Chapter 6 Data Types ISBN 0 321 33025
Chapter 6 Data Types ISBN 0 -321 -33025 -0
Chapter 6 Topics • • • Introduction Primitive Data Types Character String Types User-Defined Ordinal Types Array Types Associative Arrays Record Types Union Types Pointer and Reference Types Copyright © 2006 Addison-Wesley. All rights reserved. 6 -2
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? Copyright © 2006 Addison-Wesley. All rights reserved. 6 -3
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, e. g. integer types • Others require a little non-hardware support for their implementation Copyright © 2006 Addison-Wesley. All rights reserved. 6 -4
Primitive Data Types: Integer • Almost always an exact reflection of the hardware so the mapping is trivial • Java’s signed integer sizes: byte, short, int, long Type Size(bits) Def. value Min. value Max. value Byte 8 0 -128 +127 Short 16 0 -32768 +32767 Int 32 0 -2147483648 +2147483647 Long 64 0 -9223372036854775808 +9223372036854775807 • A signed integer value is represented by a string of bits with usually the leftmost bit representing the sign. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -5
Primitive Data Types: Floating Point • Model real numbers, but only as approximations, e. g. Pi • Languages for scientific use support at least two floating-point types, e. g. , float(usually stored in 4 bytes of memory) and double(8 bytes of memory, provides larger fractional part); • Copyright © 2006 Addison-Wesley. All rights reserved. 6 -6
Primitive Data Types: Decimal • For business applications (money) – Essential to COBOL – C# offers a decimal data type • Store a fixed number of decimal digits, with the decimal point at a fixed position in the value, unlike float or double data types which store approximation many times • Stored like character string, using binary codes for the decimal digits • More information about decimal data type: – http: //msdn 2. microsoft. com/enus/library/aa 691147(VS. 71). aspx – http: //technet. microsoft. com/en-us/library/ms 187912. aspx Copyright © 2006 Addison-Wesley. All rights reserved. 6 -7
Primitive Data Types: Decimal • Advantage: accuracy • Disadvantages: – limited range (no exponents are allowed) – wastes memory, stored one digit per byte, sometimes two digits per byte. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -8
Primitive Data Types: Boolean • Simplest of all • Range of values: two elements, one for “true” and one for “false” • C 89: all operands with nonzero values are considered true, and zero is considered false when used in conditionals. • C 99 and C++ have a Boolean type, but they also allow numeric expressions to be used as if they were Boolean. (Not the case in Java and C#) • Often used to represent switches or flags • More readable than using integers Copyright © 2006 Addison-Wesley. All rights reserved. 6 -9
Primitive Data Types: Character • Stored as numeric codings • Most commonly used coding: ASCII – Uses 0 to 127 to code 128 different characters • An alternative, 16 -bit coding: Unicode – – Includes characters from most natural languages First 128 characters are identical to ASCII Originally used in Java C# and Java. Script also support Unicode Copyright © 2006 Addison-Wesley. All rights reserved. 6 -10
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? Copyright © 2006 Addison-Wesley. All rights reserved. 6 -11
Character String Type in Certain Languages • C and C++ – Not primitive – Use char arrays and a library of functions that provide operations – Character strings are terminated with a special character, null, which is represented with zero. – The library operations simply carry out their operations until the null character being operated on. Library functions that produce strings often supply the null character. – Common library functions: • strcpy, strcat, strcmp, strlen Copyright © 2006 Addison-Wesley. All rights reserved. 6 -12
Character String Type in Certain Languages • C and C++ – String manipulation functions of the C standard library is unsafe, as they don’t guard against overflowing the destination. E. g. : strcpy(src, dest); If the length of dest is 20 and the length of src is 50, strcpy will write over the 30 bytes that follow dest, which is often in the run-time stack. - C++ programmer should use the string class from the standard library Copyright © 2006 Addison-Wesley. All rights reserved. 6 -13
Character String Types Operations • Java: – Strings are supported as a primitive type by the String class [string a=“abcd” is the same as String a = new String(“abcd”)], whose values are constant strings (each time when the value is changed, a new String object is created). – String. Buffer class: values of a string is changeable. • C#: – Similar to Java • C++: – C-style strings and strings in its standard class library which is similar to that of Java Copyright © 2006 Addison-Wesley. All rights reserved. 6 -14
Character String Types Operations • Pattern matching – Fundamental character string operation – Often called regular expressions – E. g. , /[A-Za-z][A-Za-zd]+ matches string that begin with a letter, followed by one or more letters or digits – Perl, Java. Script and PHP have built-in pattern matching operations – Java, C++ and C# have pattern matching capabilities in the class libraries, e. g. Java: Pattern p = Pattern. compile("a*b"); Matcher m = p. matcher("aaaaab"); boolean b = m. matches(); Copyright © 2006 Addison-Wesley. All rights reserved. 6 -15
Character String Length Options • Static Length: COBOL, Java’s String class – Length is set when the string is created, fixed length • Limited Dynamic Length: C and C++ – Length is set to have a maximum, varying length • Dynamic Length(no maximum): Perl, Java. Script – No length limit, varying length. Required overhead of dynamic storage allocation and deallocation but provides maximum flexibility • Ada supports all three string length options: – String: static – Bounded_String: limited dynamic – Unbounded_String: dynamic Copyright © 2006 Addison-Wesley. All rights reserved. 6 -16
