problem with linked list

**nickman** · 06-09-2010

Hi

I have created a simple linked list program but it is printing only the first element of list.

The code is as follow:

Code:

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

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

int main()
{
	struct link *head;
	struct link *temp,*node;
	node = NULL;
	temp = NULL;
	head = NULL;

    int ch = 1;
	while(ch)
	{
		printf("\n Enter your Choice");
		printf("\n 1: Insert");
		printf("\n 2: Display");
		printf("\n 3: Exit");
		scanf("%d",&ch);

		switch(ch)
		{
		case 1:


			int info;
			printf("\n Enter the number");
			scanf("%d",&info);

			
			
            
			 node = (link *)malloc(sizeof(link));
			if( head == NULL )
			{
				node->data = info;
				node->next = NULL;
				head = node;
			}
			else
			{
				temp = head;
				while( temp->next != NULL )
					temp=temp->next;

                 node->data = info;  	
				 node->next = NULL;
      			 temp = node;
				
				  
    		}
			break;

		case 2:
          
			 temp = head;
			 while ( temp !=NULL ){
				 printf("\n %d",temp->data);
				 temp = temp->next;
			 }
			 break;

		case 3:
			exit(0);
			break;
		}
	}
}

Can any body tell me where is the problem?

Thanks

**MK27** · 06-09-2010

It's in your insert:

Code:

			else
			{
				temp = head;
				while( temp->next != NULL )
					temp=temp->next;

                 node->data = info;  	
				 node->next = NULL;
      			 temp = node;

But temp is not attached to anything! It's just a variable name you are assigning to. You need to attach to a next pointer somewhere. To do that, you need to keep a "previous" pointer:

Code:

			else
			{
				temp = head;
                                previous = NULL;
				while(temp != NULL ) {
                                        previous = temp;
					temp=temp->next;
                                }
                                prev->next = node;

See how that works?

BTW, you should assign the data to the node first, so you have that code in one place instead of two or more (important concept!).

**Bayint Naung** · 06-09-2010

If you are comfortable with pointer,

Code:

struct link **href;
node = malloc( sizeof(*node) );
if(node==NULL ) { 
  /* handle error */
}
node->data = info;
node->next = NULL;
href = &head;

while( *href != NULL ) {
   href = &(*href)->next;
}
*href = node;

**nickman** · 06-09-2010

Thanks for the reply......

I have one more question....as If I have to add more nodes at end , then every time while loop will run.

Is there any other way by which I can avoid this looping.

I tried with some temp pointers variable but doesn't work.

Thanks

**Bayint Naung** · 06-09-2010

You can have tail pointer to keep track of the end of list.

Code:

head = tail = NULL;
/* code */
/* insert */
if( head == NULL ) {   
 head = tail = node;
} else {
  tail->next = node;
  tail = node;
 }

**MK27** · 06-09-2010

Originally Posted by nickman

I have one more question....as If I have to add more nodes at end , then every time while loop will run.

Is there any other way by which I can avoid this looping.

Add to the beginning and not the end. This is easier and faster, and always the same:

Code:

temp = head;
head = node;
node->next = temp;

Even if head is currently NULL (ie, this is first node added) that's fine.

However, it is obviously not a good idea if the list is time ordered.

**nickman** · 06-09-2010

Hi

I have created the the same program using functions,but this time it is not working.
Pointer head has null value.
I can understand that this is due to scoping problem,since head and other pointers are declared inside main and I am passing them as variable to functions.( Please correct me if I am wrong)

The alternate sol. can be that I declare head and other variable as global; but that will be a bad practice.

Can anybody help me to know how to do it?

