Circular List help

[Corum]

The circular list code is incomplete program with lots of feature missing, it's only one specific section that I require help with but I thought it would better to show the entirety of it to help you understand better.

The section surrounded by -------------------------- is where I'm stuck, basically the print() method should start at the first element and go through the circular list printing each element until it returns to the first element. The program then rotates the list and prints each element again.

I feel that it maybe the "do...while" loop is causing the program since when I changed the "while" expression to "(c = while)", obviously wrong as it never ended, it successfully ran through the program and printed each element but when it's "(c != while)" it fails.

If anyone can offer any assistance as to what I've done wrong then it would be greatly appreciated, I've racked my brains for long time now! (I suck)

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

struct listitem {
  int contents;
  struct listitem* next;
  struct listitem* prev;
};

class rotatableList {
protected:
  listitem *first;
  int size;
public:
  rotatableList() {
    first=NULL;
    size=0;
  }
};

class singleRotateList : public rotatableList {
public:
  void addFirst(int i) {
// Adds a new list element with contents i
// at the start of the list

    struct listitem* newItem=new listitem();

    newItem->contents=i;

    if (!first) {
      newItem->next=newItem;
      newItem->prev=newItem;
      first=newItem;
    }
    else {
       newItem->next=first;
       newItem->prev=first->prev;
       first->prev->next=newItem;
       first->prev=newItem;
       first=newItem;

   }

// Omitted: code to increment "size"
    size++;
  }
//---------------------------------------------------------------------
 void print() {
      listitem *c=first;

    if (c) {
        printf("%d\n", *c);
        listitem *c=first=first->next;

        do{
            printf("%d\n", *c);
            listitem *c=first=first->next;
        }while (c != first);
    }
  }
// Omitted: code to
// - print out contents of list element referred to by c
//   (followed by a new line);
// - make c point to the next element in the list;
// - Do the following while c doesn't refer to the same list
//   element referred to by first:
//   - print out contents of list element referred to by c
//     (followed by a new line);
//   - make c point to the next element in the list;
//----------------------------------------------------------------------

  void rotate() {
// Make first refer to the next item in the list
    if (first) first=first->next;
  }
};

class randomRotateList : public rotatableList {
public:
  void rotate() {
    if (first) {
    }
  }
};

main() {
  srand(time(NULL)); // This initialises the random number generator

  printf("Creating a singleRotateList and rotating it:\n");
  singleRotateList* l=new singleRotateList();
  l->addFirst(3);
  l->addFirst(4);
  l->addFirst(5);
  l->addFirst(6);
  l->print();

  l->rotate();
  printf("\nAfter rotation:\n");
  l->print();

  delete l;

  printf("\nNow using a randomRotateList:\n");
}

[Salem]

Well you probably shouldn't be modifying first, just to print it.

[Duoas]

You have three cs there:
line 51
line 55
line 59
Your loop will never terminate because every c is always == first (so no matter which of the three the compiler chooses to use you've got an infinite loop).

Make sure to use just one c.

Also, watch how many times you change first. You want to bump it just once (if I understand you right).

void print() {
    listitem *c=first;

    if (c) {
        printf("%d\n", *c);
        c=first=first->next;

        do{
            printf("%d\n", *c);
            c=c->next;
        }while (c != first);
    }
  }

Hope this helps.

[edit] BTW. What if your circular list has just one item? (It will get printed twice.)

[Corum]

Thanks a lot for helping me understand where I was going wrong and explaining it so well you've been a huge help.

The circular list always have 4 items (in main() you see them added to addFirst()) so it shouldn't be a problem.

Thank you again!

[Corum]

Last problem I promise! I've succesfully got the singleRotateList() to work and output but after that's executed I want it execute the randomRotateList() and output accordingly.

The problem is the main() method I have calling it two different rotate() methods but it keeps referring to the first rotate(), ideally I want it to call to the first for the singleRotateList() and the second rotate() for randomRotateList().

I'm not to hot on abstract and overridable methods so if anyone can give me tips as to how I can achieve this it would be much appreciated, again!

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

struct listitem {
  int contents;
  struct listitem* next;
  struct listitem* prev;
};

class rotatableList {

protected:
  listitem *first;
  int size;
public:
  rotatableList() {
    first=NULL;
    size=0;
  }

void addFirst(int i) {

    struct listitem* newItem=new listitem();

    newItem->contents=i;

    if (!first) {
        newItem->next=newItem;
        newItem->prev=newItem;
        first=newItem;
    }
    else {
        newItem->next=first;
        newItem->prev=first->prev;
        first->prev->next=newItem;
        first->prev=newItem;
        first=newItem;
    }

  size++;

  }

  void print() {
      listitem *c=first;

    if (c) {
        printf("%d\n", *c);
        c=first->next;
        do{
            printf("%d\n", *c);
            c=c->next;
        }while (c != first);
    }
  }

    void rotate() {

    if (first) first=first->next;

    }

};

class singleRotateList : public rotatableList {
    public:


};

class randomRotateList : public rotatableList {
public:
  void rotate() {
    if (first) {

        int numberOfTimes = rand() % 4;
        printf("The list will be rotated %d times.\n", numberOfTimes);
        for (int i = 0; i<numberOfTimes; i++){
            first=first->next;
        }
    }
  }
};
---------------------------------------------------------------------------------
main() {
  srand(time(NULL)); // This initialises the random number generator

  printf("Creating a singleRotateList and rotating it:\n");
  rotatableList* l=new singleRotateList();
  l->addFirst(3);
  l->addFirst(4);
  l->addFirst(5);
  l->addFirst(6);
  l->print();

  l->rotate();
  printf("\nAfter rotation:\n");
  l->print();

  delete l;

  printf("\nNow using a randomRotateList:\n");

  l=new randomRotateList();
  l->addFirst(3);
  l->addFirst(4);
  l->addFirst(5);
  l->addFirst(6);
  l->print();

  l->rotate();
  printf("\nAfter rotation:\n");
  l->print();
---------------------------------------------------------------------------------
}

[ArkM]

Use magical word virtual in

class rotatableList {
...
virtual void rotate () ...
...
};

and try again...

[Corum]

That indeed is a magical word, thanks for the help! Hugely appreciated.