Transcript Ch06Part1
Chapter 6
Data Types
ISBN 0-321-33025-0
Chapter 6 Topics
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Introduction
Primitive Data Types
Character String Types
User-Defined Ordinal Types
Array Types
Associative Arrays
Record Types
Union Types
Pointer and Reference Types
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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?
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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
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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.
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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);
•
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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://msdn2.microsoft.com/enus/library/aa691147(VS.71).aspx
– http://technet.microsoft.com/en-us/library/ms187912.aspx
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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.
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Primitive Data Types: Boolean
• Simplest of all
• Range of values: two elements, one for
“true” and one for “false”
• C89: all operands with nonzero values are
considered true, and zero is considered
false when used in conditionals.
• C99 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
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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 JavaScript also support Unicode
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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?
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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
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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
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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).
– StringBuffer 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
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Character String Types Operations
• Pattern matching
– Fundamental character string operation
– Often called regular expressions
– E.g., /[A-Za-z][A-Za-z\d]+ matches string that
begin with a letter, followed by one or more
letters or digits
– Perl, JavaScript 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();
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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, JavaScript
– 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
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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.
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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
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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.
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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
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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?
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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.
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Design
• In C++, we could have
enum colors {red, blue, green, yellow, black};
colors myColor = blue, yourColor = red;
The enumeration values are coerced to int when
they are put in integer context. E.g. myColor++
would assign green to myColor.
• In Java, all enumeration types are implicitly
subclasses of the predefined class Enum. They can
have instance data fields, constructors and
methods.
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Design
– Java Example
Enumeration days;
Vector dayNames = new Vector();
dayNames.add("Monday");
…
dayNames.add("Friday");
days = dayNames.elements();
while (days.hasMoreElements())
System.out.println(days.nextElement());
• 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.
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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
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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
integer types
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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;
Day1: Days;
Day2: Weekday;
Day2 := Day1; //legal if Day1 it not sat or sun
Compatible with its parent type.
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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
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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
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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.
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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?
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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
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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
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