Intro to C++ Powerpoint on Pointers.

1. Unit 6
Week 10 & 11
Pointers
and
Dynamic Arrays

2. Unit Objectives
 Introducing Pointers
 Pointer Variables
 Basic Memory Management
 Static, Dynamic, and Automatic
Variables
 Pointer Arithmetic
 Dynamic Arrays
 Array Variables and Pointer Variables

3. 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.

4. 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.

5. 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. 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 statement
yptr = &y;
assigns the address of the variable y to pointer variable
yptr. Variable yptr is then said to “point to” y.

7. 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”.

8. 6.1.2 (Cont..)
 Consider the following memory locations
int y =5; int *yptr;
5 100
100 120
y yptr
102 122
104 124
106

9. 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.

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

11. 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;
}

12. 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

13. 6.1.3 References and Pointers
 There are 3 ways in C++ to pass arguments to a
function
2. call-by-value
3. call-by-reference with reference argument
4. call-by-reference with pointer argument
The 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.

14. 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.

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

16. 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 ( )

17. 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)

18. 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 ];

19. 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;
}

20. Example (Cont..)
void printarray ( int *ptrgrade, int MAX)
{
int constant = 10;
for (int i = 0; i < MAX; i++)
*ptrgrade++ += constant;
}

21. 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

22. 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”;

23. Example (Cont..)
printc (string);
cout << endl;
return 0;
}
void printc (const char *sptr)
{ // sptr is “read-only” pointer
for ( ; *sptr !=’0’; sptr ++) // no initialization
cout << *sptr;
}

24. 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.

25. Example
// converting lowercase letter to uppercase letters
// using non-constant pointer
#include <iostream>
#include <ctype>
using namespace std;
void convert ( char *);
int main ( )
{

26. 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)

27. Example (Cont..)
{
while (*sptr !=’0’)
{
if ( *sptr >= ’a’ && *sptr <= ’z’)
*sptr = toupper ( *sptr); // convert to uppercase
++sptr; // move sptr address to the next
character
}
}

28. 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

29. 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 -= )

30. 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

31. 6.3 (Cont..)
For example, the statement
ptrarr +=2 =100+2*2=104
ptrarr -=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

32. 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

33. 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;
}

34. 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

35. 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.

36. 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.

37. 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] = “ “;

38. 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;
}

39. 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

40. 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.

41. 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”;

42. Example (Cont..)
cout << ”strcmp(s1, s2) = “ << strcmp (s1, s2)
<< "n strcmp (s1, s3) = " << strcmp (s1, s3)
<< ”n strcmp (s3, s1) = “ << strcmp (s3, s1);
cout << ”nn 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;

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

44. 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)

45. 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

46. 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 ( )
{

47. 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++)
{

48. Example (Cont..)
for ( int j= 0; j< 3; j++)
cout << *(*(c+i) +j) << setw(3);
cout << endl;
cout << ”nn”;
}
return 0;
}

49. 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);
}

50. 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.

51. 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

52. 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”};

53. 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):”;

54. 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;
}

55. 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

56. 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”};

57. 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)
{

58. Example (Cont..)
n = (n%12)/3;
cout << ”The month entered is a “ << *(ys +
n)
<<” month.” <<endl;
}

59. 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.