Recursive delete problems

Discussion in 'C++' started by Andrew Edwards, Jul 19, 2003.

  1. The following function results in an infinite loop! After 5+ hours of
    debugging, I am still unable to decipher what I am incorrectly. Any
    assistance is greatly appreciated.

    Thanks in advance,
    Andrew

    ==========>Code<==========

    //--------------------------------------------------------------------
    //
    // Recursive cRemove() function implemented in In-lab Exercise 3
    //
    //--------------------------------------------------------------------

    template < class DT >
    void List<DT>:: cRemove()

    // Recursively removes all occurrences of the character 'c' from a list
    // of characters. Moves cursor to the beginning of the list.

    {
    cRemoveSub(head);
    cursor = head;
    }

    // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    template < class DT >
    void List<DT>:: cRemoveSub ( ListNode<DT>*& p )

    // Recursive partner to the cRemove() function.

    {
    ListNode<DT>* delete_node;
    ListNode<DT>* prior_node;

    if ( p == NULL )
    {
    return;
    }
    else if ((p->dataItem == 'c' || p->dataItem == 'C') && (p == head))
    {
    delete_node = p;
    head = p = p->next;
    delete delete_node;
    cRemoveSub(p);
    }
    else if ((p->dataItem == 'c' || p->dataItem == 'C') && (p->next != 0))
    {
    delete_node = p;
    p = p->next;
    prior_node = head;
    while(prior_node->next->dataItem != 'c' &&
    prior_node->next->dataItem != 'C')
    {
    prior_node = prior_node->next;
    }
    delete delete_node;
    prior_node->next = p;
    cRemoveSub(p);
    }
    else if (p->next == 0)
    {
    delete p;
    p = 0;
    }
    else
    {
    cout << "Else: " << '[' << p->dataItem
    << "]->[" << p->next->dataItem << ']' << endl;
    cRemoveSub(p->next);
    }
    }
    Andrew Edwards, Jul 19, 2003
    #1
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  2. "Andrew Edwards" <> wrote...
    > The following function results in an infinite loop! After 5+ hours of
    > debugging, I am still unable to decipher what I am incorrectly. Any
    > assistance is greatly appreciated.
    >
    > Thanks in advance,
    > Andrew
    >
    > ==========>Code<==========
    >
    > //--------------------------------------------------------------------
    > //
    > // Recursive cRemove() function implemented in In-lab Exercise 3
    > //
    > //--------------------------------------------------------------------
    >
    > template < class DT >
    > void List<DT>:: cRemove()


    Well, without seeing what 'List<>' is I wouldn't dare to recommend
    anything. However, the usual scheme for deleting a singly-linked
    list is:

    void deleteListTailFirst(ListNode *plistnode)
    {
    if (plistnode->next != NULL)
    deleteListTailFirst(plistnode->next); // recurse

    deleteAllDataIn(plistnode); // non-recursive part
    }

    I recommend scrapping your solution and rewriting it afresh.

    Victor
    Victor Bazarov, Jul 19, 2003
    #2
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  3. On Sat, 19 Jul 2003 19:57:00 GMT, "Andrew Edwards" <> wrote:

    >The following function results in an infinite loop! After 5+ hours of
    >debugging, I am still unable to decipher what I am incorrectly.


    The question is then, what exactly are you trying to achieve?

    Details do matter.



    > Any assistance is greatly appreciated.
    >
    >Thanks in advance,
    >Andrew
    >
    >==========>Code<==========
    >
    >//--------------------------------------------------------------------
    >//
    >// Recursive cRemove() function implemented in In-lab Exercise 3
    >//
    >//--------------------------------------------------------------------
    >
    >template < class DT >
    >void List<DT>:: cRemove()


    The name "cRemove" is not very suggestive of what this routine
    is intended to achieve.

    When you can express something in code instead of comment(s),
    preferentially choose to express it in code.

    The compiler can't check comments, and they're not apparent where
    the routine is used.



    >// Recursively removes all occurrences of the character 'c' from a list
    >// of characters. Moves cursor to the beginning of the list.
    >
    >{
    > cRemoveSub(head);
    > cursor = head;
    >}


    Having a cursor (in more general terms, an iterator) built-in in the
    list abstraction isn't necessarily a good idea.

    In some cases it might make sense, for example if there should never
    be more than exactly one iterator.



    >
    >// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    >
    >template < class DT >
    >void List<DT>:: cRemoveSub ( ListNode<DT>*& p )
    >
    >// Recursive partner to the cRemove() function.
    >
    >{
    > ListNode<DT>* delete_node;
    > ListNode<DT>* prior_node;
    >
    > if ( p == NULL )
    > {
    > return;
    > }
    > else if ((p->dataItem == 'c' || p->dataItem == 'C') && (p == head))


    Here's the start of your problems. This routine should absolutely
    not have to bother with 'head' and other external things. It should
    just delete nodes.

