Learning point C language (20): Data Type-pointer

1. Get the variable address through & variable:

 
# Include <stdio. h> int main (void) {int num = 10; printf ("variable value: % d \ n", num); printf ("variable address: % d \ n ", & num); getchar (); Return 0 ;}

  
 

2. The variable indicating the variable address is a pointer:

 
# Include <stdio. h> int main (void) {int num = 10; int * P = & num; printf ("% d, % P \ n", num, P ); getchar (); Return 0 ;}

  
 

3. * The pointer is the same as the variable itself:

 
# Include <stdio. h> int main (void) {int num = 10; int * P = & num; printf ("% d, % P, % d \ n", num, P, * P); * P = 11; printf ("% d, % P, % d \ n", num, P, * P); (* P) ++; printf ("% d, % P, % d \ n", num, P, * P); num = 99; printf ("% d, % P, % d \ n ", num, P, * P); getchar (); Return 0 ;}

  
 

4. Pay attention to initialization when declaring the pointer. It is dangerous to have no initialization pointer:

 
# Include <stdio. h> int main (void) {int n1 = 11; int n2 = 22; int * P = NULL;/* Initialization is blank */P = & N1; printf ("% d, % P, % d \ n", N1, P, * P); P = & N2; printf ("% d, % P, % d \ n ", N2, P, * P); getchar (); Return 0 ;}

  
 

5. Why is it dangerous to have no initialization pointer:

 
# Include <stdio. h> int main (void) {int * P; // The above pointer P is not initialized, but it also has a spam address printf ("% P \ n ", p); // who knows what will be covered if a value is assigned to it at this time? // * P = 100;/* do not execute this */getchar (); Return 0 ;}

  
 

6. pointer to a constant: the pointer cannot be used to modify the value it points to, but the value can be modified through its variable.

# Include <stdio. h> int main (void) {int n1 = 111; int n2 = 222; const int * P = & N1;/* Note the use of const */printf ("% d, % P, % d \ n ", N1, P, * P); n1 = 333; // * P = 333;/* cannot, because the current pointer is a constant */printf ("% d, % P, % d \ n", N1, P, * P); P = & N2; /* You can change the pointer to */printf ("% d, % P, % d \ n", N2, P, * P); getchar (); Return 0 ;}

  
 

7. Constant pointer: Lock pointer pointing

 
# Include <stdio. h> int main (void) {int n1 = 111; int n2 = 222; int * const P = & N1;/* Note the use of const */printf ("% d, % P, % d \ n ", N1, P, * P); n1 = 333; // * P = 333;/* cannot, because the current pointer is a constant */printf ("% d, % P, % d \ n", N1, P, * P); // P = & N2; /* you cannot change the pointer to * // printf ("% d, % P, % d \ n", N2, P, * P ); getchar (); Return 0 ;}

  
 

8. Pointers are of the following type:

# Include <stdio. h> int main (void) {long n = 100l; float F = 1.5f; double D = 3.14159265; long * P1 = & N; float * P2 = & F; double * P3 = & D; printf ("% LD \ n", * P1); printf ("% G \ n", * P2); printf ("%. 8f \ n ", * P3); getchar (); Return 0 ;}

  
 

9. Confusing pointer definitions:

What should I do:
Int * P;
Int * P;
Int * P;

This is correct because the C language ignores the blank space, but the following example shows which one is better:

 
# Include <stdio. h> int main (void) {int N1, N2, * P;/* defines two integers (N1, N2) and an integer pointer (P) */n1 = 111; N2 = 222; P = & N1; printf ("% d, % P \ n", * p, p); P = & N2; printf ("% d, % P \ n", * p, p); getchar (); Return 0 ;}