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## PowerPoint Slideshow about ' Abstract Data Type' - gabriel-albert

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Data Types

- A data type consists of:
- A collection of data elements (a type)
- A set of operations on these data elements

- Data types in languages:
- predefined:
- any language defines a group of predefined data types
- C e.g.: int, char, float, double, …

- user-defined:
- allow programmers to define their own (new) data types
- C e.g.: struct, union, …

- predefined:

Data Type Examples

- Predefined:
- type: int
- elements: …, -2, -1, 0, 1, 2, …
- operations: +, -, *, /, %, …

- User-defined:
- type: complex
- elements: 1+3i, -5+8i, …
- operations: new, add, sub, distance, …

Concrete Data Types (CDT)

- An concrete data type:
- both concrete representations and their operations are available

- Almost all C predefined types are CDT
- For instance, “int” is a 32-bit double-word, and +, -, …
- Knowing this can do dirty hacks
- See demo…

Abstract Data Types (ADT)

- An abstract data type:
- separates data type declaration from representation
- separates function declaration (prototypes) from implementation (definitions)

- A language must some form of mechanism to support ADT
- interfaces in Java
- signatures in ML
- (roughly) header files & typedef in C

Case Study

- Suppose we’d design a new data type to represent complex number c:
- a data type “complex”
- elements: 3+4i, -5-8i, …
- operations:
- new, add, sub, distance, …

- How to represent this data type in C (CDT, ADT or …)?

Complex Number

// Recall the definition of a complex number c:

c = x + yi, where x,y \in R, and i=sqrt(-1);

// Some typical operations:

complex Complex_new (double x, double y);

complex Complex_add (complex c1, complex c2);

complex Complex_sub (complex c1, complex c2);

complex Complex_mult (complex c1, complex c2);

complex Complex_divide (complex c1, complex c2);

// Next, we’d discuss several variants of rep’s:

// CDT, ADT.

CDT of Complex:Interface—Types

// In file “complex.h”:

#ifndef COMPLEX_H

#define COMPLEX_H

struct Complex_t

{

double x;

double y;

};

typedef struct Complex_t Complex_t;

Complex_t Complex_new (double x, double y);

// other function prototypes are similar

…

#endif

Client Code

// With this interface, we can write client codes

// that manipulate complex numbers. File “main.c”:

#include “complex.h”

int main ()

{

Complex_t c1, c2, c3;

c1 = Complex_new (3.0, 4.0);

c2 = Complex_new (7.0, 6.0);

c3 = Complex_add (c1, c2);

Complex_output (c3);

return 0;

}

Do we know c1, c2, c3’s concrete representation?

How?

CDT Complex: Implementation

// In a file “complex.c”:

#include “complex.h”

Complex_t Complex_new (double x, double y)

{

Complex_t c = {.x = x, .y = y};

return c;

}

// other functions are similar. See Lab1

Problem #1

int main ()

{

Complex_t c;

c = Complex_new (3.0, 4.0);

// Want to do this: c = c + (5+i6);

// Ooooops, this is legal:

c.x += 5;

c.y += 6;

return 0;

}

Problem #2

#ifndef COMPLEX_H

#define COMPLEX_H

struct Complex_t

{

// change to a more fancy one? Anger “main”…

double a[2];

};

typedef struct Complex_t Complex_t;

Complex_t Complex_new (double x, double y);

// other function prototypes are similar

…

#endif

Problems with CDT?

- Operations are transparent.
- user code have no idea of the algorithm
- Good!

- Data representations dependence
- Problem #1: Client code can access data directly
- kick away the interface
- safe?

- Problem #2: make code rigid
- easy to change or evolve?

- Problem #1: Client code can access data directly

ADT of Complex:Interface—Types

// In file “complex.h”:

#ifndef COMPLEX_H

#define COMPLEX_H

// note that “struct complexStruct” not given

typedef struct Complex_t *Complex_t;

Complex_t Complex_new (double x, double y);

// other function prototypes are similar

…

#endif

Client Code

// With this interface, we can write client codes

// that manipulate complex numbers. File “main.c”:

#include “complex.h”

int main ()

{

Complex_t c1, c2, c3;

c1 = Complex_new (3.0, 4.0);

c2 = Complex_new (7.0, 6.0);

c3 = Complex_add (c1, c2);

Complex_output (c3);

return 0;

}

Can we still know c1, c2, c3’s concrete representation?

Why?

ADT Complex: Implementation#1—Types

// In a file “complex.c”:

#include “complex.h”

// We may choose to define complex type as:

struct Complex_t

{

double x;

double y;

};

// which is hidden in implementation.