    In addition, consider using a function that returns the uppercase of
    a given character.



    > {
    > delete_node = p;
    > head = p = p->next;


    Such multiple assignments are evil.


    > delete delete_node;
    > cRemoveSub(p);
    > }
    > else if ((p->dataItem == 'c' || p->dataItem == 'C') && (p->next != 0))
    > {
    > delete_node = p;
    > p = p->next;
    > prior_node = head;
    > while(prior_node->next->dataItem != 'c' &&
    > prior_node->next->dataItem != 'C')
    > {
    > prior_node = prior_node->next;
    > }


    What's this supposed to accomplish, except an infinite loop?


    > delete delete_node;
    > prior_node->next = p;
    > cRemoveSub(p);
    > }
    > else if (p->next == 0)
    > {
    > delete p;


    And what's the purpose of this?


    > p = 0;
    > }
    > else
    > {
    > cout << "Else: " << '[' << p->dataItem
    > << "]->[" << p->next->dataItem << ']' << endl;


    This looks like debugging/trace output.


    > cRemoveSub(p->next);


    As a general rule, don't manipulate data structures in debugging code.


    > }
    >}



    Here's one way to recursively delete all nodes containing a given
    letter (case-independent) in a singly linked zero-terminated list:


    char toUpper( char c )
    {
    return static_cast<char>(
    std::toupper( static_cast<unsigned char>( c ) )
    );
    }

    void deleteNodes( Node*& pHead, char upperCaseCh )
    {
    "Store pointer to rest of list in local variable pTail"
    if( toUpper( pHead->value ) == upperCaseCh )
    {
    "Delete the head node"
    }
    "Recursively delete the rest of the list"
    }


    You should not translate the pseudo-code to more than 4 statements.

    In "real" C++ code consider using standard library containers
    such as e.g. std::list instead.
    Alf P. Steinbach, Jul 19, 2003
    #3
  4. "Alf P. Steinbach" <> wrote:

    > The question is then, what exactly are you trying to achieve?
    >
    > Details do matter.
    >


    I'm trying to remove all occurrences of the character 'c' from list.
    Andrew Edwards, Jul 19, 2003
    #4
  5. "Alf P. Steinbach" <> wrote:

    > The name "cRemove" is not very suggestive of what this routine
    > is intended to achieve.
    >
    > When you can express something in code instead of comment(s),
    > preferentially choose to express it in code.
    >
    > The compiler can't check comments, and they're not apparent where
    > the routine is used.
    >


    Quite understandable! The function names and list interface, however, are
    not my decision. They cannot be changed because there is test code already
    written with these names to which I have no access.

    >
    > Having a cursor (in more general terms, an iterator) built-in in the
    > list abstraction isn't necessarily a good idea.
    >
    > In some cases it might make sense, for example if there should never
    > be more than exactly one iterator.
    >


    Again, the interface is predesigned and cannot be changed.

    > Here's the start of your problems. This routine should absolutely
    > not have to bother with 'head' and other external things. It should
    > just delete nodes.
    >
    > In addition, consider using a function that returns the uppercase of
    > a given character.
    >


    I'm trying to remove all occurrence of 'c' or 'C' form the list. That, I
    thought, requires that I begin at the head of the list.


    > Such multiple assignments are evil.


    Point taken!

    >
    > What's this supposed to accomplish, except an infinite loop?
    >


    It's supposed to remove the character from the "middle" of the list.

    > And what's the purpose of this?


    Delete the character if it's at the end of the list.


    > This looks like debugging/trace output.

    <snip>
    > As a general rule, don't manipulate data structures in debugging code.


    What is the best way to do debugging? I have a text editor and Borland's C++
    Builder commandlinetools v5.5.1. I'm new to programming so my art of
    debugging is severely underdeveloped.
    Andrew Edwards, Jul 19, 2003
    #5
  6. "Andrew Edwards" <> wrote...
    >
    > "Victor Bazarov" <> wrote:
    >
    > > Well, without seeing what 'List<>' is I wouldn't dare to recommend
    > > anything.

    >
    > Here are the two constructors I use:
    >
    > template < class DT >
    > ListNode<DT>:: ListNode ( const DT& initData, ListNode* nextPtr )
    > : dataItem(initData)
    > , next(nextPtr)
    > {
    > }
    >
    > template < class DT >
    > List<DT>:: List(int ignored)
    > // Constructor. Creates and empty list. The argument is provided
    > // for call compatability with the array implementation and is
    > // ignored.
    > {
    > cursor = head = NULL;
    > }
    >
    >
    > > However, the usual scheme for deleting a singly-linked list is:

    >
    > Note that I'm not trying to delete the list. I can do that. I'm trying to
    > remove all occurrences of 'c' from the list.