Character String Type Evaluation • Aid to writability • As a primitive type with static length, they are inexpensive to provide • Simple pattern matching and catenation are essential, should be included • Dynamic length is most flexible, but overhead of implementation must be weighed. Often included only in languages that are interpreted. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -17
Character String Implementation • Static length: compile-time descriptor type of string address of the first character • Limited dynamic length: may need a runtime descriptor for length (but not in C and C++) type of string address of the first character Copyright © 2006 Addison-Wesley. All rights reserved. 6 -18
Character String Implementation • Dynamic length: need run-time descriptor; allocation/de-allocation is the biggest implementation problem. Two approches: – String is stored in a linked list, so when a string grows, the newly required cells can come from anywhere in the heap. Drawback – extra storage occupied by the links in the list representation and the necessary complexity of string operations. But allocation and deallocation process is simple. – Store complete strings in adjacent storage cells. When the storage for the adjacent cell is not available(when the string grows), a new area of memory is found to store the complete new string. Faster string operation and requires less storage, but slower allocation and deallocation process. This approach is typically used. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -19
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 Copyright © 2006 Addison-Wesley. All rights reserved. 6 -20
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}; • The enumeration constants are typically implicitly assigned the integer values, 0, 1, …, but can be explicitly assigned any integer literal in the type’s definition • 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? Copyright © 2006 Addison-Wesley. All rights reserved. 6 -21
Design • In languages that do not have enumeration types, programmers usually simulate them with integer values. E. g. Fortran 77, use 0 to represent blue and 1 to represent red: INTEGER RED, BLUE DATA RED, BLUE/0, 1/ Problem: there is no type checking when they are used. It would be legal to add two together. Or they can be assigned any integer value thus destroying the relationship with the colors. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -22
Design • In C++, we could have enum colors {red, blue, green, yellow, black}; colors my. Color = blue, your. Color = red; The enumeration values are coerced to int when they are put in integer context. E. g. my. Color++ would assign green to my. Color. • In Java, all enumeration types are implicitly subclasses of the predefined class Enum. They can have instance data fields, constructors and methods. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -23
Design – Java Example Enumeration days; Vector day. Names = new Vector(); day. Names. add("Monday"); … day. Names. add("Friday"); days = day. Names. elements(); while (days. has. More. Elements()) System. out. println(days. next. Element()); • C# enumeration types are like those of C++ except that they are never coerced to integer. Operations are restricted to those that make sense. The range of values is restricted to that of the particular enumeration type. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -24
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, e. g. if the colors type has 10 enumeration constants and uses 0. . 9 as its internal values, no number greater than 9 can be assigned to a colors type variable. – Ada, C#, and Java 5. 0 provide better support for enumeration than C++ because enumeration type variables in these languages are not Copyright © 2006 Addison-Wesley. All rights reserved. 6 -25 coerced into integer types
Evaluation of Enumerated Type • C treats enumeration variables like integer variables; it does not provide either of the two advantages. • C++ is better. Numeric values can be assigned to enumeration type variables only if they are cast to the type of the assigned variable. Numeric values are checked to determine in they are in the range of the internal values. However if the user uses a wide range of explicitly assigned values, this checking is not effective. E. g. – enum colors {red = 1, blue = 100, green = 100000} A value assigned to a variable of colors type will only be checked to determine whether it is in the range of 1. . 100000. • Java 5. 0, C# and Ada are better, as variables are never coerced to integer types Copyright © 2006 Addison-Wesley. All rights reserved. 6 -26
Subrange Types • An ordered contiguous subsequence of an ordinal type • Not a new type, but a restricted existing 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; Day 1: Days; Day 2: Weekday; Day 2 : = Day 1; //legal if Day 1 it not sat or sun Compatible with its parent type. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -27
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 Copyright © 2006 Addison-Wesley. All rights reserved. 6 -28
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 Copyright © 2006 Addison-Wesley. All rights reserved. 6 -29
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. Copyright © 2006 Addison-Wesley. All rights reserved. 6 -30
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? • Are ragged or rectangular multidimensioned arrays allowed, or both? • Can arrays be initialized when they have their storage allocated? • Are any kind of slices allowed? Copyright © 2006 Addison-Wesley. All rights reserved. 6 -31
Array Indexing • Indexing (or subscripting) is a mapping from indices to elements array_name (index_value_list) • Index Syntax an element – 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 Copyright © 2006 Addison-Wesley. All rights reserved. 6 -32
Arrays Index (Subscript) Types • FORTRAN, C: integer only • Pascal: any ordinal type (integer, Boolean, char, enumeration) • Ada: integer or enumeration (includes Boolean and char) • Java: integer types only • C, C++, Perl, and Fortran do not specify range checking • Java, ML, C# specify range checking Copyright © 2006 Addison-Wesley. All rights reserved. 6 -33
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