9. pointer, pointer & function
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The document discusses pointers in C programming. Some key points: - Pointers store the address of a variable in memory. Pointer variables hold the address of other variables. - To access the value of the variable a pointer points to, you dereference the pointer using *. - Pointer variables are passed by value like other variables in C, so any changes made to the pointer or value it points to inside a function will not be reflected outside the function.
![ 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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-9-320.jpg)
![ 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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-10-320.jpg)
![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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-11-320.jpg)
![ 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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-12-320.jpg)
![ 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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-14-320.jpg)
![ 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 ;](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-15-320.jpg)
![ 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?](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-16-320.jpg)
![ 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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-17-320.jpg)
![ 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](https://image.slidesharecdn.com/9-pointerpointerfunction-130428233947-phpapp02/85/9-pointer-pointer-function-18-320.jpg)
9. pointer, pointer & function
- 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