Programming Languages Pragmatics

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

4

See excellent detailed explanation of floating pointrepresentation in the following video: http://www.youtube.com/watch?v=t-8fMtUNX1A

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>


index type



addressaddress

<array><array>


index type



addressaddress

<array><array>


index type



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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Programming Languages Pragmatics

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