Question

I am building a priority queue, but rather than filling the PQ array with integers I am assigning a pointer to a struct. Heres the code for the two structures, intializer and insertion function for PQ:

typedef struct HeapStruct *PriorityQueue;
typedef struct Customer *CustomerP;

struct HeapStruct {
    int Capacity;
    int Size;
    int *Elements;
};

struct Customer {
    float arrivalT;
    float waitT;
    float departureT;
    float currentT;
    CustomerP *siblingR; //Used for linked list
};

PriorityQueue Initialize() {
    PriorityQueue H = (PriorityQueue) malloc(sizeof (struct HeapStruct));
    CustomerP sentinal = malloc(sizeof (struct Customer));
    sentinal->currentT = MinData;
    H->Capacity = 101;
    H->Size = 0;
    H->Elements[0] = &sentinal; //Syntax Error
    return H;
}

void Insert(CustomerP X, PriorityQueue H) {
    int i;
    if (IsFull(H)) {
        printf("Priority queue is full");
        return;
    }
    //Syntax errors
    for (i = ++H->Size; H->Elements[i/2]->currentT > X->currentT; i /= 2)
    H->Elements[i] = H->Elements[i/2];
    H->Elements[i] = X;
}

So I am trying to place a pointer in the Int array and doing comparisons such as H->Elements[i]->currentT, but I don’t know how to handle a pointer to a struct in an array and access the struct from there.

Could anyone help me out with the syntax of this? I’ll gladly provide more info if needed.

Answer 1

The Elements field in your HeapStruct needs to be defined appropriately based on what you want to store in your heap. Then, you need to allocate memory for it before you use it.

So the first question, what do want in the heap? You say you want customers (not ints, which is what you have), but do you want the structs themselves (Customer) or pointers to the structs (Customer * or CustomerP). Assuming the latter you want:

struct HeapStruct {
    int Capacity;
    int Size;
    CustomerP *Elements;
};

Then you need to allocate space for it properly:

H = (PriorityQueue) malloc(sizeof (struct HeapStruct));
CustomerP sentinal = malloc(sizeof (struct Customer));
sentinal->currentT = MinData;
H->Capacity = 101;
H->Size = 0;
H->Elements = malloc(sizeof(CustomerP) * H->Capacity);
H->Elements[0] = sentinal;