Post on 09-Jan-2016
description
Programming Languages Pragmatics
Data TypesData Types
Simple typesSimple types
integer floating point binary-coded decimal
character boolean user-defined types
usually in hardware
usually in software
not composed of other types hardware or software implemented
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IntegerInteger 2’s complement unsigned operations exact within range range depends on size of virtual cell- typical size: 1, 2, 4, 8 bytes
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Floating Point (FP)Floating Point (FP) Approximate real numbers but not dense, not even
“equally sparse” Languages may support at least two FP types: float and
double May follow the IEEE FP-754 Standard (Java) representations and operations are approximate range and precision depend on size of virtual cell
(usually 4 or 8 bytes)
1 11 52 bits mantissaexponent
sign
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See excellent detailed explanation of floating pointrepresentation in the following video: http://www.youtube.com/watch?v=t-8fMtUNX1A
Binary Coded DecimalBinary Coded Decimal
‘exact’ decimal arithmetic, space costly decimal digits in 4 bit code range and precision depend on size of
virtual cell – 2 digits per byte
4 4
5 9 0 5 1 8 7 8
defined decimal pointBytes
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CharacterCharacter
ASCII – 128 character set – 1 byte Unicode – 2 byte extension usually coded as unsigned integer
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BooleanBoolean
1 bit is sufficient but... no bit-wise addressability in hardware store in a byte – space inefficient store 8 per byte – execution
inefficient c: 0=false, non-zero=true
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User-defined typesUser-defined types implemented (like character and
boolean usually are) as a coding of unsigned integer
enumerated type: (Pascal example)type suit = (club, diamond, heart, spade);
var lead: suit;
lead := heart;
internally represented as { 0, 1, 2, 3 }
operations:
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User-defined typesUser-defined types
implemented as a restricted range of integer
subrange type: (Ada example)subtype CENTURY20 is INTEGER range 1900..1999;
BIRTHYEAR: CENTURY20;
BIRTHYEAR := 1981;
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User-defined typesUser-defined types
Type compatibility issues:-can two enumerated types contain
same constant?-can defined types be coerced with
integer, with each other?
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When Should Two Types Be When Should Two Types Be Considered Equivalent?Considered Equivalent? Type equivalence Two principal forms
StructuralStructural Two types are equivalent if they consist of
the same components Name equivalence Name equivalence
Every type declaration defines a new type so two types are the same if they have the same name
More popular in more modern languages
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ExampleExample
typedef struct {int a;int b;
} Point;
typedef struct {int a;int b;
} Pair;
Java uses name equivalence ML is more-or-less structural C hybrid (structural except for structs)
Point x;Pair y;X = y;
Legal?
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Memory management introMemory management intro The parser creates a symbol table of
identifiers including variables: Some information, name plus more, is
bound at this time and as the program is compiled by storage in symbol table:e.g. int x;
--> x type: intaddr: offset
namenametypetype addressaddress 13
StringsStrings First use: output formatting only Quasi-primitive type in most
languages (not just arrays of character)
- operations: initialization, substring, catenation, comparison
The length problem: fixed or varying? No standard string model
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c
char *s = “abc”;
int len = strlen(s);
array of char with terminal:
extended syntax
library of methods
Strings - examplesStrings - examples
JAVA
String s = “abc”+x;
s = s.substring(0,2);
fixed length array
extended syntax
class with 70 methods
a b c 0
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Strings - representationsStrings - representations fixed length and content
(static) fixed length and varying
content (FORTRAN) varying length and content by
reallocation (java String) varying length and content by
extension (java StringBuffer) Varying length and content(C)
Static strLengthAddress
Dynamic strMaxLengthCurrLengthAddress
char*Address
In symbol table
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Compound (1)Compound (1) Arrays Arrays collection of elements of one type access to individual elements is
computed at execution time by position, O(1), or O(dim)
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Arrays – design decisionsArrays – design decisions indexing:
dimensions – limit? recursive?types – int, other, user defined?first index: 0, 1, variablerange checking – no(C),
yes(java)syntax for subscript operator (),
[]?
