ABHINAV RANA ABHINAV RANA - 26 days ago 8
C Question

Why does pointer value is being used as . instead of -> in this

struct AdjListNode
{
int dest;
struct AdjListNode* next;
};

// A structure to represent an adjacency list
struct AdjList
{
struct AdjListNode *head; // pointer to head node of list
};

// A structure to represent a graph. A graph is an array of adjacency lists.
// Size of array will be V (number of vertices in graph)
struct Graph
{
int V;
struct AdjList* array;
};


struct Graph* createGraph(int V)
{
struct Graph* graph = (struct Graph*) malloc(sizeof(struct Graph));
graph->V = V;

// Create an array of adjacency lists. Size of array will be V
graph->array = (struct AdjList*) malloc(V * sizeof(struct AdjList));

// Initialize each adjacency list as empty by making head as NULL
int i;
for (i = 0; i < V; ++i)
graph->array[i].head = NULL;

return graph;
}

// Adds an edge to an undirected graph
void addEdge(struct Graph* graph, int src, int dest)
{
// Add an edge from src to dest. A new node is added to the adjacency
// list of src. The node is added at the begining
struct AdjListNode* newNode = newAdjListNode(dest);
newNode->next = graph->array[src].head;
graph->array[src].head = newNode;

// Since graph is undirected, add an edge from dest to src also
newNode = newAdjListNode(src);
newNode->next = grap`enter code here`h->array[dest].head;
graph->array[dest].head = newNode;
}


Specifically

graph->array[i].head = NULL;



  • Ok so the graph is a pointer of type Graph

  • hence uses -> to access array which is an object of graph of type AdjList
    which has a
    pointer of type AdjListNode which is also a pointer.



Since *array member in Graph is a pointer to AdjList then why is

graph->array[i]->head = NULL;


not used ?

I know that
.
operator is used when we access a member of struct which is not a pointer.

somestructpointer -> itsmember


is basically sugar coating

(*somestructpointer).itsmember


I don't understand what's happening. HELP.

Answer

graph is a pointer.
graph->arraydereferences the above pointer to to get to the array variable.
graph->array[i] uses array notation to reach a member at position i.

The member we reach is of type struct AdjList, which is not a pointer. It's a struct. So a dot(.) is appropriate.

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