Structures. Heterogeneous Structures Collection of values of possibly different types. Name the...

Structures

Heterogeneous Structures

Collection of values of possibly different types.

Name the collection.Name the components.

Example : Student recordSinghal

name "V Singhal"rollno "00CS1001"classtest 14midterm 78final 73grade ‘B

Structure : terminology A struct is a group of items (variables)

which may be of different types. Each item is identified by its own

identifier, each of which is known as a member or field of the structure.

A struct is sometimes called a record or structure.

Structs are the basis of classes in C++ and Java.

Structure declaration

struct {char first[10];char midinit;char last[20];

} sname, ename;

This declaration createstwo structure variables,sname and ename, eachof which contains 3members.We can use sname.first,ename.midinit, etc.

Members To access the members of a

structure, we use the member access operator “.”.strcpy (sname.first, “Sudeshna”);sname.midinit = ‘K’;strcpy (sname.last, “Sarkar”) ;

Tagged structure

struct nametype {char first[10];char midinit;char last[20];

};struct nametype sname, ename;typedef struct nametype

NTYPE;NTYPE aname, bname;

This definition createsa structure tag nametypecontaining 3 members:first, midinit, last.Variables may be declaredof type struct <tagname>.

typedef is normally usedto give names to a struct type.

typedeftypedef struct {

char first[10];char midinit;char last[20];

} NAMETYPE;NAMETYPE

sname,ename;

Another example#define MAX_NAME 40typedef struct {

char name[MAX_NAME+1];char rollno[10];int classtest;int midterm;int final;char grade;

} StudentRecord;Defines a new data type called StudentRecord.

Does not declare a variable.

Declaring struct variables

/* typedef structs go at top of program */. . .int .....float ....StudentRecord s1;StudentRecord singhal ;/* StudentRecord is a type; s1 and singhal are variables*/

struct nametype aname;/* struct nametype is a type; aname is a variable */

Things you can and can't do

You canUse = to assign whole struct variables

You can Have a struct as a function return type

You cannot Use == to directly compare struct

variables; can compare fields directly You cannot

Directly scanf or printf structs; can read fields one by one.

Struct initializers

/* typedef structs go on top */StudentRecord s1 = {"V Singhal",

"00CS1002", 15, 78, 73, 'B'};

Using components of struct variables

s1.classtest = 46;s1.midterm = 78;scanf ("%d", &s1.rollno) ;

Assigning whole structs

s1 = singhal;is equivalent to

strcpy(s1.name, singhal.name) ; strcpy(s1.rollno, singhal.rollno; s1.classtest = singhal.classtest; s1.midterm = singhal.midterm; s1.final = singhal.final; s1.grade = singhal.grade;

Within a given structure, the member names must be unique.

However, members in different structures may have the same name.

A member is always accessed through a structure identifier.struct fruit {

char name[20];int calories;

};struct vegetable { char name[30];

int calories;};

struct fruit mango;struct vegetable potato;It is clear that we can access mango.calories andpotato.calories withoutany ambiguity.

Complicated structures A member of a structure can be an array or

another structure.struct grocerylist {

struct fruit flist[10];struct vegetable vlist[20];

} ; You can have an array of structures.

struct card {int pips;char suit;

} deck[52] ;

A function using struct array

int fail (StudentRecord slist []) {int i, cnt=0;for (i=0; i<CLASS_SIZE; i++)

cnt += slist[i].grade == ‘F’;return cnt;

}

Using structures with functions Structures can be passed as arguments to

functions. Structures can be returned from functions. Call by value is used if a structure is a function

parameter, meaning that a local copy is made for use in the body of the function. If a member of the structure is an array, then the array gets copied as well.

If the structure is large, passing the structure as an argument can be relatively inefficient. An address of th structure may be used as the parameter.

Union A union is like a structure, except that the

members of a union share the same space in memory.union int_or_float {

int i;float f;

}; It is the programmer’s responsibility to know

which representation is currently stored in a union variable.

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.

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

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

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;

}

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]);

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);

}

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) ;

}

Dynamic Memory Allocation,Structure pointers

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.

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.

Memory Allocation Process in C

Local variables

Free memory

Global variables

InstructionsPermanent storage area

Stack

Heap

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

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.

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));

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) ;

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);}

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; ........}

Arrays of Pointers Array elements can be of any type

array of structures array of pointers

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;

}

w

0

1

2

3

17

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

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;}

w

w[i]

f o r \0

a p p l e \0

Before swapping

w[j]

w

w[i]

f o r \0

a p p l e \0

After swapping

w[j]

Pointers to Structure

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 ;

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.

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.

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.

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);}

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

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

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

}

Structure and list processing

Dynamic allocation: review Variables in C are allocated in one of 3

spots: the run-time stack : variables declared local to

functions are allocated during execution the global data section : Global variables are

allocated here and are accessible by all parts of a program.

the heap : Dynamically allocated data items malloc, calloc, realloc manage the heap region of

the mmory. If the allocation is not successful a NULL value is returned.

Bad Pointers When a pointer is first allocated, it does not

have a pointee. The pointer is uninitialized or bad. A dereference operation on a bad pointer is a

serious runtime error. Each pointer must be assigned a pointee before

it can support dereference operations. int * numPtr;

Every pointer starts out with a bad value. Correct code overwrites the bad value.

