FP 201 - Unit 6

FUNDAMENTALS OF
C++ PROGRAMMING
6.0 POINTER (PENUDING)

6.1 Understand the concept of pointer

Learning Outcome
 At the end of the class, student should be able
to:
 Define pointer and explain its function
 Declare pointer
 Explain new and delete operators

What is pointer?
 Pointer is the memory address of a variable.
 Pointer is a variables that are used to store the
addresses of other variables.

What is pointer?

Memory Blocks

0x8f8dfff12 0x8f8dfff13 0x8f8dfff14

Address of Memory Blocks

Declaring Pointer

 Syntax:
data_type *pointer_name;

Example
double *p;

The * and & operators
 * operator
 Reference/dereference operator

 Produce the variable to which it point

 Refer to “value pointed by”

 Pointers are said to "point to"

 Example of usage:
 double *p; // declare pointer to double variable

 void func(int *p) // declare p to be a pointer value
parameter

The * and & operators
 & operator
 Address-of operator
 Produce address of variable
 Example:

double *p, v;
p = &v; // p point to address of v

Pointer Operator

int *my_pointer;
int my_variable;
my_pointer = &my_variable;

my_pointer =
&my_variable

Pointer Memory location
variables Address
operator

How it will look like?
#include<iostream>
using namespace std;
void main()
{
int i;
int *j;
j = &i; //pointer to i
i=4;

cout<<"i = " <<i<<endl;
cout<<"j = " <<j<<endl;
}

j = &i; //pointer to address i
i=4;

i 4

j 0012FF60

What is the output???
int i;
int *j;
j = &i; //pointer to i
i=4;

cout<<"i = " <<i<<endl;
cout<<"j = " <<*j<<endl;

j = &i; //pointer to address i
i=4;

i 4

j 0012FF60

cout<<"j = " <<*j

j 4

#include<iostream>  Output:
void main()
v1=42
{
int *p1, v1; p1=42

v1 = 0;  Why?
p1 = &v1; //pointer to v1 - As long as p1
*p1 = 42; //value pointed by p1 = 42 contains address to
v1, then both refer
cout<<"v1="<<v1<<endl; to the same
cout<<"p1="<<*p1<<endl; variable
}

v1 = 0;

v1 0

p1 = &v1;

&v
p1
1

*p1 = 42; //value pointed by p1 = 42

p1 42

v1 42

See difference?
p1 = p2;

p1 8 p1 8

p2 9 p2 9

BEFORE AFTER
*p1 = *p2;

p1 8 p1 9

p2 9 p2 9

Pointer Arithmetic
 Pointer can only used two arithmetic operation
addition and subtraction.

int a, int *p; W hat happen is:
p=&a;  pointer move over bytes
p=p+2;  not adding 2 to value of a
 it point to the last 2 x int of the integer
a

Position of pointer p Position of pointer p after
before operation p+2 operation

Pointer Arithmetic
 Example: its equivalent to:
#include<iostream>
using namespace std; #include<iostream>
void main() using namespace std;
{ void main()
int arraysize=4; {
int d []={1,2,3,4}; int arraysize=4;
for (int i = 0; i < arraysize; + int d []={1,2,3,4};
+i) for (int i = 0; i < arraysize; ++i)
cout << *(d + i) << " "; cout << d[i] << " ";
} }

Pointer & character string
 Use standard function strcmp() uses two pointers to compare
two strings:

strcmp (char *s1, char *s2)
{
while (*s1)
if (*s1 - *s2)
return *s1 - *s2; //if the string are not equal
else
{ s1++;
s2++;
}
return 0;
}

new and delete operator
 new operator creates new dynamic variable of
specific type
 It also return a pointer that point to this new
variable
 Eg:
int *n;
n = new int(17); // initialize *n to 17

int *p1, *p2;
p1 ?

p2 ?
p1 = new int;
*p1 = 42;
p1

p2 ?

p1
42

p2 ?

p2 = p1; p2 = new int;
p1 ?
p1
42
p2 p2 53

*p2 = 53; *p1 = 88;
p1 p1 88
53
p2 p2 53

 delete operator eliminates dynamic variable
 It also returns memory that the dynamic
variable occupied in freestore.
 Eg:
delete p;
// p will be undefined
 Good practice:
 Everytime you delete a pointer variable put it to
NULL to ensure it does not become dangling
pointer!

#include<iostream>
void main()
{
int *p1;
p1 = new int;
*p1 = 10;
cout << "*p1 t = "<< *p1 << endl;
delete p1;
cout << "*p1 t = "<< *p1 << endl;
}

In Class Exercise 1
 What is the output?
p1 = p2;
int *p1, *p2;
cout << *p1 << "t"<< *p2 << endl;
p1 = new int;
*p1 = 30;
p2 = new int;
cout << *p1 << "t"<< *p2 << endl;
*p1 = *p2;
*p1 = 10;
cout << *p1 << "t"<< *p2 << endl;
*p2 = 20;
cout << *p1 << "t"<< *p2 << endl;

int *p1, *p2;
p1 ?

p2 ?
p1 = new int;
p2 = new int;
p1

p2

*p1 = 10;
*p2 = 20; p1 10

p2 20

p1 = p2;
p1 10

p2 20

*p1 = 30; p1 10

p2 30

*p1 = *p2; p1 30

p2 30

In Class Exercise 2
 What is the different between the following
variable?
int *intPtr1, intPtr2;

 Write a declaration for a pointer variable
named my_new_ptr that points to dynamic
variable of type double.

