Just need a nice push in the right direction

[Donna]

Hi lovelies,
I am a little bit at a dead-end with the project I have been working on. I would just like to be pushed in the right direction on how a linkedlist would work on changing the element already in a list to a different position. Sort of a swap in a way.

For example, I already inserted 1 2 3 4
I want to swap position 1 and 2
1 3 2 4

I thought is was simple enough but for some reason when I try I come up with errors.
Thank you <3!

cout << myList.change(1,2) << endl;

  bool List::change(int first_pos, int second_pos)
{
    

}







//Header file 


#include <iostream>
using namespace std;

typedef int ElementType;

class List
{ 
public:
	List();  
	~List(); 
	List(const List & original); 
	void insert(ElementType item, int pos);
	void erase(ElementType item);
	void traverse();
	bool change(int first_pos, int second_pos); 
	void display(ostream & out) const;

private:
	class Node
	{
	public:
		ElementType data;
		Node * next;
		Node * value;
		Node()
			: next(NULL)
		{ }
		Node(ElementType dataValue)
			: data(dataValue), next(NULL)
		{ }
	};
	Node * first;
	Node * second;
	Node * head;
	int mySize;
};

#endif 

[Donna]

just something simple.

[giblit]

You need to use a temp variable.

Something like this

int temp = first_pos;
first_pos = second_pos;
second_pos = temp;

[Donna]

Thank you I did something like that, but it doesn't affect the code at all. Do I have to do something with Node?

Here is my insert

void List::insert(ElementType dataVal, int index)
{
	if (index < 0 || index > mySize)
	{
		cerr << "Illegal location to insert -- " << index << endl;
		return;
	}

	mySize++;
	Node * newPtr = new Node(dataVal);
	Node * predPtr = first;
	if (index == 0)
	{
		newPtr->next = first;
		first = newPtr;
	}
	else
	{
		for(int i = 1; i < index; i++)
			predPtr = predPtr->next;
		newPtr->next = predPtr->next;
		predPtr->next = newPtr;
	}
}

Like I did here

[JLBorges]

Something like this:

bool List::change(int first_pos, int second_pos)
{
    if( first_pos > second_pos ) std::swap(first_pos,second_pos) ;
    
    if( first_pos < 0 || second_pos > mySize) return false ; 
    if( first_pos == second_pos ) return true ;
    
    int cnt = 0 ;
    Node* n = first ;

    for( ; cnt < first_pos ; ++cnt ) n = n->next ;
    Node* first_node = n ;

    for( ; cnt < second_pos ; ++cnt ) n = n->next ;
    Node* second_node = n ;
    
    std::swap( first_node->value, second_node->value ) ;
    return true ;
}

[Donna]

Where is value coming from?

[giblit]

This might help towards the middle of the page.
http://www.codeproject.com/Articles/24684/How-to-create-Linked-list-using-C-C


I haven't made a linked list yet sorry can't be of too much help but yours looks pretty similar to that website. What errors are you coming up with anyways?

[Donna]

The errors I was properly figuring out how to use the nodes, and that I could not directly compare them to the positions given. Ha confusing

[Donna]

@JLBorges Unfortunately the code still doesn't swap the two numbers

[JLBorges]

#include <iostream>
#include <algorithm>

struct List
{
    struct Node { int value ; Node* next ; };
    int mySize = 0 ;
    Node* first ;
    bool change(int first_pos, int second_pos) ;
};

bool List::change(int first_pos, int second_pos)
{
    if( first_pos > second_pos ) std::swap(first_pos,second_pos) ;

    if( first_pos < 0 || second_pos > mySize) return false ;
    if( first_pos == second_pos ) return true ;

    int cnt = 0 ;
    Node* n = first ;

    for( ; cnt < first_pos ; ++cnt ) n = n->next ;
    Node* first_node = n ;

    for( ; cnt < second_pos ; ++cnt ) n = n->next ;
    Node* second_node = n ;

    std::swap( first_node->value, second_node->value ) ;
    return true ;
}

int main()
{
    List lst ;
    using Node = List::Node ;
    lst.first = new Node{ 0, new Node{ 1, new Node{ 2, new Node{ 3,
                           new Node{ 4, new Node{5,nullptr} }}}}} ;
    lst.mySize = 6 ;

    for( Node* n = lst.first ; n ; n = n->next ) std::cout << n->value << ' ' ;
    std::cout << '\n' ;

    lst.change(1,4) ;
    for( Node* n = lst.first ; n ; n = n->next ) std::cout << n->value << ' ' ;
    std::cout << '\n' ;

    lst.change(2,3) ;
    for( Node* n = lst.first ; n ; n = n->next ) std::cout << n->value << ' ' ;
    std::cout << '\n' ;
}

http://coliru.stacked-crooked.com/a/97ee2bc051468ef9

[Donna]

Thank you but I can not change my class list, I thank you for your effort. All of you, thank you.

[giblit]

He was giving you an example of how one works. So you can learn from it and modify your code from what you learned he didn't want you do copy/paste his so yours would work :P

[Donna]

No worries, I have been trying to manipulate it.

[Donna]

Hmm its seems I can't figure it out.