Insertion sort algorithm using doubly-linked lists in c - inserting nodes

Question

I am very new to coding and I'm trying to write a function in C using the insertion sort algorithm on a doubly linked list. The function takes an element off the old list and inserts it in alphabetical order on a new list.

I'm having problems looping the function when an element is greater than (ie further along in the alphabet) the first element in the new/sorted list. Right now it is only adding elements that can be appended or prepended. Why is this? Here is my function:

    #include "sort.h"
    #include <stdio.h>
    #include <stdlib.h>
    #include "linked_list.h"

/* sort linked list *list using merge insertion sort. */
/* upon success, the elements in *list will be sorted. */
/* return silently if *list is uninitialised or empty. */
/* the compare argument is a pointer to a function which returns */
/* less than 0, 0, or greater than 0 if first argument is */
/* less than, equal to, or greater than second argument respectively. */
void insertion_sort(LinkedList *list, int (*compare)(void *, void *))
{
    void *d, *B_node_data;
    LinkedList *B;
    Node *B_node, *new_node;

    if(!list)
        return;

    if(!list->head)
        return;

    B = initialise_linked_list();
    d = list->head->data;
    remove_head_linked_list(list);
    prepend_linked_list(B, d);


    while(list->head)
    {
        d = list->head->data;
        B_node = B->head;
        remove_head_linked_list(list);

        while(B_node)
        {
        B_node_data = B_node->data;
            if((*compare)(d, B_node_data) <= 0)
            {
                if(B_node->prev)
                {
                    new_node = initialise_node();
                    new_node->next = B_node;
                    new_node->prev = B_node->prev;
                    new_node->data = d;
                    break;
                }
                if(B->head->data == B_node_data)
                {
                    prepend_linked_list(B, d);
                    break;
                }
            }                    
            if(!B_node->next)
                append_linked_list(B, d);
        else{
                B_node = B_node->next;
            }
        }
    } 
list->head = B->head; 
list->tail = B->tail;

B->head = NULL;
B->tail = NULL;
free_linked_list(B);


}

here is the definitions of the functions i called from another file:

    #include "linked_list.h"

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

Node *initialise_node(void)
{
    Node *node;

    if(!(node = (Node *) malloc(sizeof(Node)))) {
        fprintf(stderr, "error: unable to initialise node.\n");
        exit(EXIT_FAILURE);
    }
    node->next = node->prev = node->data = NULL;

    return node;
}

void free_node(Node *node)
{
    if(!node)
        return;
    free(node);
}

LinkedList *initialise_linked_list(void)
{
    LinkedList *list;

    if(!(list = (LinkedList *) malloc(sizeof(LinkedList)))) {
        fprintf(stderr, "error: unable to initialise linked list.\n");
        exit(EXIT_FAILURE);
    }
    list->head = list->tail = NULL;

    return list;
}

void free_linked_list(LinkedList *list)
{
    Node *next;

    while(list->head) {
        next = list->head->next;
        free_node(list->head);
        list->head = next;
    }
    free(list);
}

void append_linked_list(LinkedList *list, void *data)
{
    Node *node;

    node = initialise_node();

    node->data = data;
    node->prev = list->tail;
    if(list->tail) {
        list->tail->next = node;
    }
    list->tail = node;
    if(!list->head)
        list->head = node;
}

void prepend_linked_list(LinkedList *list, void *data)
{
    Node *node;

    node = initialise_node();

    node->data = data;
    node->next = list->head;
    if(list->head) {
        list->head->prev = node;
    }
    list->head = node;
    if(!list->tail)
        list->tail = node;
}

void remove_head_linked_list(LinkedList *list)
{
    Node *head;

    if(!list->head)
        return;
    head = list->head->next;
    free(list->head);
    list->head = head;
    if(list->head)
        list->head->prev = NULL;
    else
        list->tail = NULL;