The code I have wrote is:

Code:

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

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

void insert(static struct link *head)
{
			struct link *temp,*node;
            int info =0;
			printf("\n Enter the number");
			scanf("%d",&info);
            
			 node = (link *)malloc(sizeof(link));
			if( head == NULL )
			{
				node->data = info;
				node->next = NULL;
				head = node;
			}
			else
			{
				temp = head;
				while( temp->next != NULL )
					temp=temp->next;

                 node->data = info;  	
				 node->next = NULL;
				 temp->next = node;
			}
}

void display(static struct link *head)
{
	struct link *temp;
	temp = head;

	while ( temp !=NULL ){
	 printf("\n %d",temp->data);
	 temp = temp->next;
	}
}

int main()
{
	struct link *head;
	struct link *temp,*node;
	node = NULL;
	temp = NULL;
	head = NULL;

    int ch = 1;
	while(ch)
	{
		printf("\n Enter your Choice");
		printf("\n 1: Insert");
		printf("\n 2: Display");
		printf("\n 3: Exit");
		scanf("%d",&ch);

		switch(ch)
		{
		case 1:
             insert(head);
             break;

		case 2:
             display(head);
			 
			 break;

		case 3:
			exit(0);
			break;
		}
	}
}

Thanks

**Bayint Naung** · 06-09-2010

Question 4.8
why static ?void insert(static struct link *head) ?
Can you even compile without any error?
which compiler? what option?

**nickman** · 06-09-2010

I am sorry.....

It's not static....

Code:

void insert(struct link *head) {}

**MK27** · 06-09-2010

Originally Posted by nickman

I can understand that this is due to scoping problem,since head and other pointers are declared inside main and I am passing them as variable to functions.( Please correct me if I am wrong)

That is an issue, you are right. The reason is the same as the reason assigning to temp didn't add to the list -- because in the insert function, "head" is just the name of a variable. It is assigned a value (the parameter you pass), but if you reassign it:

Code:

void insert(struct link *head) {
         head = whatever;  // reassignment!

Now head is not the same as the head of the list from main. To deal with this you need to use a pointer to the pointer:

Code:

void insert(struct link **head) {
         *head = whatever;  // dereference then assign

Will work, but you have to call it with the address of head (nb, I removed the static qualifier, that is not the answer):

Code:

insert(&head);

The exact same as the principle here:

Code:

void somefunct (int *ptr) {
         *ptr = 5;  // dereference then assign
}
int x;
somefunct (&x);

Hopefully you understand that?

The alternate sol. can be that I declare head and other variable as global; but that will be a bad practice.

Definitely not necessary!

**nickman** · 06-09-2010

Hi

I tried it but it's crashing.

I have modified as follow:

Code:

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

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

void insert( struct link **head)
{
	       
			struct link *temp,*node;
            int info =0;
			printf("\n Enter the number");
			scanf("%d",&info);
            
			 node = (link *)malloc(sizeof(link));
			if( head == NULL )
			{
				node->data = info;
				node->next = NULL;
				*head = node;
			}
			else
			{
				temp = *head;
				while( temp->next != NULL )
					temp=temp->next;

                 node->data = info;  	
				 node->next = NULL;
				 temp->next = node;
			}
}

void display( struct link **head)
{
	struct link *temp;
	temp = *head;

	while ( temp !=NULL ){
	 printf("\n %d",temp->data);
	 temp = temp->next;
	}
}

int main()
{
	struct link *head;
	struct link *temp,*node;
	node = NULL;
	temp = NULL;
	head = NULL;

    int ch = 1;
	while(ch)
	{
		printf("\n Enter your Choice");
		printf("\n 1: Insert");
		printf("\n 2: Display");
		printf("\n 3: Exit");
		scanf("%d",&ch);

		switch(ch)
		{
		case 1:
             insert(&head);
             break;

		case 2:
             display(&head);
			 
			 break;

		case 3:
			exit(0);
			break;
		}
	}
}

Can anybody correct it?

Thanks

**Bayint Naung** · 06-09-2010

No, find yourself.

**MK27** · 06-09-2010

Originally Posted by nickman

Can anybody correct it?

You need to remember to dereference head appropriately, the same as if you were accessing the value in an int pointer:

Code:

if (*x == 5)
if (*head == NULL)  // you still have if (head == NULL)

This is because "head" in the function is a pointer to the pointer from main (whereas head in main is a pointer to a node):

struct link *head is a pointer to a struct
struct link **head is a pointer to a pointer to a struct

Get it? The second one can be NULL on two different levels. You want the level of the *dereferenced value.

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