ADT Complex: Implementation Continued

// In a file “complex.c”:

#include “complex.h”

Complex_t Complex_new (double x, double y)

{

Complex_t c;

c = malloc (sizeof (*c));

c->x = x;

c->y = y;

return c;

}

// other functions are similar. See Lab1

ADT Summary

- Yes, that’s ADT!
- Algorithm is hidden
- Data representation is hidden
- client code can NOT access it
- thus, client code independent of the impl’

- Interface and implementation
- Do Lab1

Polymorphism

- To explain polymorphism, we start with a new data type “tuple”
- A tuple is of the form: (x, y)
- xA, yB (aka: A*B)
- A, B may be unknown in advance and may be different

- E.g:
- A=int, B=int:
- (2, 3), (4, 6), (9, 7), …

- A=char *, B=double:
- (“Bob”, 145.8), (“Alice”, 90.5), …

- A=int, B=int:

Polymorphism

- From the data type point of view, two types:
- A, B

- operations:
- new (x, y); // create a new tuple with x and y
- equals (t1, t2); // equality testing
- first (t); // get the first element of t
- second (t); // get the second element of t
- …

- How to represent this type in computers (using C)?

Monomorphic Version

- We start by studying a monomorphic tuple type called “intTuple”:
- both the first and second components are of “int” type
- (2, 3), (8, 9), …

- The intTuple ADT:
- type: intTuple
- elements: (2, 3), (8, 9), …
- Operations:
- tuple new (int x, int y);
- int first (int t);
- int second (tuple t);
- int equals (tuple t1, tuple t2);
- …

“IntTuple” CDT

// in a file “int-tuple.h”

#ifndef INT_TUPLE_H

#define INT_TUPLE_H

struct IntTuple_t

{

int x;

int y;

};

typedef struct IntTuple_t IntTuple_t;

IntTuple_t IntTuple_new (int n1, int n2);

int IntTuple_first (IntTuple_t t);

…

#endif

Or the “IntTuple” ADT

// in a file “int-tuple.h”

#ifndef INT_TUPLE_H

#define INT_TUPLE_H

typedef struct IntTuple_t *IntTuple_t;

IntTuple_t IntTuple_new (int n1, int n2);

int IntTuple_first (IntTuple_t t);

int IntTuple_equals (IntTuple_t t1,

IntTuple_t t2);

…

#endif

// We only discuss “tupleEquals ()”. All others

// functions left to you.

t2

x

x

y

y

Equality Testing// in a file “int-tuple.c”

int Tuple_equals (IntTuple_t t1, IntTuple_t t2)

{

return ((t1->x == t2->x) && (t1->y==t2->y));

}

Problems?

- It’s ok if we only design “IntTuple”
- But we if we’ll design these tuples:
- (int, double), (int, char *), (double, double), …

- Same code exists everywhere, no means to maintain and evolve
- Nightmares for programmers
- Remember: never duplicate code!

Polymorphism

- Now, we consider a polymorphic tuple type called “tuple”:
- “poly”: may take various forms
- Every element of the type “tuple” may be of different types
- (2, 3.14), (“8”, ‘a’), (‘\0’, 99), …

- The “tuple” ADT:
- type: tuple
- elements: (2, 3.14), (“8”, ‘a’), (‘\0’, 99), …

The Tuple ADT

- What about operations?
- tuple new (??? x, ??? y);
- ??? first (tuple t);
- ??? second (tuple t);
- int equals (tuple t1, tuple t2);
- …

Polymorphic Type

- To resove this, C dedicates a special polymorphic type “void *”
- “void *” is a pointer which can point to “any” concrete types (i.e., it’s compatible with any pointer type),
- very poly…

- long history of practice, initially “char *”
- can not be used directly, use ugly cast
- similar to constructs in others language, such as “Object”

- “void *” is a pointer which can point to “any” concrete types (i.e., it’s compatible with any pointer type),

The Tuple ADT

- What about operations?
- tuple newTuple (void *x, void *y);
- void *first (tuple t);
- void *second (tuple t);
- int equals (tuple t1, tuple t2);
- …

“tuple” Interface

// in a file “tuple.h”

#ifndef TUPLE_H

#define TUPLE_H

typedef void *poly;

typedef struct Tuple_t * Tuple_t;

Tuple_t Tuple_new (poly x, poly y);

poly first (Tuple_t t);

poly second (Tuple_t t);

int equals (Tuple_t t1, Tuple_t t2);

#endif TUPLE_H

Client Code

// file “main.c”

#include “tuple.h”

int main ()

{

int i = 8;

Tuple_t t1 = Tuple_new (&i, “hello”);

return 0;

}

x

y

“tuple” ADT Implementation// in a file “tuple.c”

#include <stdlib.h>

#include “tuple.h”

struct Tuple_t

{

poly x;

poly y;

};

Tuple_t Tuple_new (poly x, poly y)

{

tuple t = malloc (sizeof (*t));

t->x = x;

t->y = y;

return t;

}

x

y

“tuple” ADT Implementation// in a file “tuple.c”

#include <stdlib.h>

#include “tuple.h”

struct Tuple_t

{

poly x;

poly y;

};

poly Tuple_first (Tuple_t t)

{

return t->x;

}

Client Code

#include “complex.h” // ADT version

#include “tuple.h”

int main ()

{

int i = 8;

Tuple_t t1 = Tuple_new (&i, “hello”);

// type cast

int *p = (int *)Tuple_first (t1);

return 0;

}

x

y

Equality Testingstruct Tuple_t

{

poly x;

poly y;

};

// The #1 try:

int Tuple_equals (Tuple_t t1, Tuple_t t2)

{

return ((t1->x == t2->x)

&& (t1->y == t2->y));

// Wrong!!

