double pointers w/o array notation-CodePudding

I'm trying to iterate through a double pointer without using Array notation. I've been given these guidelines through my instructor.

In .h file:

int** A, B, C;

In .c file

void initNumbers() {

    int** a = A;
    int** b = B;
    int** c = C;

    int* p;// = *a;
    int* q;// = *b;
    int* r;// = *c; 

    for (int i = 0; i < 10; i  ) {
        p = *a;
        q = *b;
        r = *c;
        for (int j = 0; j < 10; j  ) {
            *r = rand() % 50;
            *p = rand() % 50;
            *q = rand() % 50;
            p  ;
            q  ;
            r  ;
        }
        a  ;
        b  ;
        c  ;
    }
}

I've allocated memory via.

void initNumSpace() {
    A = malloc(sizeof(int*) * 10);
    B = malloc(sizeof(int*) * 10);
    C = malloc(sizeof(int*) * 10);

    int** a = A;
    int** b = B;
    int** c = C;

    for (int i = 0; i < 10; i  ) {
        a = malloc(sizeof(int) * 10);
        b = malloc(sizeof(int) * 10);
        c = malloc(sizeof(int) * 10);

        a  ;
        b  ;
        c  ;
    }
}

The code gets no syntax errors, but there is a segmentation fault at the very first iteration of the imbedded loop of initNumbers. Why is this happening? What is the best way to iterate through a pointer array without array notation e.g., indices or offset values.

CodePudding user response：

The error is in your init function. The double pointers get set to the newly allocated space, but the pointer array A does not get set to the value of a. Try this instead:

for (int i = 0; i < 10; i  ) {
    *a = malloc(sizeof(int) * 10);
    *b = malloc(sizeof(int) * 10);
    *c = malloc(sizeof(int) * 10);

    a  ;
    b  ;
    c  ;
}

CodePudding user response：

Your code has a few problems.

First of all, you shouldn't DEFINE variables in header files; you should only DECLARE them. That is, you if you wanted to declare an integer N, then you shouldn't write int N; but instead extern int N; and then go to your source file and write int N; to define it. C (and C ) have the multiple declaration-one definition rule: you can declare something as many times as you like (you can write 'extern int A;five times in a row and the compiler won't mind), but you have to define each variable and function exactly once (you should writeint A;` exactly once in a source file.

Secondly, there's a slight problem with your header file. In C, writing int** A,B,C; is the same as int **A;int B;int C;. A is a double pointer, but B and C are integers. So, you should write int **A,**B,**C; (or even better, write each one on a separate line).

Your code should now look like this:

HEADER:

int** A;
int** B;
int** C;

SOURCE:

void initNumSpace() {
    A = malloc(sizeof(int*) * 10);
    B = malloc(sizeof(int*) * 10);
    C = malloc(sizeof(int*) * 10);
    
    int** a = A;
    int** b = B;
    int** c = C;

    for (int i = 0; i < 10; i  ) {
        //you should dereference these btw
        *a = malloc(sizeof(int) * 10);
        *b = malloc(sizeof(int) * 10); 
        *c = malloc(sizeof(int) * 10); 

        a  ;
        b  ;
        c  ;
    }
}

void initNumbers() {

    int** a = A;
    int** b = B;
    int** c = C;

    int* p;// = *a;
    int* q;// = *b;
    int* r;// = *c; 

    for (int i = 0; i < 10; i  ) {
        p = *a;
        q = *b;
        r = *c;
        for (int j = 0; j < 10; j  ) {
            *r = rand() % 50;
            *p = rand() % 50;
            *q = rand() % 50;
            p  ;
            q  ;
            r  ;
        }
        a  ;
        b  ;
        c  ;
    }
}

PS: why are you doing this way? It's not wrong, but it's really comlicated and hard to read. Why would you not want to use array indexing ([]).