This document provides an overview of pointers and dynamic arrays in C++. It discusses pointer variables, memory management, pointer arithmetic, array variables as pointers, functions for manipulating strings like strcpy and strcmp, and advanced pointer notation for multi-dimensional arrays. Code examples are provided to demonstrate concepts like passing pointers to functions, dereferencing pointers, pointer arithmetic on arrays, and using string functions. The overall objective is to introduce pointers and how they enable dynamic memory allocation and manipulation of data structures in C++.

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.