Example pointer code.

int * numPtr;int num = 42;numPtr = &num;*numPtr = 73;numPtr = malloc (sizeof (int));*numPtr = 73;

int a=1, b=2, c=3;

int *p, *q;1a

3c

2b

xxx

xxx

p

q

p = &a ;q = &b ;

1a

3c

2b

p

q

c = *p ;p = q ;*p = 13 ;

1a

1c

13b

p

q

Bad pointer Examplevoid BadPointer () {

int *p;*p = 42;

}int * Bad2 () {

int num, *p;num = 42;p = &num;return p;

}

x x xp

X

A function call malloc(size) allocates a block of mrmory in the heap and returns a pointer to the new block. size is the integer size of the block in bytes. Heap memory is not deallocated when the creating function exits.

malloc generates a generic pointer to a generic data item (void *) or NULL if it cannot fulfill the request.

Type cast the pointer returned by malloc to the type of variable we are assigning it to.

free : takes as its parameter a pointer to an allocated region and de-allocates memory space.

Dynamic memory allocation: review

typedef struct {int hiTemp;int loTemp;double precip;

} WeatherData;main () {

int numdays;WeatherData * days;scanf (“%d”, &numdays) ;days=(WeatherData *)malloc (sizeof(WeatherData)*numdays);if (days == NULL) printf (“Insufficient memory”);...

free (days) ;}

Self-referential structures Dynamic data structures : Structures

with pointer members that refer to the same structure. Arrays and other simple variables are

allocated at block entry. But dynamic data structures require

storage management routine to explicitly obtain and release memory.

Self-referential structures

struct list {int data ;struct list * next ;

} ;

The pointer variable next is called a link.Each structure is linked to a succeeding structureby next.

Pictorial representationA structure of type struct list

data next

The pointer variable next contains either • an address of the location in memory of the successor list element• or the special value NULL defined as 0.

NULL is used to denote the end of the list.

struct list a, b, c;a.data = 1;b.data = 2;c.data = 3;a.next = b.next = c.next = NULL;

1 NULL

data next

a2 NULL

data next

b3 NULL

data next

c

Chaining these togethera.next = &b;b.next = &c;

1

data next

a2

data next

b3

data next

cNULL

What are the values of : • a.next->data• a.next->next->data

23

Linear Linked Lists A head pointer addresses the first

element of the list. Each element points at a successor

element. The last element has a link value

NULL.

Header file : list.h#include <stdio.h>#include <stdlib.h>typedef char DATA;struct list {

DATA d;struct list * next;

};typedef struct list ELEMENT;typedef ELEMENT * LINK;

Storage allocationLINK head ;

head = malloc (sizeof(ELEMENT));

head->d = ‘n’;

head->next = NULL;

creates a single element list.

n NULLhead

Storage allocationhead->next = malloc (sizeof(ELEMENT));head->next->d = ‘e’;head->next->next = NULL;

A second element is added.

nhead e NULL

Storage allocationhead->next=>next = malloc

(sizeof(ELEMENT));head->next->next->d = ‘e’;head->next->next-> = NULL;

We have a 3 element list pointed to by head.The list ends when next has the sentinel value NULL.

nhead e w NULL

List operations Create a list Count the elements Look up an element Concatenate two lists Insert an element Delete an element

Produce a list from a string (recursive version)#include “list.h”LINK StrToList (char s[]) {

LINK head ;if (s[0] == ‘\0’)

return NULL ;else {

head = malloc (sizeof(ELEMENT));head->d = s[0];head->next = StrToList (s+1);return head;

}}

#include “list.h”LINK SToL (char s[]) {

LINK head = NULL, tail;int i;if (s[0] != ‘\0’) {

head = malloc (sizeof(ELEMENT));head->d = s[0];tail = head;for (i=1; s[i] != ‘\0’; i++) { tail->next = malloc(sizeof(ELEMENT)); tail = tail->next; tail->d = s[i];}tail->next = NULL;

}return head;

}

list from a string (iterative version)

?Ahead

tail

1. A one-element list

2. A second element is attached

Ahead

tail

??

3. Updating the tail

Ahead

tail

?B

4. after assigning NULL

Ahead

tail

NULLB

/* Count a list recursively */int count (LINK head) {

if (head == NULL)return 0;

return 1+count(head->next);}

/* Count a list iteratively */int count (LINK head) {

int cnt = 0;for ( ; head != NULL; head=head->next)

++cnt;return cnt;

}

/* Print a List */void PrintList (LINK head) {

if (head == NULL)printf (“NULL”) ;

else {printf (“%c --> “, head->d) ;PrintList (head->next);

}}

/* Concatenate two Lists */

void concatenate (LINK ahead, LINK bhead) {

if (ahead->next == NULL)ahead->next = bhead ;

elseconcatenate (ahead->next, bhead);

}

Insertion Insertion in a list takes a fixed amount of

time once the position in the list is found.

A C

p2p1

Bq

Before Insertion

Insertion/* Inserting an element in a linked list. */void insert (LINK p1, LINK p2, LINK q) {

p1->next = q;q->next = p2;

}

A C

p2p1

Bq

After Insertion

DeletionBefore deletion

1 2 3

p

p->next = p->next->next;

After deletion

1 2 3

p garbage

DeletionBefore deletion

1 2 3p

q = p->next;p->next = p->next->next;

After deletion

1 2 3

p

qfree (q) ;

Delete a list and free memory/* Recursive deletion of a list */void delete_list (LINK head) {

if (head != NULL) {delete_list (head->next) ;free (head) ; /* Release

storage */}