14/3/02 sudeshna sarkar, cse, iit kharagpur1 structures, adt lecture 25 14/3/2002

14/3/02 Sudeshna Sarkar, CSE, IIT Kharagpur

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Structures, ADT

Lecture 2514/3/2002


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Announcements Lab Test 2 on the week of 18th –

22nd March Syllabus for lab test : arrays,

structures, data types, …..


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The List ADT A list : <A1, A2, ... , AN> of size N. Special list of size 0 : an empty list Operations:

makenull () : returns an empty list makelist (elem) : makes a list containing a single

element printlist (list) search(elem, list) : searches whether a key is in

the list insert (elem, list) delete (elem, list) findKth (list)


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Array Implementation of Listtypedef int ETYPE;typedef struct {

ETYPE elements[MAXS];int size;

} LIST;LIST makenull () ;LIST makeList (ETYPE) ;void printList (LIST) ;int IsEmpty (LIST) ;int search (ETYPE, LIST) ;void delete (ETYPE, LIST * );void insert (ETYPE, LIST * )


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Complex Number ADTtypedef struct {

float real;float imag;

} COMPLEX;COMPLEX makecomplex (float, float) ;COMPLEX addc (COMPLEX, COMPLEX);COMPLEX subc (COMPLEX, COMPLEX);COMPLEX multc (COMPLEX, COMPLEX);COMPLEX divc (COMPLEX, COMPLEX);


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SET ADT Interface functions (1):SET makenullset () ;int member (ETYPE, SET)

;SET adjoin (ETYPE, SET);SET union (SET, SET) ;SET intersection (SET,

SET);Void printset (SET) ;

Interface functions (2):SET makenullset () ;int member (ETYPE, SET) ;void adjoin(ETYPE, SET *);void union (SET, SET, SET*);void intersection (SET, SET, SET*);Void printset (SET) ;


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Concrete implementation of SET ADT

typedef struct {ETYPE elem[MAX];int size;

} SET;

Implementation 1 : sorted arrayadjoin : Sorted insertmember : Binary searchdelete : ?union : merge 2 sorted arrays intersection : ?


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Concrete implementation of SET ADT

typedef struct {ETYPE elem[MAX];int size;

} SET;

Implementation 2 : unsorted arraykeep the elements in the array unsorted.

adjoin : Insert at the endmember : Search till found or till the enddelete : Go through the array sequentially until

element is found, or reach the end.Then left shift the array.

union , intersection ?


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Arrays of Structures A struct represents a single record. Typically structs are used to deal with

collections of such records Examples : student records, employee

records, book records, ... In each case we will hav multiple instances

of the struct type.Arrays of structs are the natural way to do

this.


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Arrays of structs : declaration & use

Each declaration below declares an array, where each array element is a structure:point corner_points[10] ;StudentRecord btech01[MAXS] ;

We access a field of a struct in an array by specifying the array element and then the field :btech01[i].namecorner_points[4].x


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Naming in struct Arrayspoint pentagon[5];

xy

xy

xy

xy

xy

pentagon : an array of points

pentagon[1] : a point structure

pentagon[4].x : a double


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Using Arrays of structs

StudentRecord class[MAXS];...for (i=0; i<nstudents; i++) {

scanf (“%d%d”, &class[i].midterm, &class[i].final);

class[i].grade = (double)(class[i].midterm+class[i].final)/50.0;

}


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struct Array elements as parameters

void draw_line (point p1, point p2) { ... }...point pentagon[5];...for (i=0;i<4;i++)

draw_line (pentagon[i], pentagon[i+1]);

draw_line (pentagon[4], pentagon[0]);


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structs as Parameters

A single struct is passed by value. all of its components are copied from

the argument (actual parameter) to initialize the (formal) parameter.

point set_midpt (point a, point b) { ... }int main (void) {

point p1, p2, m;...m = set_midpt(p1, p2);

}


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Passing Arrays of structs An array of structs is an array. When any array is an argument (actual parameter), it

is passed by reference, not copied [As for any array] The parameter is an alias of the actual array

argument.int avg (StudentRec class[MAX]) { ... }int main (void) {

StudentRec bt01[MAX];int average;...average = avg_midpt(bt01) ;

}


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Dynamic Memory Allocation,Structure pointers

Lecture 2614.3.2002.


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Basic Idea Many a time we face situations where

data is dynamic in nature. Amount of data cannot be predicted

beforehand. Number of data item keeps changing

during program execution. Such situations can be handled more

easily and effectively using dynamic memory management techniques.


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C language requires the number of elements in an array to be specified at compile time. Often leads to wastage or memory

space or program failure. Dynamic Memory Allocation

Memory space required can be specified at the time of execution.

