•Are subscripting expressions range checked?
•Most contemporary languages do not specify range checking but Java, ML, C#
•Unusual case: Perl

•Static: subscript ranges are statically bound and storage allocation is static
–Advantage: efficiency (no dynamic allocation)
–Disadvantage: storage is bound all the time
•Fixed stack-dynamic: subscript ranges are statically bound, but the allocation is done at declaration time
–Advantage: space efficiency
–Disadvantage: dynamic allocation
Subscript Binding & Array Categories

•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)

•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
•Ada arrays can be stack-dynamic
Get(List_Len);
declare
List : array (1..List_Len) of Integer;
begin
...
end;

•C and C++ provide fixed heap-dynamic arrays
–malloc and free, new and delete
•All arrays in Java are fixed heap-dynamic
•C# includes a array class ArrayList that provides heap-dynamic
ArrayList intList = new ArrayList();
intList.Add(nextOne);
•Perl and JavaScript 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"};

Arrays Operations
•Assignment, catenation, comparison for equality, and slices
•C-based languages do not provide any array operations
•Java, C++ and C# use methods to manipulate

Rectangular and Jagged Arrays
•C, C++, Java, C#: jagged arrays
myArray[3][7]
•Fortran, Ada, C#: rectangular array
myArray[3,7] rectangular
jagged

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
•E.g. Python
vector = [2, 4, 6, 8, 10, 12, 14, 16]
mat = [[1, 2, 3],[4, 5, 6],[7, 8, 9]]
vector[3:6], mat[1], mat[0][0:2]

Implementation of Arrays
•Access function maps subscript expressions to an address in the array
•Single-dimensioned: list of adjacent memory cells
•Access function for single-dimensioned arrays:
address(list[k]) = address (list[lower_bound])
+ ((k-lower_bound) * element_size)

Accessing Multi-dimensioned Arrays a[ 0, 0 ]
a[ 0, 1 ]
a[ 0, 2 ]
a[ 1, 0 ]
a[ 1, 1 ]
a[ 1, 2 ] a[ 0, 0 ]
a[ 1, 0 ]
a[ 0, 1 ]
a[ 1, 1 ]
a[ 0, 2 ]
a[ 1, 2 ]
Row major order – used in most languages
Column major order – used in Fortran

• General format Location (a[I,j]) = address of a [row_lb,col_lb] + (((I - row_lb) * n) + (j - col_lb)) * element_size
Accessing Multi-dimensioned Arrays

Compile-Time Descriptors
Single-dimensioned array
Multi-dimensional array

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
•Similar to Map in Scala
•Design issues: What is the form of references to elements

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:
–heterogeneous aggregate of data elements
–individual elements are identified by names
•Popular in most languages, OO languages use objects as records
•Design issues:
–What is the syntactic form of references to the field?
–Are elliptical references allowed

Definition of Records in Ada
•Record structures are indicated in an orthogonal way
type Emp_Name_Type is record
First: String (1..20);
Mid: String (1..10);
Last: String (1..20);
end record;
type Emp_Record_Type is record
Emp_Name: Emp_Name_Type;
Hourly_Rate: Float;
end record;
Emp_Rec: Emp_Rec_Type;

References to Records
•Most language use dot notation
Emp_Rec.Emp_Name.Mid
•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 fields which have the same name

•Straight forward and safe design
•Arrays are used when all data values have the same type and/or are processed in the same way
•Records are opposite
•Arrays: dynamic subscripting
•Records: static subscripting
Evaluation

Offset address relative to the beginning of the records is associated with each field
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
A discriminated union of three shape variables

Evaluation of Unions
•Potentially unsafe construct
–Do not allow type checking
•Java and C# do not support unions
–Reflective of growing concerns for safety in programming language

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

Pointer Operations
•Pointer points to a record in C/C++
–Explicit: (*p).name
–Implicit: p -> name
•Management of heap use explicit allocation
–C: subprogram malloc
–C++: new and delete operators

Problems with Pointers
•Dangling pointers (dangerous)
–A pointer points to a heap-dynamic variable that has been de-allocated
•Lost heap-dynamic variable
–An allocated heap-dynamic variable that is no longer accessible to the user program (often called garbage)

Pointers in Ada
•access types
•Some dangling pointers are disallowed because dynamic objects can be automatically de-allocated at the end of pointer's type scope
•The lost heap-dynamic variable problem is not eliminated by Ada

Pointers in C and C++
int *ptr;
int count, init;
...
ptr = &init;
count = *ptr;
•Extremely flexible but must be used with care
•Pointers can point at any variable regardless of when it was allocated
•Used for dynamic storage management and addressing

Pointers in C and C++
•Pointer arithmetic is possible
int list [10]; int *ptr; ptr = list;
*(ptr + 1)
*(ptr + index)
*ptr[index]
•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)

•C++ includes a special kind of pointer type called a reference type that is used primarily for formal parameters
•Java extends C++’s reference variables and allows them to replace pointers entirely
–References refer to call instances
•C# includes both the references of Java and the pointers of C++
Reference Types

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
•Essential in some kinds of programming applications, e.g. device drivers
•Using references provide some of the flexibility and capabilities of pointers, without the hazards

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

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

Type Checking (continued)
•If all type bindings are static, nearly all type checking can be static
•If type bindings are dynamic, type checking must be dynamic
•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: EQUIVALENCE
–C and C++ are not: unions are not type checked
–Java, C#: strongly typed

Strong Typing (continued)
•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

Name Type Equivalence
•Name type equivalence 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 Equivalence
•Structure type equivalence means that two variables have equivalent types if their types have identical structures
•More flexible, but harder to implement

Structure Type Equivalence
•Consider the problem of two structured types:
–Are two record types equivalent if they are structurally the same but use different field names?
–Are two array types equivalent if they are the same except that the subscripts are different?
(e.g. [1..10] and [0..9])
–Are two enumeration types equivalent if their components are spelled differently?
–With structural type equivalence, you cannot differentiate between types of the same structure (e.g. different units of speed, both float)

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
•Strong typing means detecting all type errors

Datatype

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Datatype