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cpointersstructure

Error while accessing array of pointers to structures

I am trying to make an array of structures in which each array elements itself points to another structure of the same type. In order to better explain the problem, I have made a schematic for it! The names of the variables are according to the code (shown below)

But I am getting an error while I am freeing the memory! Can someone provide some hints for solving this! Thanks.

array of structures

Error:

tempCodeRunnerFile.c:36:42: error: member reference type 'Vertex *' (aka 'struct Node *') is a pointer; did you mean to use '->'?
        Vertex* ptr = (*(vertexarray+ i)).next;
                      ~~~~~~~~~~~~~~~~~~~^
                                         ->
tempCodeRunnerFile.c:57:14: warning: incompatible pointer types passing 'Vertex [4]' to parameter of type 'Vertex **' (aka 'struct Node **') [-Wincompatible-pointer-types]
    finalize(vertexarray);
             ^~~~~~~~~~~
tempCodeRunnerFile.c:30:24: note: passing argument to parameter 'vertexarray' here
void finalize(Vertex** vertexarray){ // free the dynamic memmory 
                       ^
1 warning and 1 error generated.

Code:

#include <stdio.h>
#include <stdlib.h>

// Data structure to store a linked list node
typedef struct Node
{   int data;
    struct Node* next;
}Vertex;

Vertex* initialize(); //Vertex** vertexarray
void finalize(Vertex** vertexarray);

Vertex* initialize(){ // initialise the elements inside the array
    
    Vertex* node = (Vertex*)malloc(sizeof(Vertex));

    if(node ==NULL){
        printf("Error in initialising"); // If node cannot be created due to memmory issues.
        return NULL; // listhead is NULL
    }
    else{
    node->data = -1; // The initial node is empty (contains data value -1, symbolising empty).
    node->next = NULL; // The list at the time of initialisation is empty and hence no node to further point on.
    }
    return node;

}

void finalize(Vertex** vertexarray){ // free the dynamic memmory 

    int i;

    for (i=0; i<4;i++){ // free all the memmory
        
        Vertex* ptr = (*(vertexarray+ i)).next; 
        Vertex* tmp;

        while( ptr!= NULL) // Termination condition: At the end, the pointer is not pointing to any node, but a NULL value
        {
            tmp = ptr; // make tmp the current node starting from listhead.
            ptr = ptr->next; // update the pointer to the next pointer
            free(tmp); // free the previous pointer
        }   
    }

}

int main(void)
{
    Vertex vertexarray[4];
    int i;

    for (i=0; i<4;i++){
        vertexarray[i].next = initialize();
    }
    finalize(vertexarray);
    return 0;
}
Solution

  • As the finalize method would need to free every element of the array you could write it like this

    void finalize(Vertex vertexarray[], size_t len)
{ // free the dynamic memmory

  int i;

  for (i = 0; i < len; i++)
  { // free all the memmory

    Vertex *ptr = vertexarray[i].next;
    Vertex *tmp;

    while (ptr != NULL) // Termination condition: At the end, the pointer is not pointing to any node, but a NULL value
    {
      tmp = ptr;       // make tmp the current node starting from listhead.
      ptr = ptr->next; // update the pointer to the next pointer
      free(tmp);       // free the previous pointer
    }
  }
}

    And call it in main as

    finalize(vertexarray, 4);

    It will still work if vertexarray will allocated dynamically as you still need to keep track of how many elements has. Unless you change the vertexarray to also be a list