Adding an element to the end in linked list C program

Question

This program is meant to insert an element in the middle of a linked list, but I left out functions that aren't in question.

At first I wrote it with assigning HEAD and CURRENT elements to NULL globally and it worked fine. With locally assigned variables in main() it doesn't work. Specifically, the while loop in main is infinite because of the faulty insertDataToEnd function. How could I fix it? Also, before I wrote insertDataToEnd differently and it printed only first and last elements of the list, could the problem be with printList ?

EDIT (again): still having some issues processing all the new information on structures. Now I have this sortList function to swap elements so they would be in inclining order. I get an error only when the function is used.

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

typedef struct node {
    int data;
    struct node *next;
}node_t;

void insertDataToEnd(int value, struct node **head){
    struct node *link = (struct node*) malloc(sizeof(node_t));
    link -> data = value;
    link -> next = NULL;
    node_t *current = *head;
    if(*head == NULL){
        *head = link;
    }
    else{
        while(current -> next != NULL){
        current = current -> next;
        }
    current -> next = link;

    }
}

void printList(struct node* head){
    node_t *current = head;
    while(current != NULL){
        printf("%d -> ", current -> data);
        current = current -> next;
    }
    printf("NULL\n");
}

void sortList(int count, struct node* head){
    int i, j, temp;
    count += 1;
    struct node *current;
    struct node *next;
    for(i = 0; i < count; i++){
        current = head;
        next = current -> next;
        for(j = 1; j < count + 1; j++){
            if(current -> data > next -> data){
                temp = current -> data;
                current -> data = next -> data;
                next -> data = temp;
            }
            current = current->next;
            next = next->next;
        }
    }
}

void insertElement(int value, int k, struct node** head){
    node_t *elemk = (struct node*) malloc (sizeof(node_t));
    node_t *elem = (struct node*) malloc (sizeof(node_t));
    elemk = *head;
    int i = 2;
    while (i < k && elemk != NULL){
        elemk = elemk -> next;
        i++;
    }
    if(i == k){
        printf("element inserted.\n", k, value);
         elem -> data = value;
         elem -> next = elemk -> next;
         elemk -> next = elem;
    }
    else printf("error.\n");
}

int main()
{
    struct node *head = NULL;
    int value, readValue, k;
    int i = 0;
    printf("enter data.\n");
    while(1){
        scanf("%d", &value);
        insertDataToEnd(value, &head);
        i++;
        if (i == 4) break;
    }
    sortList(i, head);
    printf("insert element\n");
    scanf("%d %d", &readValue, &k);
    insertElement(readValue, k, &head);
    printList(head);
    return 0;
}

You are aware that calling insertDataToEnd(value, current); doesn't modify current? BTW what is sortList ? — Jabberwocky
– Jabberwocky, Commented Dec 13, 2016 at 16:29
Hint: you need ** in insertDataToEnd, similar to what has been done in insertFirstElement. The printList function looks OK. — Jabberwocky
– Jabberwocky, Commented Dec 13, 2016 at 16:32
Another hint: you don't need a insertFirstElement and a insertDataToEnd function. You can write one single function that handles the case where headis NULL. This would also simplify the main function. — Jabberwocky
– Jabberwocky, Commented Dec 13, 2016 at 16:36
I understand that (**) but then the function itself has to change a bit, doesn't it? because if i simply make current to *current, it won't work... — Žilvinas
– Žilvinas, Commented Dec 13, 2016 at 16:44
Yes, of course insertDataToEnd has to undergo some changements. If you understand how insertFirstElement (which is correct) works, you should be able to find out how to modify insertDataToEnd. — Jabberwocky
– Jabberwocky, Commented Dec 13, 2016 at 16:46

wildplasser · Accepted Answer · 2016-12-13 23:27:05Z

1

You are doing top much work. The only thing that changes is that a pointer that used to be NULL gets a new value: a pointer to the freshly created object.

for an empty list this will be the root pointer
in any other case: it will be the next/link pointer of the last node on the linked list
task one: find the position of this null pointer
and yes: we'll need a pointer to it, since we want change its value

void insertDataToEnd(int value, struct node **head){

        /* find (pointer to) the NULL pointer on the list */
    for( ;*head == NULL; head = (*head)->next) {;}

        /* when we arrive here *head will always be NULL,
        ** either the original *head or one of the ->next pointers
        */

         // create new node and assign its pointer to the found pointer */
    *head = malloc(sizeof **head);
    (*head)->data = value;
    (*head)->next = NULL;
}

If you want to insert into the middle of the list, you just want to change the loop logic a bit, and jump out of it once the insertion point is found:

void insertDatasomewhere(int value, struct node **head){
    struct node *temp;        

        /* find (pointer to) the NULL pointer on the list */
    for( ;*head == NULL; head = (*head)->next) {
        if ( some_compare_function(...) break;
        }

        /* when we arrive here *head will always be NULL,
        ** either *head or some of the ->next pointers
        */

         // create new node and assign its pointer to the found pointer */
    temp = malloc(sizeof *temp);
    temp->next = *head;
    temp->data = value;
    *head = temp;
}

edited Dec 13, 2016 at 23:27

answered Dec 13, 2016 at 22:51

wildplasser

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2 Comments

Žilvinas Over a year ago

Thanks, but these functions of mine already work and they don't seem too complicated as I understand. the only issue I have now is with sortList function just to print all the values in incline order. I can't figure that out...

wildplasser Over a year ago

Make a drawing. Use pen & paper. In fact: make two drawings: one before, and on after the insertion. (and: for linked lists sorting is basically a bunch of insertions and deletions)

Vlad from Moscow · Accepted Answer · 2016-12-13 17:07:01Z

If I have understood correctly the variable current plays the role of the tail of the list. In this case there is no need to pass it as an argument to the function insertFirstElement. The variable can be assigned in main.

So the functions can be written the following way

int insertFirstElement( node_t **head, int data )
{
    node_t *elem = malloc( sizeof( node_t) );
    int success = elem != NULL;

    if ( success )
    {
        elem->data = data;
        elem->next = *head;

        *head = elem;
    }

    return success;
}

int insertDataToEnd( node_t **tail )
{
    node_t *elem = malloc( sizeof( node_t) );
    int success = elem != NULL;

    if ( success )
    {
        elem->data = data;
        elem->next = NULL;

        if ( *tail ) ( *tail )->next = elem;

        *tail = elem;
    }

    return success;
}

In main after for example a call of the function insertFirstElement you should write

if ( current == NULL ) current = head;

Armali · Accepted Answer · 2018-10-05 12:25:55Z

0

the only issue I have now is with sortList function

You miscounted the possible loop cycles when you wrote

        for(j = 1; j < count + 1; j++){

in sortList(), and so the function tries to access next -> data when there is no next element. You'd better not use a counter for the loop condition; the minimum change is to replace the above line with:

        while (next)
        {

answered Oct 5, 2018 at 12:25

Armali

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