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PPP2 Chapter 17 - Link class member functions

DragonOsman (715)
The Link class member functions add, erase, find and advance. I need help with them. I wrote the helper function version of the functions, but I'm having modifying them for the member function version. Here's what I have right now:
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Link *Link::add(Link *n)
{
	if (n == nullptr)
	{
		return this;
	}
	if (this == nullptr)
	{
		return n;
	}
	n->prev = this;
	if (succ)
	{
		succ->prev = n;
	}
	n->succ = succ;
	succ = n;
	return n;
}

Link *Link::erase()
{
	if (this == nullptr)
	{
		return nullptr;
	}
	if (succ)
	{
		succ->prev = prev;
	}
	if (prev)
	{
		prev->succ = succ;
	}
	return succ;
}

Link *Link::find(const std::string &s)
{
	while (this != nullptr)
	{
		if (value == s)
		{
			return this;
		}
		this->succ;
	}
	return nullptr;
}

const Link *Link::find(const std::string &s) const
{
	while (this != nullptr)
	{
		if (value == s)
		{
			return this;
		}
		this->succ;
	}
	return nullptr;
}

Link *Link::advance(int n) const
{
	if (this == nullptr)
	{
		return nullptr;
	}
	if (n > 0)
	{
		while (n--)
		{
			if (succ == nullptr)
			{
				return nullptr;
			}
		}
	}
	else if (n < 0)
	{
		while (n++)
		{
			if (prev == nullptr)
			{
				return nullptr;
			}
		}
	}
	return const_cast<Link *>(this);
}


These are the ones in the book:
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Link* add(Link* p, Link* n) // insert n after p; return n
{
    // much like insert (see exercise 11)
}

Link* erase(Link* p) // remove *p from list; return p’s successor
{
    if (p==nullptr) return nullptr;
    if (p–>succ) p–>succ–>prev = p–>prev;
    if (p–>prev) p–>prev–>succ = p–>succ;
    return p–>succ;
}

Link* find(Link* p, const string& s) // find s in list;
                                     // return nullptr for “not found”
{
    while (p) {
    if (p–>value == s) return p;
    p = p–>succ;
    }
    return nullptr;
}

Link* advance(Link* p, int n) // move n positions in list
                              // return nullptr for “not found”
                              // positive n moves forward, negative backward
{
    if (p==nullptr) return nullptr;
    if (0<n){
        while (n––){
            if (p–>succ == nullptr) return nullptr;
            p = p–>succ;
        }
    }
    else if (n<0){
        while (n++){
            if (p–>prev == nullptr) return nullptr;
            p = p–>prev;
        }
    }
    return p;
}


How should I modify these to the member functions where we have access to the this pointer? And wat would be the this pointer equivalent of the line p = p->prev;?

I hope someone can help me. Thanks in advance.
Last edited on
ne555 (10692)
you need to differentiate a node from the list.
for example, ¿why does a node have a search function?, ¿what does advance mean? ¿are insert[i] and [i]erase responsibilities of a node or of a list?

> this == nullptr
that makes no sense, you are in a member function, ¿how may be this null then?

> this->succ;
statement has no effect (you probably have a warning with a similar message)
you are doing nothing there, no operation is performed.
DragonOsman (715)
Like I said, I need to know a good equivalent for the line p = p->prev; for that version of the function. And yeah, you're right about the text for null. I'll take that out.

This is part of a Link class. The rest of it, including the insert() function, were given in the book (I tried to modify that to write the add() function, but I'm not sure if I did it right). The book said at the end of the chapter that this is only the "link" part of a linked list. The "list" part will come later. It's like a re-implementation of the standard library "list" for teaching purposes.

Edit: Here's the whole program code:
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// Osman Zakir
// 9 / 7 / 2017
// Bjarne Stroustrup: Programming: Principles and Practice Using C++ 2nd Edition
// Chapter 17 Section 17.9.4
// Doubly Linked Lists

#include <iostream>
#include "../../cust_std_lib_facilities.h"

class Link
{
public:
	std::string value;

	Link(const std::string &v, Link *p = nullptr, Link *s = nullptr);

	Link *insert(Link *n);    // insert n before this object
	Link *add(Link *n);	  // insert n after this object
	Link *erase();		 // remove this object from list
	Link *find(const std::string &s);    // find s in list
	const Link *find(const std::string &s) const;    // find s in const list

	Link *advance(int n) const;    // advance n positions in list
	
	Link *next() const { return succ; }
	Link *previous() const { return prev; }

private:
	Link *prev;
	Link *succ;
};

void print_all(Link *p);

int main()
{
	Link *norse_gods = new Link{ "Thor" };
	norse_gods = norse_gods->insert(new Link{ "Odin" });
	norse_gods = norse_gods->insert(new Link{ "Zeus" });
	norse_gods = norse_gods->insert(new Link{ "Freia" });

	Link *greek_gods = new Link{ "Hera" };
	greek_gods = greek_gods->insert(new Link{ "Athena" });
	greek_gods = greek_gods->insert(new Link{ "Mars" });
	greek_gods = greek_gods->insert(new Link{ "Poseidon" });

	Link *p = greek_gods->find("Mars");
	if (p)
	{
		p->value = "Ares";
	}

	Link *p2 = norse_gods->find("Zeus");
	if (p2)
	{
		if (norse_gods)
		{
			norse_gods = p2->next();
			p2->erase();
			greek_gods = greek_gods->insert(p2);
		}
	}

	print_all(norse_gods);
	print_all(greek_gods);

	keep_window_open();
}

Link::Link(const std::string &v, Link *p, Link *s)
	: value{ v }, prev{ p }, succ{ s }
{

}

Link *Link::insert(Link *n)
{
	if (n == nullptr)
	{
		return this;
	}
	n->succ = this;
	if (prev)
	{
		prev->succ = n;
	}
	n->prev = prev;
	prev = n;
	return n;
}

Link *Link::add(Link *n)
{
	if (n == nullptr)
	{
		return this;
	}
	n->prev = this;
	if (succ)
	{
		succ->prev = n;
	}
	n->succ = succ;
	succ = n;
	return n;
}

Link *Link::erase()
{
	if (succ)
	{
		succ->prev = prev;
	}
	if (prev)
	{
		prev->succ = succ;
	}
	return succ;
}

Link *Link::find(const std::string &s)
{
	std::size_t i = 0;
	while (succ != nullptr)
	{
		if (value == s)
		{
			return this;
		}
		succ++;
		++i;
	}
	return nullptr;
}

const Link *Link::find(const std::string &s) const
{
	std::size_t i = 0;
	while (succ != nullptr)
	{
		if (value == s)
		{
			return this;
		}
		++i;
	}
	return nullptr;
}

Link *Link::advance(int n) const
{
	if (n > 0)
	{
		while (n--)
		{
			if (succ == nullptr)
			{
				return nullptr;
			}
		}
	}
	else if (n < 0)
	{
		while (n++)
		{
			if (prev == nullptr)
			{
				return nullptr;
			}
		}
	}
	return const_cast<Link *>(this);
}

void print_all(Link *p)
{
	std::cout << "{ ";
	while (p)
	{
		std::cout << p->value;
		if (p == p->next())
		{
			std::cout << ", ";
		}
		std::cout << " }";
	}
}


I hope people can help me better now. The add(), advance(), erase() and find() functions are the ones I need help on. Mainly.
Last edited on
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