Unit 6 pointers

Unit 6Unit 6Week 10 & 11Week 10 & 11

Pointers Pointers and and

Dynamic ArraysDynamic Arrays

Unit Objectives

Introducing Pointers Pointer Variables Basic Memory Management Static, Dynamic, and Automatic

Variables Pointer Arithmetic

Dynamic Arrays Array Variables and Pointer Variables

6.1 Pointers

Pointers are symbolic representation of addresses. Pointers enable programs to simulate call-by-reference and to create and manipulate dynamic data structures.

6.1.1 Pointer Variable Declaration and Initialization

Pointer variables contain memory addresses as their values.

A variable directly references a value, and a pointer indirectly references a value.

Referencing a value through a pointer is called indirection.

The is called indirection operator.

6.1.1 (Cont..)

Pointer are declared same as variable before you can use them as follows:

int xptr, yptr, count;Declares the variable xptr, a pointer to an

integer value or double dptr;Declares the variable dptr, a pointer to a

double value.

6.1.2 Pointer Operations The & or address operator, is a unary operator that returns

the address of its operand. For example,

int y =5;int *xptr, *yptr, Z;and statementyptr = &y; assigns the address of the variable y to pointer variable yptr. Variable yptr is then said to “point to” y.

6.1.2 (Cont..)

The statement

z = *yptr //will return to content of where yptr “point to” z = 5

The operator referred to a dereferencing operator which returns the content where a pointer variable “points to”.

6.1.2 (Cont..)

Consider the following memory locations

int y =5; int *yptr;

5 100100y

102

104

106

120yptr

122

124

6.1.2 (Cont..)

Note: You must assign an address to a pointer.

Dereferencing a pointer that has not been properly initialized, could cause a fatal execution time error, or it could cause accidentally modify important data and allow the program to run to completion providing incorrect results.

An attempt to dereference a non-pointer is a syntax error.

Example// showing pointer application#include <iostream>using namespace std;int main ( ){ int a; int *aptr; // aptr is a pointer to an integer a = 7;

Example (Cont..)

aptr = &a; // aptr set to address to a

cout << ”The address of a is” << &a

<< ”\n The value of aptr is” << aptr;

cout << ”\n The value of a is” << a

<< “\n The value of *aptr is << *aptr << endl;

return 0;

}

Example (Cont..)

The output of the above program is:

The address of a is 0X0064FDF4

The value of aptr is 0X0064FDF4

The value of a is 7

The value of *aptr is 7

6.1.3 References and Pointers

There are 3 ways in C++ to pass arguments to a function

1. call-by-value 2. call-by-reference with reference argument 3. call-by-reference with pointer argumentThe differences between the last two are: The address named as a reference can not be

altered. But because pointer is a variable, the address in the pointer can be changed.

6.1.3 (Cont..)

References do not require the indirection operator to locate the final value being accessed, whereas pointers do. References are automatically or implicitly dereferenced, whereas pointers must be explicitly dereferenced.

References provide a simpler notational interface and are usually preferred. Pointers are used for dynamically allocating new sections of memory for additional variables as a program is running or using alternative to array notation.

Example

// Find a cube of a variable with a pointer argument#include <iostream>using namespace std;void cube (int *); // prototypeint main ( ){int number = 5;cout << ”The original value of number is” << number <<

endl;cube (&number);

Example (Cont..)

cout << ”\n The new value of number is “<< number <<endl;

return 0;}void cube ( int *nptr){ *nptr =(*nptr)*(*nptr)*(*nptr); }

\\note that there is no return from function cube to main ( )

6.2 Array Name as Pointers

An array name is a pointer constant. The value of the pointer constant is the

address of the first element. Thus, if val is the name of an array, val and

&val [ 0 ] can be used interchangeably.

Creating an array also creates a pointer

int grade [ 2 ] grade [ 0 ] grade [ 1 ]

*grade *(grade + 1)

Example

// pointer to arrays#include <iostream>using namespace std;void printarray ( int*, int)int main ( ){ const int arraysize = 5; int grade [arraysize] = { 93, 88, 94, 91, 82}; int *ptr; ptr = grade; // or ptr=&grade [ 0 ];

Example (Cont..)

printarray (grade, arraysize);

for ( int i = 0; i < 5; i++)

cout << ”\n Element “<< i <<” is “ << *ptr++;

// or *(grade+i)

cout << endl;

return 0;

}

Example (Cont..)

void printarray ( int *ptrgrade, int MAX)

{

int constant = 10;

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

*ptrgrade++ += constant;

}

Example (Cont..)

