CODEWITHSUNDEEP
cmemory-managementdata-structureslinked-listmalloc
dizzy_robot
dizzy_robot

Reputation: 21

linked list of strings in C

I am trying to create a linked list of strings in C and have had problems adding the first Node into the list. For whatever reason my program prints NULL even though I reference the head variable to newNode but it does not copy the string from struct pointer to struct pointer. Any help is appreciated. Thanks! 

#include "stdafx.h"
#include <stdlib.h>
#include <string.h>

typedef struct stringData {
    char *s;
    struct stringData *next;
} Node;

Node *createNode(char *s) {
    Node *newNode = (Node *)malloc(sizeof(Node));
    newNode->s = s;
    newNode->next = NULL;
    return newNode;
}


void insert(Node *head, Node *newNode) {
    if (head == NULL) {
        head->s = newNode->s;
        head = newNode;
    }
}

void printList(Node *head) {
    while (head != NULL) {
        printf("%s\n", head->s);
        head = head->next;
    }
}



int main()
{
    Node *head = createNode(NULL);

    Node *a = createNode("A");
    insert(head, a);

    printList(head);
    return 0;
}

Upvotes: 1

Views: 7776

Answers (2)

Ian Abbott
Ian Abbott

Reputation: 17403

Here is a modified version of the code that gives an example of inserting new nodes at both the start of a list and the end of a list. In fact, the insert function could be used to insert a new node at any position in the list, since all it needs is a pointer to a link and a pointer to the node to be inserted.

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

typedef struct stringData {
    char *s;
    struct stringData *next;
} Node;

Node *createNode(char *s) {
    Node *newNode = (Node *)malloc(sizeof(Node));
    newNode->s = s;
    newNode->next = NULL;
    return newNode;
}


void insert(Node **link, Node *newNode) {
    newNode->next = *link;
    *link = newNode;
}

void printList(Node *head) {
    while (head != NULL) {
        printf("%s\n", head->s);
        head = head->next;
    }
}



int main(void)
{
    Node *head = NULL;
    Node *tail = NULL;
    Node *n;

    n = createNode("B");
    // First node at start of list - head is updated.
    insert(&head, n);
    // First node is also the tail.
    tail = n;

    n = createNode("A");
    // Insert node at start of list - head is updated.
    insert(&head, n);

    n = createNode("C");
    // Insert node at end of list.
    insert(&tail->next, n);
    // Update tail.
    tail = n;

    printList(head);
    return 0;
}

Upvotes: 1

Mohit Jain
Mohit Jain

Reputation: 30489

Following code snippet is wrong:

void insert(Node *head, Node *newNode) {...}
...
insert(head, a);

You need to pass the pointer by reference. Currently you are changing local copy (argument).

Fix
Change your insert as:

void insert(Node **head, Node *newNode) {...}

And call as:

insert(&head, a);

What else
Atleast insert (and possibly) more functions are not fool-proof (guaranteed null pointer dereference, else case not handled etc). You need to debug and fix many such cases. Working your approach properly on paper before coding may help.

Upvotes: 2

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