    Sorry, I must have misunderstood. Why do you need recursion, then?
    Just traverse the list and extract all the 'c' elements. Here is
    pseudocode:

    void removeFromList(ListNode* p, ListNode* pprev)
    {
    if (pprev)
    pprev->next = p->next;
    delete p;
    }

    void removeFromListAllThatHave(const ValueType& value)
    {
    while (head && head->getValue() == value) // remove the head
    {
    ListNode* newhead = head->next;
    removeFromList(head, 0);
    head = newhead;
    }

    if (head) // something remains in the list
    {
    ListNode* p = head;
    while (p->next)
    {
    if (p->next->getValue() == value)
    removeFromList(p->next, p);
    else
    p = p->next;
    }
    }
    }

    I think that should do it...

    Victor
    Victor Bazarov, Jul 19, 2003
    #6
  7. "Victor Bazarov" <> wrote:

    > Sorry, I must have misunderstood. Why do you need recursion, then?


    Using iteration over recursion is not an option. This must be done with
    recursion.
    Andrew Edwards, Jul 19, 2003
    #7
  8. Andrew Edwards

    David White Guest

    "Andrew Edwards" <> wrote in message
    news:gohSa.253958$...
    > The following function results in an infinite loop! After 5+ hours of
    > debugging, I am still unable to decipher what I am incorrectly. Any
    > assistance is greatly appreciated.
    >
    > Thanks in advance,
    > Andrew
    >
    > ==========>Code<==========
    >
    > //--------------------------------------------------------------------
    > //
    > // Recursive cRemove() function implemented in In-lab Exercise 3
    > //
    > //--------------------------------------------------------------------
    >
    > template < class DT >
    > void List<DT>:: cRemove()
    >
    > // Recursively removes all occurrences of the character 'c' from a list
    > // of characters. Moves cursor to the beginning of the list.
    >
    > {
    > cRemoveSub(head);
    > cursor = head;
    > }
    >
    > // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    >
    > template < class DT >
    > void List<DT>:: cRemoveSub ( ListNode<DT>*& p )
    >
    > // Recursive partner to the cRemove() function.
    >
    > {
    > ListNode<DT>* delete_node;
    > ListNode<DT>* prior_node;
    >
    > if ( p == NULL )
    > {
    > return;
    > }
    > else if ((p->dataItem == 'c' || p->dataItem == 'C') && (p == head))
    > {
    > delete_node = p;
    > head = p = p->next;
    > delete delete_node;
    > cRemoveSub(p);
    > }
    > else if ((p->dataItem == 'c' || p->dataItem == 'C') && (p->next != 0))
    > {
    > delete_node = p;
    > p = p->next;


    The above line changes either 'head' or the 'next' member of a node, because
    'p' is a reference to one of those. This means that you have inadvertently
    taken the node with the 'c' out of the list.

    > prior_node = head;
    > while(prior_node->next->dataItem != 'c' &&
    > prior_node->next->dataItem != 'C')
    > {
    > prior_node = prior_node->next;
    > }


    The reference problem causes this loop to find the the second node with a
    'c'. If there isn't one it will just fly off into space.

    > delete delete_node;
    > prior_node->next = p;
    > cRemoveSub(p);
    > }
    > else if (p->next == 0)
    > {
    > delete p;


    Here you are deleting the tail node whether it has a 'c' or not.

    > p = 0;
    > }
    > else
    > {
    > cout << "Else: " << '[' << p->dataItem
    > << "]->[" << p->next->dataItem << ']' << endl;
    > cRemoveSub(p->next);
    > }
    > }
    >


    DW
    David White, Jul 19, 2003
    #8
  9. Andrew Edwards <> wrote in message
    news:jniSa.313568$...
    >
    > "Victor Bazarov" <> wrote:
    >
    > > Sorry, I must have misunderstood. Why do you need recursion, then?

    >
    > Using iteration over recursion is not an option. This must be done with
    > recursion.


    void List::remove_node(node *n, node *parent) // private member
    function
    {
    if(parent) parent->next = n->next;
    else m_head = n->next;
    delete n;
    }

    void List::remove_all_with_value(const value_type& val) // public member
    function
    {
    remove_all_with_value_recurse(val, m_head, NULL);
    }

    void List::remove_all_with_value_recurse(const value_type& val, node *n,
    node *parent) // private member function
    {
    assert(n != NULL);
    if(n->next) remove_all_with_value_recurse(n->next, n);
    if(n->val == val) remove_node(n, parent);
    }

    I think this works (I haven't got access to a compiler for a couple of days
    so checking it is problematic). I've skipped all the usual template stuff to
    make it easier to understand.

    HTH,

    Stuart.
    Stuart Golodetz, Jul 20, 2003
    #9
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