Question

I’m trying to implement a skew heap in C, but my code doesn’t compile. I’m not that experienced in C and never created any type of heap in C. That is why I don’t know how to fix it, I’m hoping someone can point me the right direction. I have been reading articles about the skew heap and this is what I got so far using the algorithms I have found online. Thanks in Advance.

typedef struct node
{
int value;
struct node * root;
struct node * leftchild;
struct node * rightchild;
} Node;

struct skewHeap
{
    struct node * root;
};

void skewHeapInit (struct skewHeap * sk)
{
    sk->root = 0;
}

void skewHeapAdd (struct skewHeap *sk)
{
    struct node *n = (struct node *) malloc(sizeof(struct node));
    assert(n != 0);
    n->value = 0;
    n->leftchild = 0;
    n->rightchild = 0;
    line 185. s->root = skewHeapMerge(s->root, n);
}

void skewHeapRemoveFirst (struct skewHeap *sk)
{
    struct node * n = sk->root;
    free(n);
    sk->root = skewHeapMerge(n->leftchild, n->rightchild);
}

line 196. struct node * skewHeapMerge(struct node *left, struct node *right)
{
    struct node *temp = (struct node *) malloc(sizeof(struct node));

    if (left == NULL) 
        return *right;

    if (right == NULL) 
        return *left;

    if (left->value < right-> value)
    {
        temp = left->leftchild;
        left->leftchild = skewHeapMerge(left->rightchild, right);
        left->rightchild = temp;
        return left;
    }
    else
    {
        temp = right->rightchild;
        right->rightchild = skewHeapMerge(right->leftchild, left);
        right->leftchild = temp;
        return right;
    }
}

These are the compilations errors I’m getting at the moment:

program.c: In function ‘skewHeapAdd’:
program.c:185: warning: implicit declaration of function ‘skewHeapMerge’
program.c:185: warning: assignment makes pointer from integer without a cast
program.c: In function ‘skewHeapRemoveFirst’:
program.c:191: warning: assignment makes pointer from integer without a cast
program.c: At top level:
program.c:196: error: conflicting types for ‘skewHeapMerge’
program.c:185: note: previous implicit declaration of ‘skewHeapMerge’ was here
program.c: In function ‘skewHeapMerge’:
program.c:202: error: incompatible types when returning type ‘struct node’ but ‘struct   node *’ was expected
program.c:205: error: incompatible types when returning type ‘struct node’ but ‘struct node *’ was expected

Answer 1

Regarding the compiler errors,

program.c: In function ‘skewHeapAdd’:
program.c:185: warning: implicit declaration of function ‘skewHeapMerge’
program.c:185: warning: assignment makes pointer from integer without a cast

tells you that no prototype of skewHeapMerge is in scope where skewHeapAdd is defined, hence (the compiler apparently operates in C89 mode, but thankfully warns about it), the compiler supposes an implicit declaration with return type int for skewHeapMerge.

Add a header file with prototypes for all your functions, and #include that in all *.c files where these functions are used or defined, so that the compiler knows the types of the functions.

program.c: In function ‘skewHeapRemoveFirst’:
program.c:191: warning: assignment makes pointer from integer without a cast

that should be the line

sk->root = skewHeapMerge(n->leftchild, n->rightchild);

where sk->root is a struct node*, but due to the implicit declaration of skewHeapMerge, that is assumed to return an int.

program.c: At top level:
program.c:196: error: conflicting types for ‘skewHeapMerge’
program.c:185: note: previous implicit declaration of ‘skewHeapMerge’ was here

here the compiler finds that the definition of skewHeapMerge gives a type conflicting with the one from the implicit declaration.

program.c: In function ‘skewHeapMerge’:
program.c:202: error: incompatible types when returning type ‘struct node’ but ‘struct   node *’ was expected
program.c:205: error: incompatible types when returning type ‘struct node’ but ‘struct node *’ was expected

That is for the lines

if (left == NULL) 
    return *right;

if (right == NULL) 
    return *left;

where you ought to return right resp. left instead of *right resp. *left (I overlooked that at first).

---

You have a mistake in skewHeapRemoveFirst

void skewHeapRemoveFirst (struct skewHeap *sk)
{
    struct node * n = sk->root;
    free(n);
    sk->root = skewHeapMerge(n->leftchild, n->rightchild);
}

where you use n after you freed it. You have to exchange the last two lines in that function.

And in skewHeapMerge

struct node * skewHeapMerge(struct node *left, struct node *right)
{
    struct node *temp = (struct node *) malloc(sizeof(struct node));

    if (left == NULL)
        return *right;

    if (right == NULL)
        return *left;

you are leaking memory. Remove the allocation, since if temp is used at all, you assign either left->leftchild or right->rightchild to it.