1. Pointer

2. 2
Pointer Declaration and Assignment
 Variables and address
– Structure of Memory
– Each variable is placed in a specified address
• Pointer: Address of variables
…
1000 ‘a’
1001
3.2
1005
4
…
ch, 1bytes
f, 4bytes
i, 4bytes
char ch = ‘a’ ;
float f = 3.2 ;
int i = 4 ;
address

3. 3
Pointer Declaration and Assignment
 Variables and address
– We can get pointers of variables from the variables
char ch = ‘a’ ;
float f = 3.2 ;
int i = 4 ;
printf( “%d %d %dn”, &a, &f, &i ) ;
&(name of variable) == address of variable …
1000 ‘a’
1001
3.2
1005
4
…
Address of
operator

4. 4
Pointer Declaration and Assignment
 Variables and address
– We can access the variable with the pointer of the variable
char ch ;
float f ;
int i ;
*(&ch) = ‘a’ ;
*(&f) = 3.2 ;
*(&i) = 4 ;
*(address) means the variable in the address
…
1000 ‘a’
1001
3.2
1005
4
…
char ch ;
float f ;
int i ;
ch = ‘a’ ;
f = 3.2 ;
i = 4 ;
Indirect
operator

5. 5
Pointer Declaration and Assignment
 Pointer Variable
– The variable which can hold addresses of variables
char ch = ‘a’ ;
float f = 3.2 ;
int i = 4 ;
char *pch ;
float *pf ;
int *pi ;
pch = &ch ;
pf = &f ;
pi = &i ;
printf( “%d %d %dn”, pch, pf, pi ) ;
data_type * pointer_varaible;
Variable can hold
only ‘char’ type pointer
Variable can hold
only ‘float’ type pointer
Variable can hold
only ‘int’ type pointer
char *pch = &ch ;
float *pf = &f ;
int *pi = &i ;

6. 6
Pointer Declaration and Assignment
 Pointer Variable
char ch ;
float f ;
int i ;
ch = ‘a’ ;
f = 3.2 ;
i = 4 ;
char ch ;
float f ;
int i ;
char *pch = &ch ;
float *pf = &f ;
int *pi = &i ;
*pch = ‘a’ ;
*pf = 3.2 ;
*pi = 4 ;
…
1000 ‘a’
1001
3.2
1005
4
…

7. 7
Pointer Declaration and Assignment
 Pointer Variable
– The size of all pointer variables is 4 bytes (4 byte machine)
• why??
char *pch ;
float *pf ;
int *pi ;
printf( “%dn”, sizeof(pch) ) ;
printf( “%dn”, sizeof(pf) ) ;
printf( “%dn”, sizeof(pi) ) ;
int i = 0 ;
int *pi = & i;
printf( “%dn”, i ) ;
printf( “%dn”, *pi ) ;
printf( “%dn”, &pi ) ;

8. 8
Pointer Declaration and Assignment
 Pointer Variable
– Since a pointer variable is also a variable, it has it own
address, like other variables do
int i = 0 ;
int *pi = &i;
printf( “%dn”, i ) ;
printf( “%dn”, *pi ) ;
printf( “%dn”, &pi ) ;
1000
…
1020
…
pi, 4 bytes
i, 4 bytes

9.  Example
9
Pointer Declaration and Assignment
[Ex] int *p;
int month=3;
p = &month;
Assign the address of ‘month’
to pointer variable
p month
3
p
1000
month
31000
Use an arrow instead of
writing address in a
pointer variable

10.  Example
10
Addressing and Dereferencing
[Ex] int a, b;
int *p;
a = b = 7;
p = &a;
printf(“*p = %dn”, *p);
*p = 3;
printf(“a = %dn”, a);
*p == 7
The variable pointed by p, that is a
The variable pointed by p, a, has 3
assign the address of a to p
*p = 7
a = 3

11. 11
Addressing and Dereferencing
[Ex1] int a, b;
int *p;
a = b = 7;
p = &a;
*p = 3;
p = &b;
*p = 2 * *p – a;
pa
7
b
7
p
3
a b
7
pa
3 11
b
 Example

12.  Prone to error
12
Addressing and Dereferencing
[Ex1] int x, *p;
x = 10;
*p = x;
[Ex3] int x, *p;
x = 10;
p = x;
Error!!
Can’t assign value x to pointer p
because p is not initialized.
We don’t know where p points
Error!!
Can’t assign value x to pointer p
because p cannot hold an integer variable

13. 13
Multi Pointer Variable
 Multi Pointer Variable
– i is a integer variable
– p is a pointer can hold the address of an integer variable
– q is a pointer can hold the address of a pointer variable of
integer variables
• The size of pointer variable q is 4 byte (32bit machine)
int i = 4 ;
int *p ;
int **q ;

14.  Example
14
Pointer Declaration and Assignment
[Ex] int i = 3;
int *p ;
int **q ;
p = &i ;
q = &p ;
p i
3
i
31000
p
10002000
q
20003000
q

15.  Example:
– What are the values of i and j,
respectively?
– Assume that the addresses of i,
j, p and q are 1000, 2000, 3000
and 4000, respectively.
– What are the values of p, q, r?
15
Pointer Declaration and Assignment
[Ex] int i = 3, j = 2;
int *p, *q ;
int **r ;
p = &i ;
q = &j ;
r = &p ;
*p = 4 ;
*q = 5 ;
**r = 6 ;
*r = &q ;
q =&i ;
**r = 7 ;

16.  Example
16
Swap Function
[Ex]
void swap(int i, int j) {
int temp = i ;
i = j;
j = temp;
}
int main(void) {
int a=3, b=7;
printf(“before swap : %d %dn”, a, b);
swap( a, b );
printf(“after swap : %d %dn”, a, b);
}
Can it swap values of a and b?

17.  Example
17
Swap Function
[Ex]
void swap(int *p, int *q) {
int temp = *p ;
*p = *q;
*q = temp;
}
int main(void) {
int a=3, b=7;
printf(“before swap : %d %dn”, a, b);
swap(&a, &b);
printf(“after swap : %d %dn”, a, b);
}
before swap : 3 7
after swap : 7 3

18.  Swap values of two pointer variables
18
Swap Function
[Ex]
void swap(int **pp, int **qq) {
int *temp = *pp ;
*pp = *qq;
*qq = temp;
}
int main(void) {
int a=3, b=7;
int *p = &a, *q = &b ;
printf(“before swap : %d %dn”, *p, *q);
swap(&p, &q);
printf(“after swap : %d %dn”, *p, *q);
}
before swap : 3 7
after swap : 7 3

19. 19
Call-by-Value
1 1
a in main a in function
main() function function() function
a = a + 1 ;
When function(a); is
called, value of a in main
is copied to a in function
2
After Return;
1 Variable a in main and a in function are
not identical, so that value of a in main
is not changed
1

20. 20
Call-by-Value
1 1
a in main a in function
main() function function() function
a = a + 1 ;
When function(a); is
called, value of a in main
is copied to a in function
2
After Return;
1 Variable a in main and a in function are
not identical, so that value of a in main
is not changed
1