The output of the above program is:

Element 0 is 103

Element 1 is 98

Element 2 is 104

Element 3 is 101

Element 4 is 92

Example

// Example of pointers and strings and const data

#include <iostream>

using namespace std;

void printc (const char * );

int main ( )

{

char string [ ] = “print characters of a string”;

cout << ”The string is: \n”;

Example (Cont..)

printc (string);cout << endl;return 0;}void printc (const char *sptr){ // sptr is “read-only” pointerfor ( ; *sptr !=’\0’; sptr ++) // no initialization

cout << *sptr;}

6.2 (Cont..)

If a value does not ( or should not) change in the body of a function to which it is passed, the parameter should be declared const to ensure that it is not accidentally modified.

Example

// converting lowercase letter to uppercase letters

// using non-constant pointer

#include <iostream>

#include <ctype>

using namespace std;

void convert ( char *);

int main ( )

{

Example (Cont..)

char string [ ] = “characters and $32.98”;

cout << ”the string before conversion is: ” << string;

convert (string);

cout << ”\n the string after conversion is: “ << string << endl;

return 0;

}

void convert ( char *sptr)

Example (Cont..)

{while (*sptr !=’\0’) { if ( *sptr >= ’a’ && *sptr <= ’z’)*sptr = toupper ( *sptr); // convert to uppercase

++sptr; // move sptr address to the next character

}}

Example (Cont..)

The output of the above program is:

the string before conversion is: characters and $ 32.98

the string after conversion is: CHARACTERS AND $ 32.98

6.3 Pointer Expressions and Pointer Arithmetic

A limited set of arithmetic operations may be performed on pointers. A pointer may be incremented ( ++ ) decremented ( -- ) an integer may be added to a pointer ( + or - ) an integer may be subtracted from a pointer

( - or -= )

6.3 (Cont..)

Assume that array int arr [5] has been declared and its first element is at location 100 in memory. Assume pointer int *ptrarr has been initialized to the beginning address of arr, i.e.; 100.

100 102 104 106 108

6.3 (Cont..)

For example, the statementptrarr +=2 =100+2*2=104ptrarr -=1 =104-2*1=102++ptrarr = 104

Finally, pointer variables may be subtracted from another. For example, if ptrarr 2 = 100 and ptrarr1 = 102 then

x = ptrarr2 –ptrarr1=2

6.3 (Cont..) Pointer arithmetic is meaningless unless performed

on an array.// using strcpy and strncpy#include <iostream>#include <string>using namespace std;int main ( ){ char x [ ] = “Happy Birthday to you”; char y [ 25 ], z [15 ]; cout << ”The string in array x is:”<< x

Example (Cont..)

<< ”\n The string in array y is: “ << strcpy(y, x)

<< ”\n”;

strncpy (z, x, 14); // does not copy null character

z [14] = ‘\0’;

cout << ”The string in array z is: “ << z << endl;

return 0;

}

Example (Cont..)

The output of the above program is:

The string in array x is: Happy Birthday to you

The string in array y is: Happy Birthday to you

The string in array z is: Happy Birthday

6.3 (Cont..)

Functions strcpy ( ) copies its second argument (a string) into its first argument which must be large enough to store the string and the terminating null character.

6.3 (Cont..)

strncpy ( ) is equivalent to strcpy ( ) except that strncpy ( ) specifies the number of characters to be copied from the string into the array. Note that the function strncpy ( ) does not necessarily copy the terminating null character of its second argument – a terminating null character is written only if the number of characters to be copied is at least one more than the length of the string.

Example// using strcat and strncat#include <iostream>#include <string>using namespace std;int main ( ){ char s1[20 ]=”Happy ”;char s2 [ ] = “New Year”;char s3 [40] = “ “;

Example (Cont..)

cout << ”s1= “ << s1 << ”\n s2 = “ << s2;

cout << ”\n strcat ( s1, s2) = “ << strcat (s1, s2);

cout << ”\n strncat (s3, s1, 6) = “<< strncat (s3, s1, 6);

cout << ”\n strcat (s3, s1,) = ” << strcat(s3, s1) << endl;

return 0;

}

Example (Cont..)

The output of the above program is:

s1 = Happy

s2 = New Year

strcat (s1, s2) = Happy New Year

strncat (s3, s1, 6) = Happy

strcat (s3, s1) = Happy Happy New Year

6.3 (Cont..) Function strncat ( ) appends a specified number of

characters from the second string to the first string. A terminating null character is appended to the result.

Assuming that strcmp ( ) and strncmp ( ) return 1 when their argument are equal is a logic error. Both functions return 0 ( C++’s false value ) for equality. Therefore, when testing two strings for equality, the result of the strcmp ( ) or strncmp ( ) function should be compared with 0 to determine if the strings are equal.