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Arrays – design decisionsArrays – design decisions binding times
type, index typeindex range(ie array size), space
staticfixed stack-dynamicstack-dynamicheap-dynamic
initial values of elementsat storage allocation? e.g. int[] x =
{1,2,3};
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Arrays – operationsArrays – operations
on elements – based on type on entire array as variables -
- vector and matrix operations e.g.,APL- sub array (~ substring)
subarray dimensions(slices)
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Arrays – storageArrays – storage
<array>element type, size
index typeindex lower boundindex upper bound
address
address
lower bound upper bound
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Arrays – element accessArrays – element access
<array>element type, size
index typeindex lower boundindex upper bound
address
address
lower bound i
address of a[i] =
address + (i-lower bound)*size
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Arrays - multidimensionalArrays - multidimensional
contiguous or not row major, column major order computed location of element
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Jagged arraysJagged arrays
Implemented as arrays of arrays<array>
<array>, 4index type
index lower boundindex upper bound
address
address
<array><array>
<array>, 3<array>, 3index type
index type
index lower boundindex lower bound
index upper boundindex upper bound
addressaddress
<array><array>
<array>, 7<array>, 7index type
index type
index lower boundindex lower bound
index upper boundindex upper bound
addressaddress
<array><array>
<array>, 4<array>, 4index type
index type
index lower boundindex lower bound
index upper boundindex upper bound
addressaddress
<array><array>
<array>, 5<array>, 5index type
index type
index lower boundindex lower bound
index upper boundindex upper bound
addressaddress
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(2) (2) Associative Arrays - mapsAssociative Arrays - maps
values accessed by keys,not indices no order of elements automatic growth of capacity operations: add/set, get, remove fast search for individual data slower for batch processing than
array Java classes; Perl data structure
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Associative Arrays - implementationAssociative Arrays - implementation
hash tables based on key value most operations ‘near O(1)’ expanding capacity may be O(n)
For a java class that combines features of array and associative array, see LinkedHashMap
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(3)(3) RecordsRecords
multiple elements of any type elements accessed by field name design issues:
- hierarchical definition(records within records)
- syntax of naming- scopes for elliptical (incomplete) reference to fields
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Records - implementationRecords - implementation<array> a
element type, size
index type
index lower bound
index upper bound
address
address
lower bound upper bound
<record>dept
array [1..4] of char 0 (offset)
code
address
Caddress O S C 3127
dept course
integer4
type course =
record dept : array[1..4] of char; code : integer;
end
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(4)(4) Pascal variant records Pascal variant records (unions)(unions)
type coord = (polar, cart);
point =
record
case rep : coord of
polar: ( radians : boolean;
radius : real;
angle : real);
cart: ( x : real;
y : real);
end;
Note:
•varying space requirements
•discriminant field is optional (rep)
•type checking loopholes: Ada has similar variant record but closed these loopholes
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Other unionsOther unions Fortran EQUIVALENCE c union not inside records no type checking
* unions do not cause type coercion - data is reinterpreted
Sebesta’s c example
union flextype {
int intE1;
float floatE1;
}
union flexType ell;
float x;
ell.intE1 = 27;
x = ell.floatE1;
Sebesta’s c example
union flextype {
int intE1;
float floatE1;
}
union flexType ell;
float x;
ell.intE1 = 27;
x = ell.floatE1;
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(5)(5) Sets (Pascal)Sets (Pascal)
defined on one (discrete) base type implementation imposes maximum
size (set of integer;-not possible)
type day = (M, Tu, W, Th, F, Sa, Su); dayset = set of day;var work, wknd : dayset; today : day;today = F;work = [M, Tu, W, Th, F];wknd = [Sa, Su, F];if (today in work and wknd) ...
