why insertion is not working in my link-list-CodePudding

I am learning linked list Here, I am trying to add node at the first so, I made a structure function to add but i dont know why its not working its only giving this output without adding

7 11 66 After insretion 7 11 66

and also please tell me is there any way to insert element on linked list.

// inserting element on first
#include<stdio.h>
#include<stdlib.h>

struct Node
{
    int data;
    struct Node * next;
};

void link_list_tranversal(struct Node *ptr){
    while (ptr!=NULL)
    {
        printf("%d\n",ptr->data);
        ptr=ptr->next;
    }
    
   
}


struct Node * insert_at_first(struct Node *head,int data){
    struct Node *ptr=(struct Node *)malloc(sizeof(struct Node));
    ptr->next=head;
    ptr->data=data;
    return ptr;
}

int main(){
    struct Node*head;
    struct Node*second;
    struct Node*third;
    head =(struct Node *) malloc(sizeof(struct Node));
    second =(struct Node *) malloc(sizeof(struct Node));
    third =(struct Node *) malloc(sizeof(struct Node));
   // linking first and secend node
    head ->data=7;
    head->next=second;
    // linking secend and third node
    second->data=11;
    second->next=third;
    // terminate the list at the third
    third->data=66;
    third->next=NULL;
    link_list_tranversal(head);

    insert_at_first(head,56);
    printf("After insretion\n ");
    link_list_tranversal(head);
    return 0;
}

CodePudding user response：

After insert_at_first(head,56);, you've created a new node, but head is now pointing to the second node in the list and you have lost track of the newly created node. Simplest fix is to do: head = insert_at_first(head,56);. You can also pass &head and let the function update head.

CodePudding user response：

You need to update the pointer you are storing in head after calling insert_at_first.

// inserting element on first
#include<stdio.h>
#include<stdlib.h>

struct Node
{
    int data;
    struct Node * next;
};

void link_list_tranversal(struct Node *ptr){
    while (ptr!=NULL)
    {
        printf("%d\n",ptr->data);
        ptr=ptr->next;
    }
    
   
}


struct Node * insert_at_first(struct Node *head,int data){
    struct Node *ptr=(struct Node *)malloc(sizeof(struct Node));
    ptr->next=head;
    ptr->data=data;
    return ptr;
}

int main(){
    struct Node*head;
    struct Node*second;
    struct Node*third;
    head =(struct Node *) malloc(sizeof(struct Node));
    second =(struct Node *) malloc(sizeof(struct Node));
    third =(struct Node *) malloc(sizeof(struct Node));
   // linking first and secend node
    head ->data=7;
    head->next=second;
    // linking secend and third node
    second->data=11;
    second->next=third;
    // terminate the list at the third
    third->data=66;
    third->next=NULL;
    link_list_tranversal(head);

    head = insert_at_first(head,56);
    printf("After insretion\n ");
    link_list_tranversal(head);
    return 0;
}

Output:

7
11
66
After insretion
 56
7
11
66

CodePudding user response：

The function insert_at_first returns a new value of the pointer that points to the head node. You need to assign the returned value to the pointer head to update it

head = insert_at_first(head,56);

Also there is no great sense to allocate nodes in main when you already have the function insert_at_first

You could initially write for example

struct Node *head = NULL;

head = insert_at_first( head, 66 );
head = insert_at_first( head, 11 );
head = insert_at_first( head, 7 );
head = insert_at_first( head, 56 );

Also as the function link_list_tranversal does not change the outputted list then its parameter should be declared with qualifier const

void link_list_tranversal( const struct Node *ptr );

Pay attention to that this functiondefinition

struct Node * insert_at_first(struct Node *head,int data){
    struct Node *ptr=(struct Node *)malloc(sizeof(struct Node));
    ptr->next=head;
    ptr->data=data;
    return ptr;
}

is not good because the function will invoke undefined behavior if memory for a node will not allocated successfully.

It is better to declare and define the function the following way

int insert_at_first( struct Node **head, int data )
{
    struct Node *ptr = malloc( *ptr );
    int success = ptr != NULL;

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

    return success;
}

And the function can be called like for example

struct Node *head = NULL;

insert_at_first( &head, 66 );

struct Node *head = NULL;

if ( !insert_at_first( &head, 66 ) )
{
    puts( "Error. No enough memory" );
}