6.2 Illustrate the relationship between
pointer and array

Learning Outcome
to:
 Identify relationship between pointer and array
 Write program using pointer and array

Pointer & Array
 Array is a collection of similar type of data
 Variable in an array can store memory address
instead of variable value
 Dynamic array is an array whose size is
determined during run-time.
 It is created using new operator.
 Eg:
double *new_array; //point to 1st index in the
array
new_array = new double[10] //to allocate

Pointer in Array: Example 1
#include<iostream>
void main()
{
int d []={1,2,3,4};
int *p1;
p1=d;
for (int i = 0; i < 4; ++i)
cout << *(p1 + i) << " ";
}

Pointer in Array: Example 2
#include <iostream>
void main()
{
int array_size;
cout << “Enter array size: “;
cin >> array_size;
int* my_dyn_array = new int [array_size];

cout << “Enter ” << array_size << “ number ” << endl;
for (int i = 0; i < array_size; ++i)
cin >> my_dyn_array[i];
}

Pointer and Array
 An array declared without specifying its size
can be assume as a pointer to the address of
the first element in the array.
 Eg:
int a[] = {2, 4, 6, 8, 10};
 To iterate to the next element, add operation is
used.

Pointer and Array: Example 1
#include <iostream>
int main()
{
char susun[] = "Selamat belajar";
char * pt_str;
pt_str = susun; / point to first element
/
cout << "Turutan sblm perubahan " << susun;
cout << "
nSebelum perubahan, kandungan susun[5] : " <<susun[5];
* (pt_str+5) = 'C'; /change the 6th element
/
cout << "
nSelepas perubahan, kandungan susun[5] : " <<susun[5];
cout << "
nTurutan slps perubahan" << susun;
return 0;
}

Pointer and Array: Example 2
#include <iostream>
int main()
{
int senarai[] = {1,2,3,4,5};
int * point;
point=senarai; / point to first element
/
cout << "
nTurutan sblm perubahan " << senarai[0] << " " << senarai[1];
cout << " " << senarai[2] << " " << senarai[3] << " " << senarai[4];
cout << "
nSebelum perubahan, kandungan senarai[2] : " <<senarai[2];
* (point+2)= 7; / change the 3rd element
/
cout << "
nSelepas perubahan, kandungan senarai[2] : " <<senarai[2];
cout << "
nTurutan slps perubahan " << senarai[0] << " " << senarai[1];
cout << " " << senarai[2] << " " << senarai[3] << " " << senarai[4];
cout << endl;
return 0;
}

In Class Exercise 3
 Write a declaration for pointer variable named
char_ptr that will be used to point to a dynamic
array of type char.
 What is the output for the following code?
int c[3] = {2, 3, 4};
int arraysize = 3, * p;
p = c;
for (int i = 0; i < arraysize; ++i)
cout << c[i] << “ “;
for (i = 0; i < arraysize; ++i)
cout << * (p + i) << “ “;

 Declaration
char * char_ptr; //point to 1st index in the array
char_ptr = new char[6];

 What is the output for the following code?

6.3 Illustrate the relationship between
pointer and function

Learning Outcome
to:
 Identify relationship between pointer and function
 Use pointer as function argument
 Write program using pointer and function

Pointer & Functions
 2 types of function calls:
 Call by value
 Call by reference

 Pointer can be used as parameters in both
function

Pointers & Function
#include <iostream>
using namespace std; void Change (int *a, int *b, int *c)
{
void Change (int *, int *, int *); *a = 100;
*b = 200;
*c = 300;
void main() }
{
int x=50, y=60, z=70;
cout << "nx = " << x << "ny = " << y received address
<< "nz = " << z << "nn";
Change (&x, &y, &z);
cout << "nx = " << x << "ny = " << y
<< "nz = " << z << "nn"; change value in
} variable that is point
by a,b,c

Change(&x, &y, &z);
void Change(int *a, int *b, int *c)
{ &x
50
*a = 100; x (100) 1001
*a
*b = 200; 1001 *a
&y
*c = 300; 60
y (200) 1002
} 1002
*b
*b

70 &z
z (300) 1003
*c
1003 *c

Pointers & Function
#include <iostream> void Change (int &a, int &b, int &c)
using namespace std; {
a = 100;
b = 200;
void Change (int &, int &, int &);
c = 300;
}
void main() received address
{
int x=50, y=60, z=70;
cout << "nx = " << x << "ny = " << y
<< "nz = " << z << "nn"; change value refered by
Change (x, y, z); a,b,c
cout << "nx = " << x << "ny = " << y
<< "nz = " << z << "nn";
}

Summary
 Pointer is a memory address
 Pointer variable provides way to indirectly
name variable by naming the address of
variable in memory
 new and delete operator can be use to
allocate and free the memory after pointer is
used
 Dynamic array is array whose sized is
determined during runtime
 Function can have pointer variable as its
parameter variables.

FP 201 - Unit 6

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FP 201 - Unit 6