9. pointer, pointer & function

9. pointer, pointer & function

• 1.
  Pointer
• 2.
  2 Pointer Declaration andAssignment  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 andAssignment  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 andAssignment  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 andAssignment  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 andAssignment  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 andAssignment  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 andAssignment  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 Declarationand 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 andDereferencing [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 toerror 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 Declarationand 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: – Whatare 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] voidswap(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] voidswap(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 valuesof 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 inmain 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 inmain 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