I'm messing around with linked lists and so far I was able to do a little something. However, I want to improve this code so I would like for any help.
First, I want improve the function insertFirst() so that it tests if memory allocation worked.
Second, I want improve main() so that it tests if insertFirst() returns NULL. If it does, main() should write out an error message and terminate any loop. -
#include <stdio.h>
-
#include <stddef.h>
-
#include <stdlib.h>
-
struct NODE
-
{
-
struct NODE *link;
-
int value;
-
};
-
typedef struct NODE Node;
-
Node *insertFirst( Node **head, int val )
-
{
-
Node *node = (Node *)malloc( sizeof( Node ) );
-
node->value = val;
-
node->link = *head;
-
*head = node;
-
return node;
-
}
-
void traverse( Node *p )
-
{
-
while ( p != NULL )
-
{
-
printf("%d ", p->value );
-
p = p->link;
-
}
-
}
-
void freeList( Node *p )
-
{
-
Node *temp;
-
while ( p != NULL )
-
{
-
temp = p;
-
p = p->link;
-
free( temp );
-
}
-
}
-
int main()
-
{
-
Node *head = NULL;
-
int j;
-
for ( j=0; j<13; j++ )
-
insertFirst( &head, j );
-
traverse( head );
-
freeList( head );
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return 1;
-
}
8 4185 sicarie 4,677
Recognized Expert Moderator Specialist
I'm messing around with linked lists and so far I was able to do a little something. However, I want to improve this code so I would like for any help.
First, I want improve the function insertFirst() so that it tests if memory allocation worked.
Second, I want improve main() so that it tests if insertFirst() returns NULL. If it does, main() should write out an error message and terminate any loop. -
#include <stdio.h>
-
#include <stddef.h>
-
#include <stdlib.h>
-
struct NODE
-
{
-
struct NODE *link;
-
int value;
-
};
-
typedef struct NODE Node;
-
Node *insertFirst( Node **head, int val )
-
{
-
Node *node = (Node *)malloc( sizeof( Node ) );
-
node->value = val;
-
node->link = *head;
-
*head = node;
-
return node;
-
}
-
void traverse( Node *p )
-
{
-
while ( p != NULL )
-
{
-
printf("%d ", p->value );
-
p = p->link;
-
}
-
}
-
void freeList( Node *p )
-
{
-
Node *temp;
-
while ( p != NULL )
-
{
-
temp = p;
-
p = p->link;
-
free( temp );
-
}
-
}
-
int main()
-
{
-
Node *head = NULL;
-
int j;
-
for ( j=0; j<13; j++ )
-
insertFirst( &head, j );
-
traverse( head );
-
freeList( head );
-
return 1;
-
}
DeFault-
Are you set in C, and not C++? You could do those things very easily in C++, but if you need C, you can always just put the insertFirst() inside the condition of an 'if' statement, and a break in the body: -
if ( (insertFirst(&head, j) ) {
-
break;
-
}
-
Though the infamous story of the crashing PBX that caused an almost nationwide phone outage comes to mind - and you might (if you don't want to do anything else) consider 'return 0;' instead of 'break;'
As for 'malloc' this link http://www.opengroup.o rg/onlinepubs/009695399/functions/malloc.html says that errno is set if malloc fails (or you get a null pointer, which is easy to check for). So you would need to include <errno.h> but then it's as easy as -
if ( (*node == NULL) || (errno != 0) ) {
-
printf("Malloc failed!\n");
-
}
-
PS - you can look more in to that here: http://publib.boulder. ibm.com/infocenter/iadthelp/v6r0/index.jsp?topic =/com.ibm.etools. iseries.pgmgd.d oc/cpprog416.htm
They suggest setting errno immediately before any tests and then resetting it immediately after you check.
-Sicarie
Thanks I'll give that a try.
I was also thinking about adding a function called insertLast() I haven't tried yet it but what do you think of this. -
Node* insertLast ( Node **head, int val)
-
{
-
-
Node *last;
-
Node *temp;
-
-
if (*head != NULL)
-
-
last = (Node *)malloc( sizeof( Node ));
-
last->value = val;
-
-
last->link=temp;
-
last=temp;
-
-
}
You think that will get the job then.
sicarie 4,677
Recognized Expert Moderator Specialist
DeFault-
If you're eventually going for the full functionality I would recommend creating a head and tail node that carry sentinel values to identify them, so that doing such insertions are easier.
As for inserting to the tail, it seems that you pass it a node and a value, however the 'last' node is only set to temp, not joined to any nodes in the list (such as the 'head' or any nodes after it). But it's possible that I have been staring at my screen for too long today and I'm just not seeing it - I've been known to do that ;)
If you were to add a 'previous' pointer to the nodes as well, then you could set the tail nodes pretty easily, and create iterators to go through the list backwards as well, it's all a matter of how you want to implement it (how big you want them to be, how much you want to do with them, etc...).
Is this any better? -
Node* insertLast ( Node **head, int val)
-
{
-
-
Node *current;
-
Node *previous;
-
Node *last;
-
-
-
current = (Node *)malloc(sizeof(Node));
-
-
if (current != NULL)
-
-
previous = current;
-
current = current->link;
-
-
last = (Node *)malloc( sizeof( Node ));
-
last->value = val;
-
-
last->link=current;
-
previous->link = last;
-
-
}
sicarie 4,677
Recognized Expert Moderator Specialist
Better? I think you covered 'better' when you decided to program your own linked list set to better understand the workings of the language.
The only thing I can see (with a quick glance - I will take a better look at it later, so ignore me if I'm wrong), is that you pass this **head, but then don't do anything with it. What is being passed to insertLast()? Are you passing it the current 'last' node, and the value you want it to hold? I think the only thing you're missing is something tying it to the rest of the list, something along the lines of: -
head->next = current;
-
current->previous = head;
-
current->next =
-
/* and here is where you need to decide what you are going to do,
-
* if it is going to be a sentinal 'end' node, or if you're going to tie it
-
* back around to the first node, etc...
-
*/
-
Does that make sense, or am I on something and just missed where you did that?
Better? I think you covered 'better' when you decided to program your own linked list set to better understand the workings of the language.
The only thing I can see (with a quick glance - I will take a better look at it later, so ignore me if I'm wrong), is that you pass this **head, but then don't do anything with it. What is being passed to insertLast()? Are you passing it the current 'last' node, and the value you want it to hold? I think the only thing you're missing is something tying it to the rest of the list, something along the lines of: -
head->next = current;
-
current->previous = head;
-
current->next =
-
/* and here is where you need to decide what you are going to do,
-
* if it is going to be a sentinal 'end' node, or if you're going to tie it
-
* back around to the first node, etc...
-
*/
-
Does that make sense, or am I on something and just missed where you did that?
To answer your question about what is being passed to insertLast() that is basically inserting a node containing val at the end of a linked list.
For the code you wrote I would have a sentinal 'end' node so could you so me how it would be then by your code.
sicarie 4,677
Recognized Expert Moderator Specialist
To answer your question about what is being passed to insertLast() that is basically inserting a node containing val at the end of a linked list.
For the code you wrote I would have a sentinal 'end' node so could you so me how it would be then by your code.
Hang on, I'm gonna run it and look into it - I was just spot reading that, I could be totally missing something. It's just weird to me that you are passing a node and a value - not just one or the other (a node to just insert, or a value from which insertLast will create a node to add onto the list), but I probably am missing something, I'm not feeling too well today.
This is a silly question but when I ask how to improve insertFirst() and the code you wrote I would have to put that in insertFirst() right? The same goes to the malloc part I would have to put that in main right?
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