C supports allocating and freeing memory dynamically using library routines.


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Memory Allocation Process in C

Local variables

Free memory

Global variables

InstructionsPermanent storage area

Stack

Heap


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The program instructions and the global variables are stored in a region known as permanent storage area.

The local variables are stored in another area called stack.

The memory space between these two areas is available for dynamic allocation during execution of the program. This free region is called the heap. The size of the heap keeps changing


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Memory Allocation Functions

malloc: Allocates requested number of bytes and returns a pointer to the first byte of the allocated space.

calloc: Allocates space for an array of elements, initializes them to zero and then returns a pointer to the memory.

free : Frees previously allocated space. realloc: Modifies the size of previously

allocated space.


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Dynamic Memory Allocation used to dynamically create

space for arrays, structures, etc.

int main () { int *a ; int n; .... a = (int *) calloc (n, sizeof(int)); ....}

a = malloc (n*sizeof(int));


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Space that has been dynamically allocated with either calloc() or malloc() does not get returned to the function upon function exit.

The programmer must use free() explicitly to return the space. ptr = malloc (...) ; free (ptr) ;


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void read_array (int *a, int n) ;int sum_array (int *a, int n) ;void wrt_array (int *a, int n) ;

int main () { int *a, n; printf (“Input n: “) ; scanf (“%d”, &n) ; a = calloc (n, sizeof (int)) ; read_array (a, n) ; wrt_array (a, n) ; printf (“Sum = %d\n”, sum_array(a, n);}


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void read_array (int *a, int n) { int i; for (i=0; i<n; i++)

scanf (“%d”, &a[i]) ;}void sum_array (int *a, int n) { int i, sum=0; for (i=0; i<n; i++) sum += a[i] ; return sum;}void wrt_array (int *a, int n) { int i; ........}


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Arrays of Pointers Array elements can be of any type

array of structures array of pointers


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int main (void) {char word[MAXWORD];char * w[N]; /* an array of pointers */int i, n; /* n: no of words to sort */

for (i=0; scanf(“%s”, word) == 1); ++i) { w[i] = calloc (strlen(word)+1, sizeof(char)); if (w[i] == NULL) exit(0);

strcpy (w[i], word) ;}

n = i;sortwords (w, n) ;wrt_words (w, n);return 0;

}


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w

0

1

2

3

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Input : A is for apple or alphabet pie which all get a slice of come taste it and try

A \0

i s \0

f o r \0

a p p l e \0

t r y \0


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void sort_words (char *w[], int n) { int i, j; for (i=0; i<n; ++i)

for (j=i+1; j<n; ++j)if (strcmp(w[i], w[j]) > 0) swap (&w[i], &w[j]) ;

}void swap (char **p, char **q) { char *tmp ; tmp = *p; *p = *q; *q = tmp;}


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w

w[i]

f o r \0

a p p l e \0

Before swapping

w[j]


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w

w[i]

f o r \0

a p p l e \0

After swapping

w[j]


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Pointers to Structure


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Pointers and Structures You may recall that the name of an array

stands for the address of its zero-th element. Also true for the names of arrays of structure

variables. Consider the declaration:

struct stud { int roll; char dept_code[25]; float cgpa; } class[100], *ptr ;


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The name class represents the address of the zero-th element of the structure array.

ptr is a pointer to data objects of the type struct stud.

The assignmentptr = class ;

will assign the address of class[0] to ptr. When the pointer ptr is incremented by

one (ptr++) : The value of ptr is actually increased

by sizeof(stud). It is made to point to the next record.


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Once ptr points to a structure variable, the members can be accessed as: ptr –> roll ; ptr –> dept_code ; ptr –> cgpa ;

The symbol “–>” is called the arrow operator.


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Warning When using structure pointers, we should

take care of operator precedence. Member operator “.” has higher precedence than “*”.

ptr –> roll and (*ptr).roll mean the same thing. *ptr.roll will lead to error.

The operator “–>” enjoys the highest priority among operators.

++ptr –> roll will increment roll, not ptr. (++ptr) –> roll will do the intended thing.


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Program to add two complex numbers using pointers

typedef struct { float re; float im;} complex;main() { complex a, b, c; scanf (“%f %f”, &a.re, &a.im); scanf (“%f %f”, &b.re, &b.im); add (&a, &b, &c) ; printf (“\n %f %f”, c,re, c.im);}


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void add (complex * x, complex * y, complex * t) {

t->re = x->re + y->re ;

t->im = x->im + y->im ;

}

14/3/02 sudeshna sarkar, cse, iit kharagpur1 structures, adt lecture 25 14/3/2002

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