esc101-lec21

7/31/2019 esc101-lec21

1/9

1

ESc101: Fundamentals of Com utin

2011-12-Monsoon Semester

Lecture #21, September 20, 2011

Please switch off your mobile phones.

Announcements

Mid-sem exam co ies will be returned in Wednesda Tutorial.

Lec-21 1Dheeraj Sanghi, CSE Dept., IIT Kanpur

ESc101, 2011-12-Monsoon

7/31/2019 esc101-lec21

2/9

2

Recap

Pointers

Variable declaration

Memory model and addresses

* and & operators

Arrays as pointers



Recap: Pointer

variable.

A pointer is denoted by the symbol *

int i; // i is a variable of type int

int *p; // p is a pointer to some integer

The address of a variable is denoted by the symbol &

p = &i; // p is now address of i (1 in our example)

p stores the address of i, i.e., &i

*p denotes the content pointed by p, i.e., i.

Lec-21 Dheeraj Sanghi, CSE Dept., IIT Kanpur


3

7/31/2019 esc101-lec21

3/9

3

Recap: Have we seen pointers before

Arrays are passed as address

Array names are essentially pointers

a is a pointer to the first element of the array, i.e., a[0]

*a is equivalent to a[0]

Since array elements are stored contiguously in memory

(a + i) is a pointer to a[i]

*(a + i) is equivalent to a[i]

Notice that pointer arithmetic is different from normal

arithmetic

When we add 1 to a, we are actually telling the computer to get

the address of the next integer (or whatever type array is of)



4

Recap: More on pointers and arrays

,

of (a + i) and accesses the variable in that address

If the computer has 4-byte integers, then (a+i) would

actually translate to a + 4i, where a is the address of a[0] No error checking on array boundaries is done

Therefore, a[-2], a [12], etc., become legal in C language

thou h ma be wron as er the lo ic of our ro ram

Pointers can be assigned array names:

int *p;

p = a;



5

7/31/2019 esc101-lec21

4/9

4

Recap: Swapping two integers

void swap (int *pa, int *pb)

{

int t = *pa;

* = *

*pb = t;

}

int main ( )

{

int x = 3, y = 4;

int *a, *b;

= = , , ,

a = &x; b = &y;

swap (a, b);

printf (x = %d, y = %d\n, x, y);

}



6

Pointer Operations

Integers can be added to or subtracted from a pointer

Adds (or subtracts) in units of size of type pointed to

int *p, *q, i;

p = &i; // suppose p is 2001

p++; // p is now 2005 (assuming int is 4 bytes)

q = p -2; // q is now 1997 Pointer cannot be multiplied or divided by an integer

Pointers can be compared (useful in arrays)

It signifies that pointer points to nothing

It is equivalent to 0

No variable can be allotted memory starting address 0



7

7/31/2019 esc101-lec21

5/9

5

Pointer Operations

#include

int main ( )

{

n , , p, q;

// Assigning constant to a pointer

p = 12345678; // Warning, may result in segmentation fault

printf (%d\n, *p); // when you de-reference this pointer

// Assigning NULL to a pointer

p = ;printf (%d\n, *p); // Will certainly result in segmentation fault

// Size of pointer on your machine

printf (%d\n, sizeof (p)); // Will print 8 on lab machines



8

Pointer Operations

// Printing a pointer

p = &i;

printf (%u\n, p);

ncrement ng, ecrement ng a po nter

q = p 1;

p++;

printf (%u\t%u\n, p, q);

// Comparing two pointers

printf (%d\n, (p < q));

// Storing values in memory areas pointed to by pointer variables

p = &i; q = &j;

*p = 7; *q = 8;

printf (%d\t%d\t%d\t%d\t\n, (*p), (*q), i, j);



9

7/31/2019 esc101-lec21

6/9

6

Pointer Operations

// Assigning a pointer to another pointer

p = q;

printf (%d\t%d\n, (*p), (*q));

*p = 4;

printf (%d\n, (*q));

// Subtracting a pointer from another pointer

= + 10;

printf (%d\n, (q p));

}



10

Passing arrays as pointers

#include

void read_array (int *p, int size)

{

{

printf (%d\t, *p);

p++; i++;

}

int i = 0;

while (i < size)

{

scanf (%d, p);

p++; i++;

}

}

printf (\n);

}

int main ( )

{

int a [ ] = {3, -2, 7, 19};

print_array (a, 4);

read arra (a, 4 ;

void print_array (int *p, int size)

{

int i = 0;

while (i < size)

_

print_array (a, 4);

}



11

7/31/2019 esc101-lec21

7/9

7

Passing pointers as arrays

The pointer must point to the correct array address

Pointer may point to the middle of the array, and

operations may start from there



12

Passing arrays as pointers

#include

void read_pointer (int a[ ], int size)

{

{

printf (%d\t, a[i]);

i++;

}

int i = 0;

while (i < size)

{

scanf (%d, &a[i]);

i++;

}

}

printf (\n);

}

int main ( )

{

int b [ ] = {3, -2, 7, 19};

int *p = b;

rint ointer ( , 4 ;

void print_pointer (int a[ ], int size)

{

int i = 0;

while (i < size)

_

print_pointer (p+1, 3);

read_pointer (p, 4);

print_pointer (p, 4);

}



13

7/31/2019 esc101-lec21

8/9

8

Sizes of variables

int *p; a[10];

printf (%d\n, sizeof (int));

printf (%d\n, sizeof (char));

printf (%d\n, sizeof (float));

printf (%d\n, sizeof (double));

pr nt n , s zeo ong nt ;printf (%d\n, sizeof (p));

printf (%d\n, sizeof (a));



14

String copy using arrays

void strcpy (char s [ ], char t [ ])

{

int i = 0;

while (t[i] != \0)

{

s[i] = t[i]; // copy

i++; // point to next element

}

s[i] = \0; // terminate s

}



15

7/31/2019 esc101-lec21

9/9

9

String copy using pointers

void strcpy (char *s, char *t)

{

while (*t != \0)

{

*s = *t; // copy

s++; // point to next element

t++; // point to next element

}

*s = \0; // terminate s

}



16

Any Questions?



esc101-lec21

Documents