Question

I have been working on a DisjointSet assignment in C for a couple days now. I understand the functions find (w/ and w/o path compression), using rank in the function link when performing the union on a set. But I am having issues with C syntax.

We have to make an array of records containing the rank and the key for the set. so my struct looks as:

typedef struct DisjointSet_t {
  int data;
  int key;
} DisjointSet;

My problem is declaring the array to manipulate the set. There is something wrong with me initializing the array for the set. Here’s the snippet of the CreateSet code:

static DisjointSet *S;

void CreateSet(int numElements){  
  DisjointSet *t;

  if (numElements > 0){
    t = (DisjointSet *)malloc(sizeof(DisjointSet));
    }

  if(S != NULL){
    S = t[numElements+1];
  }  
}

If I implemented this in Java I think it’d be a little easier. How can I improve this? Am I missing something about understanding how to initializing class arrays in C?

Answer 1

Hmmm … with

malloc(sizeof(DisjointSet))

you reserve space for one object of type DisjointSet. To allocate space for 20 objects you need to multiply …

malloc(20 * sizeof(DisjointSet))

and, once you have 20 objects, in an array, the array goes from 0 to 19.