Transcript Chapter 6 updated

Chapter 6
Data Types
Contents
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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6.1 Introduction
 Data type: A data type defines a collection of data values and a set of
predefined operations on those values
An object represents an instance of a user-defined (abstract data) type
 Design issues for all data types
 What operations are defined and how are they specified.
 It is convenient, both logically and concretely, to think of variables in
terms of descriptors.
 A descriptor is the collection of the attributes of a variable
 A descriptor is used for type checking, allocation and, deallocation
 Static attributes need only be available at compile-time; dynamic
attributes need to be available at run-time.
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6.2
Primitive data types
 Primitive data types are those that are not defined in terms
of other data types
 Most primitive types are abstractions for underlying hardware data
types.
 Common primitive types:
 Numeric types
Early PLs had only numeric primitive types, and still play a central
role among the collections of types supported by contemporary
languages.
1. Integers
 Almost always an exact reflection of the hardware, so the
mapping is trivial.
 For example, C, Ada, java .. allows these: short integer, integer
and long integer.
 An integer is represented by a string of bits, with the leftmost
representing the sign bit.
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6.2
2.
Primitive data types (cont.)
Floating point numbers
 Model real numbers but only as approximations
 languages for scientific use support at least two floating-point types; sometimes more.
 usually exactly like the hardware, but not always; some languages allow accuracy specs
in code e.g. (Ada)
IEEE (The Institute of Electrical and Electronics Engineers) floating-point formats: (a) Single
precision, (b) Double precision
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Primitive data types (contd.)
6.2
3.
Decimal
 for business applications (money)
 store a fixed number of decimal digits (coded)
 advantage: accuracy
 disadvantages: limited range, wastes memory
 Boolean types
 The range of values has only two elements TRUE or FALSE
 Booleans types are often used to represent switches or flags in programs
 advantage: readability
 Character types
 stored as numeric codings (usually ASCII but Unicode has appeared as an
alternative)
 A new 16-bit character set named Unicode had been developed as an
alternative. Java is the first to use Unicode
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6.3
Character String Types
 Character string type is one in which the values consist of sequences of
characters
 Design issues with the string types
 Should strings be simply a special kind of character array or a
primitive type?
 Should strings have static or dynamic length?
 String Operations
 Assignment ( Java: str1 = str2;) (C: strcpy(pstr1, pstr2);
 Comparison (=, >, etc.) BASIC: str1 < str2
 Concatenation, C: strcat (str1,str2),
(Java : str2 + str3;)
 Substring reference
 Pattern matching, C: strcmp(str1,str2);
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6.3 Character String Types contd.
 Examples
 C and C++
 not primitive
 use char arrays and a library of functions that provide operations
 Java : String class (not arrays of char)
 objects are immutable
 StringBuffer is a class for changeable string objects
 String length options
 Static – Python, Java’s String class, C++ standard class library,
Ruby’s built-in String class, and the .NET class library in C# and
F#.
 limited dynamic length – C and C++ ( up to a max length indicated
by a null character)
 dynamic –Perl, JavaScript
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6.3
Character String Types (cont.)
 Implementation
 static length - compile-time descriptor
 limited dynamic length - may need a run-time descriptor for
length (but not in C and C++ because the end of a string is
marked with the null character)
 dynamic length - need run-time descriptor;
allocation/deallocation is the biggest implementation problem
 Fig (a) Compile-time descriptor for static strings; Fig (b)
Run-time descriptor for limited dynamic strings
Compile – time descriptor for
static strings
Run-time descriptor for limited
dynamic strings
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6.4
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
 Design issue: should a symbolic constant be allowed to be in more than one type
definition?
Examples
 Java does not include an enumeration type, but provides the Enumeration
interface
 C# example
enum days {mon, tue, wed, thu, fri, sat, sun};
 Evaluation of enumeration types
 aid to readability e.g. no need to code a color as number.
 aid to reliability e.g. compiler can check
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6.5
Arrays
 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.
 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 array objects be initialized?
 Are any kind of slices allowed?
 Indexing is a mapping from indices to elements
 map(array_name, index_value_list)  an element
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6.5 Arrays (continued)
 Index Syntax
 FORTRAN, PL/I, Ada use parentheses
 Most other languages use brackets
 Subscript Types:
 FORTRAN, C - integer only
 Java - integer types only
 Categories of arrays:
1. Fixed stack dynamic - range of subscripts is statically bound,
but storage is bound at elaboration time
 e.g. Most Java locals, and C locals that are not static
 Advantage: space efficiency
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6.5
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Arrays (continued)
2.
Stack-dynamic - range and storage are dynamic, but fixed from
then on for the variable’s lifetime
 e.g. Ada declare blocks
declare
STUFF : array (1..N) of FLOAT;
begin
...
end;
 Advantage: flexibility - size need not be known until the array is about to
be used
3.
Heap-dynamic - subscript range and storage bindings are
dynamic and not fixed
 In APL, Perl, and JavaScript, arrays grow and shrink as needed
 In Java, all arrays are objects (heap-dynamic)
Array Initialization
 Some languages 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”};
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6.6
Associative Arrays
 An associative array is an unordered collection of data elements that are
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indexed by an equal number of values called keys
Also known as Hash tables
 Index by key (part of data) rather than value
 Store both key and value (take more space)
 Best when access is by data rather than index
Examples:
 Lisp alist:
 ((key1 . data1) (key2 . data2) (key3 . data3)
Design Issues
 What is the form of references to elements?
