Thread: Linked lists; getting the current elements next element

how exactly is the next element of the current element accessed in a linked list?
for some strange reason:

Code:

printf("%s", start->next->custName);

gives me a segmentation fault.

The first element element is skipped, the second one is printed, and then thats when i get the segmentation fault.

You should show how you are building your list and your list structure

hi sand_man,

i've already posted the code here

but here's a slightly different version to show my problem:

Code:

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

struct charNode
{
   double numberone;
   double numbertwo;
   struct charNode *next;
};

typedef struct charNode charNode;

void addElement(charNode **start, double numberone, double numbertwo);
void printList(charNode *start);
charNode* newNode(double numberone, double numbertwo);
void getString(charNode *start);

int main()
{
   charNode *start = NULL;

   getString(start);
   printList(start);
   return 0;
}


void getString(charNode *start)
{

     addElement(&start, 1, 1);
     addElement(&start, 2, 2);
     addElement(&start, 3, 3);
     printList(start);
}

void addElement(charNode **start, double numberone, double numbertwo)
{

   charNode *temp,*prev, *loc;

   temp = newNode(numberone, numbertwo);


   if (*start == NULL)
   {
      *start = temp;
   }

   else
   {
      loc = *start;
      prev = NULL;

      while (loc != NULL)
      {
         prev = loc;
         loc = loc->next;
      }


      temp->next = loc;
      if (prev == NULL)
      {
         *start = temp;
      }
      else
      {
         prev->next = temp;
      }
   }

}


charNode* newNode(double numberone, double numbertwo)
{

   charNode *temp;

   temp = (charNode *) malloc(sizeof(charNode));
   if (temp == NULL)
   {
      printf("WARNING - Memory allocation error\n");
      exit(EXIT_FAILURE);
   }


   temp->numberone = numberone;
   temp->numbertwo = numbertwo;


   temp->next = NULL;

   return temp;
}


void printList(charNode *start)
{

   while (start != NULL)
   {

      printf("\n%f %f\n", start->numberone, start->numbertwo);
      printf("%s %.2f %.2f %s", "next element is -+> ", start->next->numberone, start->next->numbertwo, "**OK**");
      start = start->next;

   }


/*

1.000000 1.000000
next element is -+>  2.00 2.00 **OK**
2.000000 2.000000
next element is -+>  3.00 3.00 **OK**
3.000000 3.000000
<--- BAM! segmentation fault.


notice how the "next element..." is not printed in the first line?

yes you're right but how exactly do i get a reference to the next element when its not null?

Here's what actually happens:

element one | start | next skips here for some reason
element two -> start | next points here when it's first run
element three-> next now points here second time
element four -> ???

it thinks that theres a last element because it skipped the first one and since there's no memory allocated to me in that area it throws a segmentation fault.

How do i get it to start off at element one?

ok i see, it was a simple case of:

Code:

   while (start != NULL )
   {

      printf("\n%f %f\n", start->numberone, start->numbertwo);
      if (start->next != NULL)
      {
         printf("%s %.2f %.2f %s", "next element is -+> ", start->next->numberone, start->next->numbertwo, "**OK**");
      }
      start = start->next;

   }

i guess that worked, but it's not really what i wanted. With the following modification:

Code:

 addElement(&start, 28.80, 17.60);
     addElement(&start, 5.60,  79.12);
     addElement(&start,  55.30, 49.90);
          addElement(&start,  22.22, 11.11);

printed out:

28.800000 17.600000

5.600000 79.120000

55.300000 49.900000

as you can see the last element 22.22, 11.11 is skipped
with the following code:

Code:

  while (start != NULL )
   {

      if (start->next != NULL)
      {
                printf("\n%f %f\n", start->numberone, start->numbertwo);
         //printf("%s %.2f %.2f %s", "next element is -+> ", start->next->numberone, start->next->numbertwo, "**OK**");
      }
      start = start->next;

   }

is there anyway to print it out without skipping the last node? It stops because when it hits the last element and when start->next is run, its null so the if condition fails which causes the start->numberone print statement to fail, when it's actually not null.

What about

Code:

 while (start != NULL )
   {

printf("\n%f %f\n", start->numberone, start->numbertwo);
         //printf("%s %.2f %.2f %s", "next element is -+> ", start->next->numberone, start->next->numbertwo, "**OK**");
     if (start->next == NULL)
          break;
                
      start = start->next;

   }

Yep, based on what he said i figured it out. But for some reason:

Code:

     while (start != NULL )
   {

      //printf("\n%f %f\n", start->numberone, start->numbertwo);
      if (start->next != NULL)
      {
         //printf("%s %.2f %.2f %s", "next element is -+> ", start->next->numberone, start->next->numbertwo, "**OK**");
         printf("\n%s%f%s%f ", "X is ", start->numberone, " Y is ", start->numbertwo );
         total += sqrt( ( pow( (start->numberone - start->next->numberone), 2) + pow( (start->numbertwo - start->next->numbertwo), 2) ));
      }
      start = start->next;

    printf("%s %.2f", "\nTotal is now ", total);

   }

X is 28.800000 Y is 17.600000
Total is now 65.75
X is 5.600000 Y is 79.120000
Total is now 123.40
X is 55.300000 Y is 49.900000
Total is now 174.38
Total is now 174.38

why is the last element contents:
22.22, 11.11
skipped?

That is becoz

Code:

printf("\n%s%f%s%f ", "X is ", start->numberone, " Y is ", start->numbertwo );

is inside

Code:

if (start->next != NULL)

If you do it like this. I dont see the last being skipped here. But I bet you would still have a problem??

Code:

     while (start != NULL )
   {

      printf("\n%f %f\n", start->numberone, start->numbertwo);

      if (start->next != NULL)
      {
         total += sqrt( ( pow( (start->numberone - start->next->numberone), 2) + pow( (start->numbertwo - start->next->numbertwo), 2) ));
      }
      start = start->next;

    printf("%s %.2f", "\nTotal is now ", total);

   }

Is this part importan to you?

Code:

      total += sqrt( ( pow( (start->numberone - start->next->numberone), 2) + pow( (start->numbertwo - start->next->numbertwo), 2) ));

what about put that out of the if( start-next==null). and replace the red part with zero if start-next would be null. There wouldnt be anything there anyway.


x =0;

if( start->next != null )
x = pow( (start->numbertwo - start->next->numbertwo), 2) );

Code:

      total += sqrt( ( pow( (start->numberone - start->next->numberone), 2) + x  )

yes, the total should be 183. You're right the last element is printed now but it is not being added to total for some reason.