Example

// using strcmp and strncmp#include <iostream>#include <string>using namespace std;int main ( ){

char *s1 = “Happy New Year”;char *s2 = “Happy New Year”;char *s3 = “Happy Holidays”;

Example (Cont..)

cout << ”strcmp(s1, s2) = “ << strcmp (s1, s2)

<< "\n strcmp (s1, s3) = " << strcmp (s1, s3)

<< ”\n strcmp (s3, s1) = “ << strcmp (s3, s1);

 cout << ”\n\n strncmp (s1, s3, 6) = “ << strncmp (s1, s3, 6)

<< ”\n strncmp (s1, s3, 7 ) = “ << strncmp (s1,s3, 7)

<< ”\n strncmp (s3, s1, 7) = “ << strncmp (s3, s1, 7)

<<endl;

Example (Cont..)

return 0;}The output of the above program is:strcmp (s1, s2) = 0strcmp (s1, s3) = 1strcmp (s3, s1) = -1 strncmp (s1, s3, 6) = 0strncmp (s1, s3, 7) =1strncmp (s3, s1, 7) =1

6.4 Advanced Pointer Notation

Here first, we consider pointer notation for the two-dimensional numeric arrays. consider the following declaration

int nums[2][3] = {{16,18,20},{25,26,27}}; In general,

nums[ i ][ j ] is equivalent to *(*(nums+i)+j)

6.4 (Cont..)

More specifically

Pointer Notation Array Notation Value

*(*nums) nums[ 0 ] [ 0 ] 16

*(*nums + 1) nums [ 0 ] [ 1 ] 18

*(*nums + 2) nums [ 0 ] [ 2 ] 20

*(*(nums + 1)) nums [ 1 ] [ 0 ] 25

*(*(nums + 1) +1) nums [ 1 ] [ 1 ] 26

*(*(nums + 1) +2) nums [ 1 ] [ 2 ] 27

Example

Write a program that initialize three two-dimensional array of your choice and then call a user-defined function to add these two arrays and print the result in the main ( ).

#include <iostream>#include <iomanip>using namespace std;void addarray (int(*pta)[3],int(*ptb)[3],int(*ptc)[3]);int main ( ){

Example (Cont..)

int a [2] [3] = {{1,2,3},{5,3,2}};

int b [2] [3] = {{1,3,5},{2,0,8}};

static int c [ 2 ] [ 3 ];

addarray ( a, b, c);

 for (int i=0; i < 2; i++)

{

Example (Cont..)

for ( int j= 0; j< 3; j++)

cout << *(*(c+i) +j) << setw(3);

cout << endl;

  cout << ”\n\n”;

}

return 0;

}

Example (Cont..)

void addarray ( int (*pta)[ 3 ], int *(ptb)[ 3 ], int *(ptc)[ 3 ])

{ for (int i =0; i<2; i++)

for(int j=0; j<3; j++) *(*(ptc + i)+j) = *(*(pta + i)+j) +

*(*(ptb + i)+j);}

6.5 Pointer Arrays

The declaration of an array of character pointer is an extremely useful extension to single string declaration.

For example, the declarations

char *seasons [ 4 ];

create an array of four elements, where each element is a pointer to a character.

6.5 (Cont..) As individual pointers, each pointer can be assigned

to point to a string using string assignment statements.

Thus, the statements

seasons [ 0 ] = “Winter”;

seasons [ 1 ] = “Spring”;

seasons [ 2 ] = “Summer”;

seasons [ 3 ] = “Fall”; // sting lengths may differ

6.5.1 Initializing arrays of pointers to strings

The initialization of the seasons array can also be incorporated directly within the definition of the array as follows:

char *seasons [ 4 ] = {“Winter”, “Spring”, “Summer”, “Fall”};

Example

// string and pointer example#include < iostream>using namespace std;int main ( ){int n;char *seasons [ ] = { “Winter”, “Spring”, “Summer”,

“Fall”};cout << ”\n Enter a month (use 1 for Jan, 2 for Feb,

etc):”;

Example (Cont..)

cin >> n;

n = (n % 12) / 3; // create the correct subscript

cout << ”The month entered is a << seasons [ n ]// or * (seasons + n)

<< ”month” << endl;

return 0;

}

Example (Cont..)

The output of the above program is:

Enter a month ( use 1 for Jan 2 for Feb, etc.) : 12

The month entered is a Winter month

Example

The following example is the modification to previous example in this case a function is used to print seasons.

#include <iostream>using namespace std;void st_pt ( char *ys [ ], int n);int main ( ){ int n; char *seasons [ ] = { “Winter”, “Spring”, “Summer”,

“Fall”};

Example (Cont..)

cout << ”\n Enter a month ( use 1 for Jan, etc.) : “;

cin >> n;

st_pt (seasons, n);

return 0;

}

void st_pt (char *ys [ ], int n)

{

Example (Cont..)

n = (n%12)/3;

cout << ”The month entered is a “ << *(ys + n)

<<” month.” <<endl;

}

6.5.1 (Cont..)

Pointers are exceptionally useful in constructing string-handling functions when pointer notation is used in place of subscripts to access individual characters in a string, the resulting statements are both more compact and more efficient.