Thread: help with reversing something in a linklist

Originally Posted by quzah

1) Set up a new pointer to the next item in the list, so once we change the current node's next poitner, we don't lose our list.

temp2 = temp->next;

done.

Originally Posted by quzah

4) Use the node we set up in step 1 to advance down the list we're reversing.

temp = temp2;

using the node created in step 1.

Code:

custNode *reverse(custNode *current)
{

    custNode *temp2;

    for( custNode *temp = current; temp; temp = temp2 )
    {
        temp2 = temp->next;
        temp->next = temp2;
        temp = temp2; // updated line
    }
    return temp2;

}

So you need a seperate pointer to store that.

done, temp2 the new reordered list is returned.
The for loop is not running endlessly this time, but the list is still ordered..pfft.

Hmm i see.

ugh, this is so frustrating... i've modified the code to include the reordered list by creating another reference to the list as you said. But it's still ordered.

Code:

custNode *reverse(custNode *current)
{
    custNode *temp2;
    custNode *temp3;

    for( custNode *temp = current; temp; temp = temp2 )
    {
        temp2 = temp->next;
        temp->next = temp2;
        temp3 = temp2;
    }


    return temp3;


}

Code:

custNode *start = NULL;

updateList(&start, cityName, xcoor, ycoor); // i have a while loop which scans the text file and gets each cityName xcoor etc.

printList(start); <-- works fine.

 reverse(start); // run the reverse algorithm

  printList(start);  <--- same thing is printed after the reverse function is run

what does newlist refer to? I'm assuming the list.

Code:

custNode  *newList = reverse(start);
 printList(newList);

nothing is printed.

i've managed to get it printed by sticking the printf statement inside the reverse function.

Code:

printf("\n%s", temp3->cityName)

but

city2
(null)

the last element in the list is null?

and also

Code:

custNode  *newList = reverse(start);
 printList(newList);

gives me a segmentation fault.

I've also tried it on 5 link list elements it reverses some but as usual the last element is null for some reason. This shouldn't be too hard to do, i still don't get why the last element is null.

Code:

    charNode *temp2 ;
    charNode *temp3 = NULL;

    for( charNode *temp =  current; temp; temp = temp2 )
    {
      temp2 = temp->next;
        temp->next = temp3;
        temp = temp2; // updated line

        printf("\n%s", temp3->cityName);

    }

hmm :|

(null)
(null)
(null)
(null)

to make things a little clear:

Code:

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

struct charRecord
{
   char numberone;
   char numbertwo;
   struct charRecord *next;
};

typedef struct charRecord charNode;

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

int main()
{
   charNode *start = NULL;
   start = getString(start);
   //reverse(start);
   printList(start);
   printf("\n +--+ reversing... +--+ \n");
   start = reverse(start);
   //printList(start);
   return 0;
}


charNode* getString(charNode *start)
{

     addElement(&start, 'A', 'A');
     addElement(&start, 'B', 'B');
     addElement(&start, 'C', 'C');

     return start;


}

void addElement(charNode **start, char numberone, char 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%c %c\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;

   }

}

charNode *reverse(charNode *current)
{

    charNode *temp2;
     charNode *temp3 = NULL;

    for( charNode *temp = current; temp; temp = temp2 )
    {
        temp2 = temp->next;
        temp->next = temp3;
        temp3 = temp2;
        printf("\n%c %c\n", temp3->numberone, temp3->numbertwo );
    }

    return temp2;

}

/*

A A

B B

C C

 +--+ reversing... +--+

B B

C C

*/

should be:

BB
AA
CC

yeah that's what i've done:

Code:

harNode *reverse(charNode *current)
{

    charNode *temp2;
     charNode *temp3 = NULL;

    for( charNode *temp =  current; temp; temp = temp2 )
    {
        temp2 = temp->next;
        temp->next = temp3;
        temp3 = temp;
        printf("\n%c%c\n", temp3->numberone, temp3->numbertwo);
    }

    return temp3;

}

still's still in the same order though:

+--+ r

AA

BB

CC

and also the following works but it gives me a segmentation fault at the last element:

Code:

charNode *reverse(charNode *current)
{

    charNode *temp2;
     charNode *temp3 = NULL;

    for( charNode *temp =  current; temp; temp = temp2 )
    {
        temp2 = temp->next;
        temp->next = temp3;
        temp3 = temp2;
        printf("\n%c%c\n", temp3->numberone, temp3->numbertwo);
    }

    return temp3;

}