Structures and Unions

Chapter 6

Structure

A structure is an aggregate data type Composed of two or more related variables called

member/field/element

struct tag-name{type member1;type member2;type member3;.type memberN;

} variable-list;

Structure Contd.

struct point{int x;int y;

}; Tag-name and variable-list are optional but one of them must

present Structure can be initialized

strcut point pt = {20, 30}; Member can be accessed via . operator

struct point pt;pt.x = 20;pt.y = 30;

Array of Structures

stuct student{

long id;

char name[20];

char dept;

};

struct student s[100];

Array of Structures Contd.

stuct {int id;char name[20];char dept[4];

} s[100];

scanf(“%d”, &s[0].id);scanf(“%s”, s[0].name);scanf(“%s”, s[0].dept);

Nested Structures

struct careof{char houseno[20];int roadno;char location[80];char phone[20];

};struct student_info{

int id;char name[80];struct careof address;

}s;

Nested Structures Contd.

Accessing the nested structure

scanf(“%s”, s.address.houseno);

scanf(“%d”, &s.address.roadno);

Union

union is a single piece of memory that is shared by two or more variables

union tag-name{type member1;type member2;type member3;.type memberN;

} variable-list;

Union Contd.

union u_type{

int i;

char c[2];

float d;

}sample;

1 2 3 4

d

c[0] c[1]

i

Suppose that a constant may be an int, a float, or a charstruct {

char name[20];

int utype;

union u_tag{

int ival;

float fval;

char cval;

}u;

}symtab[MAX];

Structure: Bit-fields

So far we cannot access at bit level Bit-fields are useful when you want to pack

information into the smallest possible space

struct b_type{

unsigned dept: 3;

unsigned stock: 2;

}var_list;

Structure: Bit-fields Contd.

The members are all either integer or unsigned integer For integer the left most bit will be regarded as sign bit

Can be assigned values confirming its limit A field may overlap a word boundary is

implementation-defined Fields may not be named (: and width) used for

padding Processor architecture dependent

File I/O

Chapter 9

Teach Yourself

by Herbert Schildt

Understanding Streams

Stream: the C I/O system supplies a consistent interface to the programmer for device I/O files A level of abstraction A logical interface

File: actual device providing I/O is called a file

Standard Streams

stdin stdout stderr

Types of Stream

Two types Text Binary

Text file Contains ASCII characters Some character translation So, no one-to-one correspondence between what is sent to

the stream and what is written to the file Binary file

May be used with any type of data No character translation So there is one-to-one correspondence

How to open a file?

FILE *fopen (char *filename, char *mode); Stdio.h Filename

path Mode

“r”, “w”, “a”, “rb”, “ab”, “r+” Consequence of those modes

How to close a file?

Int fclose (FILE *fp); In order to improve efficiency most file

system write data to disk one sector at a time Fclose flushes the buffer

How to know it is the end of file?Int feof (FILE *fp);

Int ferror (FILE *fp);

How to read and write a character?Int fgetc (FILE *fp);

Int fputc (int ch, FILE *fp);

And more on this later

An example: reading a text file and displaying it in the screenFILE *fp;

if ((fp = fopen (“a.txt”, “r”))==NULL){

//error and exit

}

while (! feof (fp)){

putchar (fgetc(fp));

}

fclose (fp);

Writing and reading strings and othersInt fputs (char *str, FILE *fp);

Int fgets (char *str, int num, FILE *fp);

Int fprintf (FILE *fp, format speci, variable(s));

Int fscanf (FILE *fp, format speci, address(es));

How to read/write in binary mode?size_t fread (void *buffer, size_t size, size_t num, FILE

*fp);

size_t fwrite (void *buffer, size_t size, size_t num, FILE *fp);

size_t: defined in stdio.h

unsigned long

Void pointer: pointer of any data types

An example: reading a text file and displaying it in the screenFILE *fp; char ch;if ((fp = fopen (“a.txt”, “rb”))==NULL){

//error and exit}while (! feof (fp)){

fread(&ch, sizeof (char), 1);putchar (ch);

}fclose (fp);

Writing an entire array

double d[10] = {10.2, 20.3,….};

fwrite (d, sizeof d, 1, fp); //entire

fread (d, sizeof (double), 5, fp); //only first five elements

Random Access

So far, we have seen write and read sequentially Beginning to end

Using another function we can access any point in a file Used only in binary mode (one-to-one)

int fseek (FILE *fp, long offset, int origin);

Origins are

SEEK_SET //seek from the start of file SEEK_CUR //seek from current location SEEK_END //seek from end of file

long ftell (FILE *fp);

Example: copy a file into another in reverse orderFILE *in, *out; char ch; long loc;fseek(in, 0L, SEEK_END);loc = ftell(in);loc = loc -1; //skip the end markerwhile (loc >= 0){

fseek(in, loc, SEEK_SET);ch = fgetc(in);fputc(ch, out);loc--;

}

Example??

10 double numbers are written in a file User want to access any number