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ESc101: Fundamentals of Com utin

2011-12-Monsoon Semester

Lecture #24, September 27, 2011

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Lec-24 1Dheeraj Sanghi, CSE Dept., IIT Kanpur

ESc101, 2011-12-Monsoon

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Recap

Multi-dimensional arrays

Array of pointers

Dynamic memory allocation

File input/output



Structures

Structures are customized data types

It is declared using the keyword struct

struct point

{

double x;

double y;

}

struct point is a structure having two variables x and y

Variables in a structure are called members

struct point p;

Structures can be initialized during declaration

struct point p = {4.0, -3.0};

Lec-24 Dheeraj Sanghi, CSE Dept., IIT Kanpur


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Members

Members of a structure can be explicitly assigned values

. notation to access members

structure variable.member name_ _

p.x = 4.0;

p.y = -3.0

Members behave just like ordinary variables

Size of a structure is the combined size of its members

A bit more complex could be more than this

Example: Size ofstruct point is 8 + 8 = 1 bytes



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Functions returning pointers

A function can return a structure

struct point create_point (float x, float y)

{ struct point p;

p.x = x;

p.y = y;

return p;

}

This can be used to initialize structures

q = create_point (4.0, -3.0);

opy ng can a so e one s mp y y: q = p;

A structure is different from array!

Structures cannot be compared

if (q == p) // error



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Passing structures to functions

Since structures are variables, they can be passed to functions

void modify (struct point p, double c, double d)

{

p.x = c;

p.y = d;

}

The following code prints 5.0 and -3.0

struct point q = {5.0, -3.0};

modify (q, 9.0, 1.0);

printf(%lf %lf\n, q.x, q.y);



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Pointers to structures

A pointer to a structure can be defined

struct point *ptr, p; ptr = &p;

When a pointer to structure is passed to a function, modifying the

elements of the structure inside the function becomes ermanent

void modify (struct point *p, double c, double d)

{ p->x = c;

p->y = d;

}

-- > notation to access members using pointers

structure_pointer->member_name

*- .

The following code prints 9.0 and 1.0

struct point q = {5.0, -3.0};

modify (&q, 9.0, 1.0);

printf(%lf %lf\n, q.x, q.y);



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Structure operations 1

#include

#include

struct point {

double y;

}; // defining a structure

struct point new_point (double c, double d) // structure as return value

{ struct point p;

p.x = c;

p.y = d;

}

double distance (struct point a, struct point b) // structure as parameter

{ return (sqrt (pow (a.x b.x, 2) + pow (a.y b.y), 2));

}



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void modify_wrong (struct point p, double c, double d)

{ p.x = c;

p.y = d;

} // modifications are temporary

vo mo y_r ght (struct po nt p, ouble c, ouble

{ p->x = c;

p->y = d;

} // modifications are permanentint main ( )

{ struct point p, q, s; // declaring structure variables

struct point t = {9.0, -5.0}; // initialization during declaration

double d;

struct point *ptr;

printf(%lf %lf\n, t.x, t.y); // Accessing individual members

q.x = 4.0; // modifying members in structure notation

q.y = -3.0;



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p = new_point (7.0, -1.0); // Setting structure through functions return value

d = distance (p, q); // passing structures as parameters

printf(Distance = %lf\n, d);

// s = {6.0, 2.0}; // error

modify_wrong (p, 12.0, 8.0);

printf(%lf %lf\n, p.x, p.y); // No modification would have happened

ptr = &p;

modify_right (ptr, 12.0, 8.0); // Now p would have been modified

printf(%lf %lf %lf %lf\n, p.x, p.y, ptr->x, ptr->y);

// if (q == p) // error

}



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Nested structures

A structure can have another structure as its member

struct line {

struct point p;

struct point q;

};

struct line l;

Value x of point p of variable l of type struct line can be

accessed as:

l.p.x

The . operator has left-to-right associativity



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Array of structures

struct point t [3];

An individual structure is accessed as t[0], etc.

A member of a structure is accessed as t[1].x, etc.

All operations allowed on normal arrays are allowed on

arrays of structures

Just like any other array, it is equivalent to a pointer tostructure



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Pointer in a structure

A structure can have a pointer as its member

struct student {

int roll;

char *name;

};

struct student s;

Declaring a variable of type struct student just declares the pointer

name it does not allocate space for it

strc (s.name Sudhir // wron

Memory for name has to be allocated explicitly using malloc

s.name = (char *) malloc (30 * sizeof(char));

strcpy (s.name, Sudhir); // right



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Any Questions?



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