PLC Session 4

Session 4: Data Types

A. 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.

B. Primitive Data Type

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:
Floating Point.
Double Floating Point.
Wide Character.

C. Character String Types

Values are sequences of characters.
Typical operations:
Assignment and copying.
Comparison (=, >, etc.).
Substring reference.
Pattern matching.

D. 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
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};
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.
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.

E. Array Types

An array is a homogeneous aggregate of data elements in which an individual element is identified by its position in the aggregate, relative to the first element.
Subscript Binding and Array Categories
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.
Array Initialization:
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″];

Heterogeneous Arrays
A heterogeneous array is one in which the elements need not be of the same type
Array Initialization:
int list [] = {1, 3, 5, 7};
char *names [] = {″Mike″, ″Fred″, ″Mary Lou″};
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.

Example: #include <iostream> // std::cout

#include <cstddef> // std::size_t

#include <valarray> // std::valarray, std::slice

int main ()

{

std::valarray<int> foo (12);

for (int i=0; i<12; ++i) foo[i]=i;

std::valarray<int> bar = foo[std::slice(2,3,4)];

std::cout << “slice(2,3,4):”;

for (std::size_t n=0; n<bar.size(); n++)

std::cout << ‘ ‘ << bar[n];

std::cout << ‘\n’;

return 0;

}

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
A compile-time descriptor for a multidimensional array

Compile-Time Descriptors

F. Associative Arrays

An associative array is an unordered collection of data elements that are indexed by an equal number of values called
Example:

#include <iostream>

#include <map>

#include <string>

#include <algorithm>

#include <cstdlib>

#include <iomanip>

#include <vector>

using namespace std;

int main()

{

map<string, map<string, vector<string> > > data;

data[“plants”][“trees”].push_back(“Elm”);

data[“plants”][“trees”].push_back(“Pine”);

data[“plants”][“trees”].push_back(“Ash”);

data[“plants”][“trees”].push_back(“Birch”);

data[“plants”][“trees”].push_back(“Ceder”);

vector<string>::iterator it;

for(

it = data[“plants”][“trees”].begin();

it != data[“plants”][“trees”].end();

it++)

{

cout << “Kind of tree: ” << *it << endl;

}

return 0;

}

G. Record Types

A record is a possibly heterogeneous aggregate of data elements in which the individual elements are identified by names
Example:

Student (idnumber, name, status, credits, gpa)

enum StatusType {FULLTIME, PARTTIME, NONDEGREE, GRAD} struct StudentType { int idNumber; char name[30]; StatusType status; float credits; float gpa;}; StudentType student1;StudentType student2;§ Implementation Of Record TypeØ Offset address relative to the beginning of the records is associated with each field

H. Tuple Types

§ A tuple is a data type that is similar to a record, except that the elements are not named

I. List Types

§ Lists in LISP and Scheme are delimited by parentheses and use no commas (A B C D) and (A (B C) D)§ Data and code have the same form As data, (A B C) is literally what it is As code, (A B C) is the function A applied to the parameters B and C§ The interpreter needs to know which a list is, so if it is data, we quote it with an apostrophe. ′(A B C) is data§ List Operations in SchemeØ CAR returns the first element of its list parameter (CAR ′(A B C)) returns AØ CDR returns the remainder of its list parameter after the first element has been removed(CDR ′(A B C)) returns (B C)Ø CONS puts its first parameter into its second parameter, a list, to make a new list(CONS ′A (B C)) returns (A B C)Ø LIST returns a new list of its parameters(LIST ′A ′B ′(C D)) returns (A B (C D))

J. Union Types

§ A union is a type whose variables are allowed to store different type values at different times during execution§ Implementation of Unions:type Node (Tag : Boolean) is record case Tag is when True => Count : Integer; when False => Sum : Float; end case; end record;

K. 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).§ Pointer Operations:Ø Two fundamental operations: assignment and dereferencing.Ø Assignment is used to set a pointer variable’s value to some useful address.Ø Dereferencing yields the value stored at the location represented by the pointer’s valueü Dereferencing can be explicit or implicitü C++ uses an explicit operation via * j = *ptr sets j to the value located at ptr The assignment operation j = *ptr § Problems with PointersØ Dangling pointers (dangerous)ü A pointer points to a heap-dynamic variable that has been deallocatedØ Lost heap-dynamic variableü An allocated heap-dynamic variable that is no longer accessible to the user program (often called garbage)§ Pointers in C and C++Ø Extremely flexible but must be used with careØ Pointers can point at any variable regardless of when or where it was allocatedØ Used for dynamic storage management and addressingØ Pointer arithmetic is possibleØ Explicit dereferencing and address-of operatorsØ 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 TypesØ C++ includes a special kind of pointer type called a reference type that is used primarily for formal parametersØ Java extends C++’s reference variables and allows them to replace pointers entirelyØ C# includes both the references of Java and the pointers of C++§ Representations of PointersØ Large computers use single valuesØ Intel microprocessors use segment and offset§ Dangling Pointer ProblemØ Tombstone: extra heap cell that is a pointer to the heap-dynamic variableü The actual pointer variable points only at tombstonesü When heap-dynamic variable de-allocated, tombstone remains but set to nilü Costly in time and spaceØ Locks-and-keys: Pointer values are represented as (key, address) pairsü Heap-dynamic variables are represented as variable plus cell for integer lock valueü When heap-dynamic variable allocated, lock value is created and placed in lock cell and key cell of pointer§ Heap ManagementØ A very complex run-time processØ Single-size cells vs. variable-size cellsØ Two approaches to reclaim garbageü Reference counters (eager approach): reclamation is gradualü Mark-sweep (lazy approach): reclamation occurs when the list of variable space becomes empty§ Reference CounterØ Reference counters: maintain a counter in every cell that store the number of pointers currently pointing at the cellü Disadvantages: space required, execution time required, complications for cells connected circularlyü Advantage: it is intrinsically incremental, so significant delays in the application execution are avoided

L. 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 typeØ 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§ A programming language is strongly typed if type errors are always detected§ Advantage of strong typing: allows the detection of the misuses of variables that result in type errors

M. Strong Typing

§ Language examples:Ø C and C++ are not: parameter type checking can be avoided; unions are not type checkedØ Ada is, almost (UNCHECKED CONVERSION is loophole)(Java and C# are similar to Ada)§ 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

Source(s) :

http://jcsites.juniata.edu/faculty/rhodes/cs2/ch05a.htm

http://www.dreamincode.net/forums/topic/67804-c-multidimensional-associative-arrays/

http://www.cplusplus.com/reference/valarray/slice/

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