 Is the size static or dynamic?
Structure and Operations in Perl
 Names begin with %
 Literals are delimited by parentheses, e.g.,
%hi_temps = ("Monday" => 77, "Tuesday" => 79,…);
 Subscripting is done using braces and keys, e.g.,
$hi_temps{"Wednesday"} = 83;
 Elements can be removed with delete, e.g.,
delete $hi_temps{"Tuesday"};
6.7 Records
 A record is a possibly heterogeneous aggregate of data elements in which the
individual elements are identified by names
 Design Issues
 What is the form of references? (Calling format: OFF, .)
 What unit operations are defined? (Assignment, equality, assign corresponding
filed)
 Implementation method
 Simple and efficient, because field name references are literals bound at compiletime.
 Use offsets to determine address.
 Record Definition Syntax
 COBOL uses level numbers to show nested records; others use recursive
definitions
 Record Field References
 COBOL
field_name OF record_name_1 OF ... OF record_name_n
 Others (dot notation)
record_name_1.record_name_2. ... .record_name_n.field_name
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6.7 Records (continued)
 Record Operations
 Assignment
 Pascal, Ada, and C allow it if the types are identical
 In Ada, the RHS can be an aggregate constant
 Initialization
 Allowed in Ada, using an aggregate constant
 Comparison
 In Ada, = and /=; one operand can be an aggregate constant
 MOVE CORRESPONDING
 In COBOL - it moves all fields in the source record to fields with the same
names in the destination record
 Useful operation in data processing application, where input records are
moved to output files after same modification.
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6.7 Records (continued)
 Comparing records and arrays
 Access to array elements is much
A compile-time descriptor
for a record
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.
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6.8
Unions
 A union is a type whose variables are allowed to store different type
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values at different times during execution.
Implementation:
 Allocate for largest variant
 Discriminated unions include tag field to indicate type
Example:
 Table of symbols and values
 Each value may be int, real, or string
Design Issues for unions
 Should type checking be required? Note that any such type checking must be
dynamic.
 Should unions be integrated with records?
Examples:
FORTRAN, C and C++ - free unions (no tags)
 Not part of their records
 No type checking of references
Java has neither records nor unions
6.10 Pointers
 A pointer type is a type in which the range of values consists of memory addresses and a
special value, nil (or null)
 Uses
 Addressing flexibility (support indirect addressing)
 Dynamic storage management (scoping)
 Design Issues
 What is the scope and lifetime of pointer variables?
 What is the lifetime of heap-dynamic variables?
 Are pointers restricted to pointing at a particular type?
 Are pointers used for dynamic storage management, indirect addressing, or both?
 Should a language support pointer types, reference types, or both?
Note: heap dynamic variables have no name and must be referenced by pointer variable.
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6.10 Pointers (continued)
 Fundamental Pointer Operations:
 Assignment of an address to a pointer (first binding)
 References (explicit versus implicit dereferencing)
 The assignment operation j = *ptr (second binding)
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6.10 Pointers (continued)
 Problems with pointers
 Dangling pointers (dangerous)
 A pointer points to a heap-dynamic variable that has been deallocated
 Creating one (with explicit deallocation):
Set a second pointer to the value of the first pointer
Deallocate the heap-dynamic variable, using the first pointer
 Lost Heap-Dynamic Variables (wasteful)
 A heap-dynamic variable that is no longer referenced by any program pointer
 Creating one:
 Pointer p1 is set to point to a newly created heap-dynamic variable
 p1 is later set to point to another newly created heap-dynamic variable
 The process of losing heap-dynamic variables is called memory leakage
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6.10 Pointers (continued)
• C and C++
– Used for dynamic storage management and addressing
– Explicit dereferencing (*value and &  address) and
address-of operator
– Can do address arithmetic in restricted forms
e.g. 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]
– Domain type need not be fixed (void * )
– void * - Can point to any type and can be usefull for
transferring memory from one place to another.
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6.10 Pointers (continued)
 C++ Reference Types
 Constant pointers that are implicitly dereferenced
 Example:
int result = 0;
int &ref_result = result;
……
ref_result = 100;
In this code segment, result and ref_result are aliases.
 Advantages of both pass-by-reference and pass-by-value
 Java has no pointer type, but only a reference type.
 No pointer arithmetic
 Can only point at objects (which are all on the heap)
 No explicit deallocator
 Means there can be no dangling references
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6.10 Pointers (continued)
 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
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Dealing with Lost Objects
 The lost object problem can be solved if the language implements
automatic storage management. (Java and Lisp)
 Two approaches for garbage collection :
 Reference counting (“eager” approach):
 Object maintains a counter of how many pointers reference it,
when counter is decremented to zero, the object is deallocated.
 Reference counting incurs significant overhead on each pointer
assignment, but the overhead is distributed throughout the session.
 Mark-sweep (“lazy” approach):
 Wait until all storage is allocated, then collect the garbage
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Summary
 The data types of a language are a large part of what determines that
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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
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