}

void remove_tail_linked_list(LinkedList *list)
{
    Node *tail;

    if(!list->tail)
        return;
    tail = list->tail->prev;
    free_node(list->tail);
    list->tail = tail;
    if(list->tail)
        list->tail->next = NULL;
    else
        list->head = NULL;
}

here is my output

==942== Command: ./task1_test harmonious pencil push naive jittery 
greedy billowy practise lock star appear record precede pen lace
==942== 
appear
billowy
greedy
harmonious
pencil
push
star
==942== 
==942== HEAP SUMMARY:
==942==     in use at exit: 264 bytes in 11 blocks
==942==   total heap usage: 35 allocs, 24 frees, 824 bytes allocated
==942== 
==942== 264 bytes in 11 blocks are definitely lost in loss record 1 of 1
==942==    at 0x4C29C23: malloc (vg_replace_malloc.c:299)
==942==    by 0x40084E: initialise_node (linked_list.c:11)
==942==    by 0x400CF1: insertion_sort (sort.c:43)
==942==    by 0x400815: main (task1_test.c:33)
==942== 
==942== LEAK SUMMARY:
==942==    definitely lost: 264 bytes in 11 blocks
==942==    indirectly lost: 0 bytes in 0 blocks
==942==      possibly lost: 0 bytes in 0 blocks
==942==    still reachable: 0 bytes in 0 blocks
==942==         suppressed: 0 bytes in 0 blocks
==942== 

Answer 1

I think you're doing a lot of unnecessary alloc/free.

When taking an item off the original list, it doesn't need to be deallocated, just adjust the head pointer.

When the insertion point is found, this node can be inserted directly into the destination list.

This simplifies the code quite a bit. But, your base list primitives aren't suitable for this type of node transfer from list to list.

I've coded up a few functions that can work directly on insertion of existing node pointers into a list.

I also refactored the sort function to use these.

NOTE: Because you didn't post all of your code, I didn't have a convenient way to test this. It will compile, but otherwise, no guarantees. Hopefully, it will help you get further.

void
insert_list_tail(LinkedList *list,Node *new)
{
    Node *prev;

    prev = list->tail;

    if (prev != NULL)
        prev->next = new;

    new->next = NULL;
    new->prev = prev;

    list->tail = new;

    if (list->head == prev)
        list->head = new;
}

void
insert_list_head(LinkedList *list,Node *new)
{
    Node *next;

    next = list->head;

    if (next != NULL)
        next->prev = new;

    new->prev = NULL;
    new->next = next;

    list->head = new;

    if (list->tail == next)
        list->tail = new;
}

void
insert_list_before(LinkedList *list,Node *cur,Node *new)
{
    Node *next;
    Node *prev;

    do {
        // we got to the end of the list without an insert before point, so
        // append to tail of list
        if (cur == NULL) {
            insert_list_tail(list,new);
            break;
        }

        // we do insert after [internally here] so get the previous node
        cur = cur->prev;

        // with no previous node insert at head of list
        if (cur == NULL) {
            insert_list_head(list,new);
            break;
        }

        prev = cur->prev;
        next = cur->next;

        if (prev != NULL)
            prev->next = new;
        else
            list->head = new;

        if (next != NULL)
            next->prev = new;
        else
            list->tail = new;

        new->prev = prev;
        new->next = next;
    } while (0);
}

void
insertion_sort(LinkedList *slist, int (*compare)(void *,void *))
{
    LinkedList list;
    LinkedList *dlist = &list;
    Node *src;
    Node *dst;

    if (slist == NULL)
        return;

    if (slist->head == NULL)
        return;

    dlist->head = NULL;
    dlist->tail = NULL;

    // grab first source node and insert it at destination list head
    src = slist->head;
    slist->head = src->next;
    insert_list_head(dlist,src);

    while (1) {
        src = slist->head;
        if (src == NULL)
            break;

        slist->head = src->next;

        // find node to do an insertion before (i.e. find the first destination
        // node that is higher than the source node)
        for (dst = dlist->head;  dst != NULL;  dst = dst->next) {
            if (compare(dst->data,src->data) > 0)
                break;
        }

        insert_list_before(dlist,dst,src);
    }

    slist->head = dlist->head;
    slist->tail = dlist->tail;
}