1 1 0111 0
0 0 1100 1
0 0 0100 0
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(6)(6) Pointers and references Pointers and references references are dereferenced pointers
(whatever that means) primary purpose: dynamic memory
access secondary purpose: indirect
addressing as in machine instructions
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PointersPointers (and references) (and references) data type that stores an address in
the format of the machine (usually 4 bytes) or a “null”
a pointer must be dereferenced to get the data at the address it contains
a reference is a pointer data type that is automatically dereferenced
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Dereferencing exampleDereferencing example
In c++:
double x,y;
Point p(0.0,0.0);
Point *pref;
pref = &p;
x = p.X;
y = (*pref).Y;
In Java:
Point2D.Double p;
p = new Point2D.Double(0.0,0.0);
double xCoord = p.x;
Dereferencing and field access combined
Dereferencing Field access
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Pointers hold addressesPointers hold addresses
Indirect addressingIn c: pointer to statically allocated memory
int a,b;
int *iptr, *jptr;
a = 100;
iptr = &a;
jptr = iptr;
b = *jptr;
int x, y, arr[4];
int *iptr;
iptr = arr;
arr[2] = 33;
x = iptr[2];
y = *(iptr + 2);
Security loophole…35
Pointer arithmeticPointer arithmetic
Arithmetic operations on addresses
int x;
int *iptr;
iptr = &x;
for (;;){
<< process loc (*iptr)>>
iptr++;
}
Scan through memory starting at x
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Basic dynamic memory Basic dynamic memory management model:management model: Heap (memory) manager keeps list of
available memory cells “Allocate” operation transfers cell
from list in heap to program “Deallocate” transfers cell from
program back to list in heap Tradeoffs of fixed or variable sized
cells
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Problems with pointers and Problems with pointers and dynamic memory:1dynamic memory:1
Dangling reference: pointer points to de-allocated memory
Point *q;
Point *p = new Point(0,0);
q = p;
delete p;
// q is dangling - reference to q should cause
// an error - ‘tombstones’ will do error check
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Problems with pointers and Problems with pointers and dynamic memory: 2dynamic memory: 2 Memory leakage: memory cell with no
reference to it
Point *p = new Point(0,0);
p = new Point(3,4);
// memory containing Point(0,0) object
// is inaccessible - counting references will help
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Cause of reference problemsCause of reference problems Multiple references to a memory cell Deallocation of memory cells
Where is responsibility?-automatic deallocation (garbage collection)
OR -user responsibility (explicit ‘delete’ – C++)
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User management of memoryUser management of memory
Dangling references can be detected as errors but not prevented
Memory leakage is a continuing problem – Can you think of a way to find stranded memory?
int *p =*q = 6;
p = null;
int *p =*q = 6;
p = null;
p 6
q
p 6
q
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Garbage CollectionGarbage Collection
1. Reference counting: ongoing “eager”-memory cells returned to heap as soon as all references removed.
2. Garbage collection: occasional “lazy”-let unreferenced memory cells ‘leak’ till heap is nearly empty then collect them
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Reference counting:Reference counting:
When an item is no longer referenced it may be deleted
Need to keep count of references
When p is set to null nothing refers to Association in example
Does this technique always work? No!
Illustration from http://www.brpreiss.com/books/opus5/html/page422.html
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Why Does This Fail?Why Does This Fail? What’s wrong with this?
Illustration from http://www.brpreiss.com/books/opus5/html/page423.html
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Garbage Collection:Garbage Collection: (mark-sweep) (mark-sweep)
1. All cells in memory marked inaccessible(f)
2. Follow all references in program and mark cells accessible(t);
f
t
t
‘Accessible’ marker in cell
3. Return inaccessible cells to heap
f
t
t
Classic problem:
effect on program performance
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A sloppy java exampleA sloppy java example from Main (Data Structures)public class ObjectStack{ private Object[] data; private int manyItems; .... public Object pop() { if (manyItems==0) throw new EmptyStackException(); return data[--manyItems]; //leaves reference in data }}
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Managing heap ofManaging heap ofvariable-sized cellsvariable-sized cells Necessary for objects with different
space requirements Problem: tracking cell size Problem: heap defragmentation
- keep blocks list in size order?- keep blocks list in sequence order?
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ReferencesReferences http://www.cs.laurentian.ca/dgoforth/c
osc3127/my%20ppt/wk0401datatypes.ppt
http://www.hpcnet.org/upload/directory/materials/11991_20070302114047.doc
http://www.brpreiss.com/books/opus5/
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