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Pointers

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POINTERS

Pointers are variables that contain memory

addressesas their values.

A variable name directlyreferences a value.

A pointer indirectlyreferences a value.

Referencing a value through a pointer is

called indirection.

A pointer variable must be declared before it

can be used.

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Concept of Address and

Pointers Memory can be

conceptualized as

a linear set of data

locations. Variables reference

the contents of a

locations

Pointers have a

value of the

address of a given

location

Contents1

Contents11

Contents16

ADDR1ADDR2ADDR3ADDR4ADDR5

ADDR6***

ADDR11

**

ADDR16

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POINTERS

Examples of pointer declarations:

FILE *fptr;

int *a;

float *b;

char *c;

The asterisk, when used as above in thedeclaration, tells the compiler that the variable

is to be a pointer, and the type of data that the

pointer points to, but NOT the name of the

variable pointed to.

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POINTERS Consider the statements:

#include

int main ( )

{

FILE *fptr1 , *fptr2 ; /* Declare two file pointers */

int *aptr ; /* Declare a pointer to an int */

float *bptr ; /* Declare a pointer to a float */

int a ; /* Declare an int variable */

float b ; /* Declare a float variable */

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POINTERS

/* Then consider the statements: */

aptr = &a ;

bptr = &b ;fptr2 = fopen ( "my_out_file.dat" , "w" ) ;

fptr1 = fopen ( "my_in_file.dat" , "r" ) ;

if ( fptr1 != NULL )

{

fscanf ( fptr1, "%d%f" , aptr , bptr ) ;

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POINTERS

fprintf ( fptr2, "%d %d\n" , aptr , bptr ) ;

fprintf ( fptr2, "%d %f\n" , *aptr , *bptr ) ;

fprintf ( fptr2, "%d %f\n" , a , b ) ;

fprintf ( fptr2, "%d %d\n" , &a , &b ) ;return 0 ;

}

Assuming that the above is part of a program that runswithout error and the input file does open, what wouldbe printed to the file

By the first fprintf? By the second fprintf?

By the third fprintf? By the fourth fprintf?

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Use of & and *

When is & used?

When is * used?

& is "address operator" which gives or produces

the memory address of a data variable

* is "dereferencing operator" which provides the

contents in the memory location specified by a

pointer

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POINTERSaptr = &a ;

bptr = &b ;

fptr2 = fopen ( "my_out.dat" , "w" ) ;

fptr1 = fopen ( "my_in.dat" , "r" ) ;

if ( fptr1 != NULL )

{

fscanf (fptr1, "%d%f", aptr, bptr);

fprintf (fptr2, "%d %d\n", aptr, bptr ) ;

fprintf (fptr2, "%d %f\n", *aptr, *bptr ) ;

fprintf (fptr2, "%d %f\n", a , b ) ;

fprintf (fptr2, "%d %d\n", &a , &b ) ;

return 0 ;

}

}

/* input file */

5 6.75

/* output file */

1659178974 1659178976

5 6.750000

5 6.750000

1659178974 1659178976

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Pointer Arithmeticpointer+ number pointernumber

E.g.,pointer+ 1 adds 1 something to a pointer

char *p;char a;

char b;

p = &a;

p += 1;

int *p;int a;

int b;

p = &a;

p += 1;

In each, p now points to b(Assuming compiler doesnt

reorder variables in memory)

Adds 1*sizeof(char) tothe memory address

Adds 1*sizeof(int) tothe memory address

Pointer arithmetic should be used cautiously

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Arrays and Pointers

Dirty secret:Array pointer to the initial (0th) array

element

a[i] *(a+i)

An array is passed to a function as a

pointer

The array size is lost!

Usually bad style to interchange arrays

and pointers

Avoid pointer arithmetic!

Really int *array

int

foo(int array[],

unsigned int size)

{

array[size - 1]

}

int

main(void){

int a[10], b[5];

foo(a, 10) foo(b, 5)

}

Must explicitlypass the size

Passing arrays:

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Arrays and Pointersintfoo(int array[],

unsigned int size)

{

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

}

int

main(void)

{

int a[10], b[5]; foo(a, 10) foo(b, 5)

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

}

What does this print?

What does this print?

8

40

... because arrayis reallya pointer

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Arrays and Pointers

int i;

int array[10];

for (i = 0; i < 10; i++)

{

array[i] = ;

}

int *p;

int array[10];

for (p = array; p < &array[10]; p++)

{

*p = ;

}

These two blocks of code are functionally equivalent

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Pointers and Functions

Pointers can be used to pass addresses of variables to

called functions, thus allowing the called function to

alter the values stored there.

We looked earlier at a swap function that did not

change the values stored in the main program because

only the values were passed to the function swap.

This is known as "call by value".

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Pointers and Functions

If instead of passing the values of thevariables to the called function, we pass theiraddresses, so that the called function can

change the values stored in the callingroutine. This is known as "call by reference"since we are referencingthe variables.

The following shows the swap functionmodified from a "call by value" to a "call byreference". Note that the values are now

actually swapped when the control isreturned to main function.

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Pointers with Functions

(example)

#include

void swap ( int *a, int *b ) ;

int main ( )

{int a = 5, b = 6;

printf("a=%d b=%d\n",a,b) ;

swap (&a, &b) ;

printf("a=%d b=%d\n",a,b) ;

return 0 ;

}

void swap( int *a, int *b )

{

int temp;

temp= *a; *a= *b; *b = temp ;printf ("a=%d b=%d\n", *a, *b);

}

Results:

a=5 b=6a=6 b=5

a=6 b=5

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