Question

Hello I recently asked some questions on linked lists in C.
The link was found here

First I want to thank everyone for helping me with this. But I have one issue I cannot understand. I even asked the professor but he emailed me back with little information. Basically I am writing a linked list in C (see above link). One of the things the professor gives us in the header file is this:

void list_map( INTLIST *list, void (*f)(void *) );
/*Applies a function to each element of the list */

So I emailed him about this and said:

Another question, in the header file you did not define a sorting function, do we need to write a sorting function with the prototype and finally what is list_map

And he replied with:

You are asked to implement a sorting function f, which is called through list_map(list, f). Hope it clears your doubts.

My only doubts are this was not fully taught. I can understand how to sort the linked list in fact here is some pseudo code:

tmp=head;

while(tmp!=NULL)
{
   tmp2=tmp->next; //pointer to next node
   while(tmp2!=NULL)
    {
     if (tmp2->data < tmp->data)
        {
         int x = tmp2->data;
         tmp2->data = tmp->data;
         tmp2->data = x;
        }
     tmp2=tmp2->next;
    }
   tmp=tmp->next;
}

I know the experts might say this is not the most efficient, and I understand that right now I am just learning and trying to get things working. I can clean up afterwords…so on to my question.

My question is given I have the sort function (in the professor’s case he calls it f). How would I call this sorting function when the signature is:

void list_map(INTLIST* list, void (*f) (void*));

Would I just say:

list_map(myList, f()); //apply function f to the current linked list

Or do I really need to define list_map somewhere? I am not the typical student just looking for someone to do my work. I am really trying to understand this as best I can.

Thanks to all of you.

[EDIT PORTION]

I wanted to add that one of the posters Kaleb P. said

“Thus, your job is to create a sorting
function that you will pass in to
list_map. Note that the correct syntax
for passing it in will be:”

So should my code simply be this:

in the .h file I prototype the function as:

void myCustomSort(void*);

And then in the .cpp it becomes:

void myCustomSort(void*f)
{
tmp=f->head; 

while(tmp!=NULL) 
{
   tmp2=tmp->next; //pointer to next node 
   while(tmp2!=NULL) 
   { 
     if (tmp2->data < tmp->data) 
        { 
         int x = tmp2->data; 
         tmp2->data = tmp->data; 
         tmp2->data = x; 
        } 
     tmp2=tmp2->next; 
    } 
   tmp=tmp->next; 
} 
}

And to call it in main I would just do:

list_map(myListPointer, &myCustomSort); 

But don’t I need to define list_map anywhere? Because it is in the .h file do I not have to define it?

Answer 1

Assuming list_map is implemented like this, giving f each node in sequential order,

void list_map(INTLIST *list, void (*f)(void *)) {
    INTLIST *node;
    for (node = list; node; node = node->next)
        f(node);
}

you can implement a selection sort

void list_sort(INTLIST *list) {
    list_map(list, swap_head_with_smallest);
}

where void swap_head_with_smallest(void *) swaps the datum of a given node with the smallest of the data of any nodes following it in the list.

---

As this is homework, I’m trying not to give the whole solution away.

void swap_head_with_smallest(void *list) {
    INTLIST *head = list;
    INTLIST *smallest;

    /* set smallest the smallest node of
         head, head->tail, head->tail->tail, etc. */

    /* swap head->datum and smallest->datum */
}