Questions regarding pointers

 

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In C arrays why is this true? a[5] == 5[a]

  • If p is a pointer (say int * p ), then what does [p] means ? Also what does 4[p] means ? (ie multiplying a scalar with [p] )

  • Suppose xyz is some data type defined by in the program. Then what does the 

    void (*xyz)(void);

    statement mean?

  • 4[p] means the same as p[4] . See eg http://c-faq.com/aryptr/joke.html.

  • If xyz is already a data type, then that's an error. If not, then it's the definition of a function pointer called xyz . Assuming that you meant " void " not " coid ", then cdecl.org tells us:

    declare xyz as pointer to function (void) returning void

    • if p is defined as int *p then

    [p] is wrong! cause an error: expected expression before '[' token.

    Where as 0[p] is correct!

    And 0[p] is same as p[0] , similarly p[4] is 4[p] .

    compiler convert p[4] into *(p + 4) that as *(4 + p) => 4[p]

    Additionally , suppose if you have an array say int a[10] , you can access elements of array either as a[i] or i[a]

    following example will be useful, I think: 

    int main(){
    int a[5] = {1,2,3,4,5};
    int* p;  // similar declaration of p (you asked)
    p = a;
    int i= 0; 
    for(i=0; i < 5; i++){
        printf("a[i] =  %d  and i[a] = %d n",a[i],i[a]);
    }
    printf(" using p n"); // access using pointer.  
    for(i=0; i < 5; i++){
        printf("p[i] =  %d  and i[p] = %d n",p[i],i[p]);
    }
}

    compile and execution: 

    :~$ ./a.out 
a[i] =  1  and i[a] = 1 
a[i] =  2  and i[a] = 2 
a[i] =  3  and i[a] = 3 
a[i] =  4  and i[a] = 4 
a[i] =  5  and i[a] = 5 
 using p 
p[i] =  1  and i[p] = 1 
p[i] =  2  and i[p] = 2 
p[i] =  3  and i[p] = 3 
p[i] =  4  and i[p] = 4 
p[i] =  5  and i[p] = 5

    [ANSWER-2 ]

    A declaration void (*xyz)(void); creates xyz a pointer to function that returns void and arguments are void . ( xyz is not a data-type but a pointer variable) eg 

    void function(void){
 // definition 
}

void (*xyz)(void);

    then xyz can be assigned address of function : 

    xyz = function;

    And using xyz() you can call function() , A example for void (*xyz)(void) : 

    #include<stdio.h>
void function(void){
    printf("n An Examplen");
} 
int main(){
    void (*xyz)(void);   
    xyz = function;    
    xyz();
}

    Now compile and execute it: 

    :~$ gcc  x.c
:~$ ./a.out 

 An Example
:~$

    what does [p] mean?

    Nothing in itself.

    Also what does 4[p] mean?

    Due to pointer arithmetic, 4[p] means *(4 + p) , which is, given that addition is commutative, equivalent to *(p + 4) , which in turn can be written as p[4] , ie it's the 5th element of an array pointed to by p .

    If xyz is a data type, then what does void (*xyz)(void); statement mean?

    It's a syntax error then.

    If xyz is not a data type, then it declares xyz to be a function pointer taking and returning void (ie "nothing").

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