Pointers+(2)

C++ Spring 2000 Arrays

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Pointers in C

Pointers

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A pointer is a variable that holds the address of something else.

...

...

MEMORY

012345

813458134681347

Address

int foo;int *x;

foo = 123;x = &foo;

foo

x

123

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Pointers

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Example: int *x=i;

x is a pointer to an integer.

“the int x points to”

Pointers4

In C you can get the address of a variable with the “&” operator.

...

...

MEMORY

012345

Address

&foo means “the address of foo”

foo

int foo;

foo = 123;x = &foo; 123

Assigning a value to a dereferenced pointer

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A pointer must have a value before you can dereference it (follow the pointer).

int *x;*x=3;

int foo;int *x;x = &foo;*x=3;

ERROR!!!

x doesn’t point to anything!!!

this is fine

x points to foo

Pointers to anything

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int *x;

int **y;

double *z;

x some int

some *int some inty

z some double

Pointer arithmetic

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Integer math operations can be used with pointers.

If you increment a pointer, it will be increased by the size of whatever it points to.

int a[5];

a[0] a[1] a[2] a[3] a[4]

int *ptr = a;

*ptr

*(ptr+2)*(ptr+4)

Operations on Pointers

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Can be done, Valid Operations Addition/Subtraction of a number to/from pointer Subtraction of two pointers Comparison of two pointers Assignment of a pointer to another pointer

Can NOT be done, Invalid Operations Addition of two pointers Multiplication/Division of a pointer with a number

Pointer arithmetic

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You can use the integer x points to in a C expression like this:

y = *x + 17;

*x = *x +1;

Q&A

Q. Are the expression *ptr++ and ++*ptr same?

A.No. *ptr++ increment the pointer and not the value pointed by it, whereas ++*ptr increments the value pointed to by ptr.

Q. Can you write any another expression which does the same job as ++*ptr?

A. (*ptr)++

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Pointers and Arrays

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An array name is basically a const pointer.You can use the [] operator with a pointer:

int *x;

int a[10];

x = &a[2];x is “the address of a[2] ”

Printing an array using Pointer

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void print_array(int a[], int len) {for (int i=0;i<len;i++)

cout << "[" << i << "] = " << a[i] << endl;

}

void print_array(int *a, int len) {

for (int i=0;i<len;i++)

cout << "[" << i << "] = "

<< *a++ << endl;

}

pointer version

array version

Q&A

What would be the output of the following?main()

{int arr[]={12,13,14,15,16};printf(“%d %d” %d”, sizeof(arr),sizeof(*arr),sizeof(arr[0])); }

A. 10 2 2B. 2 2 2C. 10 10 10D. 2 10 2

Answer: A

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Q&A

Q. What would be equivalent expression for referring the same element as a[i][j][k][l]?

A. *(*(*(*(a+i)+j)+k)+l)

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Pointers and Functions

Call by Value

main( ){

int a = 4;myFunc (a);

}myFunc (int b){

b = b * 2;}

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Address (2 bytes)65514

6551565516655176551865519655206552165522655236552465525

a 4

b 48

main

myFunc

Call by Reference

main( ){

int a = 4;myFunc (&a);

}myFunc (int *b){

*b = *b * 2;}

17Data Word Size 1 byte (assume)Address Word Size

(2 bytes) 655146551565516655176551865519655206552165522655236552465525

a 4 main

myFunc

8

b65520

Passing pointers as parameters

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void swap( int *x, int *y) {

int temp;

temp = *x;

*x = *y;

*y = temp;

}

Pointer Parameters

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Pointers are passed by value (the value of a pointer is the address it holds).

If we change what the pointer points to the caller will see the change.

If we change the pointer itself, the caller won't see the change (we get a copy of the pointer)

Q&A

main(){

int a[3][4]={1,2,3,4,5,6,7,8,7,8,9,0 };

int *ptr;ptr=&a[0][0];func(&ptr); }

void func(int **p){ printf("%d",**p);}

Answer: 1

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Pointers and String

Declared and initialized as: char str[6]=“c prog”; // compiler insert null

‘\0’ char at the end

Or char *str=“c prog”; //stores address in the

pointer variable

Or char *str; str=“hello”; //Not a String copy

Display the content as: printf(“%s”,str);

Or puts(str);

Pointers and String

Length of string char name[20]; char *str;

str=name;while(ptr!=‘\0’)// condition true until end of the string reached, then ptr

holds the address of null character.

So,int length=ptr-str;

Pointers to Structures

struct part {float price ;char name [10] ;

} ;struct part *p , thing;p = &thing;/* The following three statements are equivalent */thing.price = 50;(*p).price = 50; /* () around *p is needed */p -> price = 50;

Pointer as Structure Memberstruct node{

int data;struct node *next;

};struct node a,b,c;a.next = &b;b.next = &c;c.next = NULL;

NULL

a b c

a.data = 1;a.next->data = 2;/* b.data =2 */a.next->next->data = 3;/* c.data = 3 */c.next = (struct node *)

malloc(sizeof(struct node));……

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