}

x

y

Equality Testingstruct Tuple_t

{

poly x;

poly y;

};

// The #2 try:

int Tuple_equals (Tuple_t t1, Tuple_t t2)

{

return (*(t1->x) == *(t2->x)

&& *(t1->y) == *(t2->y));

// Problem?

}

x

y

Equality Testingstruct Tuple_t

{

poly x;

poly y;

};

// The #3 try:

int Tuple_equals (Tuple_t t1, Tuple_t t2)

{

return (equalsXXX(t1->x, t2->x)

&&equalsYYY(t1->y, t2->y));

// but what are “equalsXXX” and “equalsYYY”?

}

Function as Arguments

// So in the body of “equals” function, instead

// of guessing the types of t->x and t->y, we

// require the callers of “equals” supply the

// necessary equality testing functions.

// The #4 try:

typedef int (*tf)(poly, poly);

int Tuple_equals (tuple t1, tuple t2,

tf eqx, tf eqy)

{

return(eqx (t1->x, t2->x)

&&eqy (t1->y, t2->y));

}

Change to “tuple” Interface

// in file “tuple.h”

#ifndef TUPLE_H

#define TUPLE_H

typedef void *poly;

typedef int (*tf)(poly, poly);

typedef struct Tuple_t *Tuple_t;

Tuple_t Tuple_new (poly x, poly y);

poly Tuple_first (Tuple_t t);

poly Tuple_second (Tuple_t t);

int Tuple_equals (Tuple_t t1, Tuple_t t2,

tf eqx, tf eqy);

#endif TUPLE_H

Client Code

// in file “main.c”

#include “tuple.h”

int main ()

{

int i=8, j=8, k=7, m=7;

Tuple_t t1 = Tuple_new (&i, &k);

Tuple_t t2 = Tuple_new (&j, &k);

Tuple_equals (t1, t2, Int_equals, Int_equals);

return 0;

}

Moral

- void* serves as polymorphic type in C
- mask all pointer types (think Object type in Java)

- Pros:
- code reuse: write once, used in arbitrary context
- we’d see more examples later in this course

- Cons:
- Polymorphism doesn’t come for free
- boxed data: data heap-allocated (to cope with void *)

- no static or runtime checking (at least in C)
- clumsy code
- extra function pointer arguments

- Polymorphism doesn’t come for free

Function-Carrying Data

- Why we can NOT make use of data, such as passed as function arguments, when it’s of type “void *”?
- Better idea:
- Let data carry functions themselves, instead passing function pointers
- such kind of data called objects

Function Pointer in Data

int Tuple_equals (Tuple_t t1, Tuple_t t2)

{

// note that if t1->x or t1->y has carried the

//equality testing functions, thenthe code

// could just be written as:

return (t1->x->equals (t1->x, t2->x)

&& t1->y->equals (t1->y, t2->y));

}

equals

equals_x

……

t1

x

equals_y

y

equals

……

n

x

y

Function Pointer in Data// To cope with this, we should modify other

// modules. For instance, the “complex” ADT:

struct Complex_t

{

int (*equals) (poly, poly);

double a[2];

};

Complex_t Complex_new (double x, double y)

{

Complex_t c = malloc (sizeof (*c));

c->equals = Complex_equals;

…;

return n;

}

Function Call

int Tuple_equals (Tuple_t t1, Tuple_t t2)

{

return(t1->x->equals (t1->x, t2->x)

&& t1->y->equals (t1->y,t2->y));

}

equals

t2

t1

x

a[0]

a[0]

x

y

a[1]

a[1]

y

Client Code

// in file “main.c”

#include “complex.h”

#include “tuple.h”

int main ()

{

Complex_t c1 = Complex_new (1.0, 2.0);

Complex_t c2 = Complex_new (1.0, 2.0);

Tuple_t t1 = Tuple_new (c1, c2);

Tuple_t t2 = Tuple_new (c1, c2);

Tuple_equals (t1, t2); // dirty simple! :-P

return 0;

}

Object

- Data elements with function pointers is the simplest form of objects
- object = virtual functions + private data

- With such facilities, we can in principal model object oriented programming
- In fact, early C++ compilers compiles to C
- That’s partly why I don’t love object-oriented languages

Summary

- Abstract data types enable modular programming
- clear separation between interface and implementation
- interface and implementation should design and evolve together

- Polymorphism enables code reuse
- Object = data + function pointers

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