basic linked list program problem

This is a discussion on basic linked list program problem within the C++ Programming forums, part of the General Programming Boards category; I want to create a link list that has two record, and insert another record in between the two. However ...

  1. #1
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    basic linked list program problem

    I want to create a link list that has two record,
    and insert another record in between the two.
    However my program is not working properly in the sense that the linked list did not really re-link after I commanded it to.

    Code:
    /* This program will attempt to insert a record in between two records */
    
    #include <iostream>
    
    using namespace std;
    
    struct list{
    
    char event[20];
    int year;
    list *next;
    }* root, event1, event2;
    
    
    void insert(list*, char incident[20], int);
    
    int main()
    {
     char event[20];
     int year;
    
     list event1 = { "World War I", 1914};
     list event2 = { "World War II", 1939};
    
     root = &event1;
     event1.next = &event2;
     event2.next = NULL;
    
     cout<<"Please enter an event and a date in between 1914 and 1939."<<endl;
     cin>>event>>year;
    
     insert(root, event, year);
    
     return 0;
    }
    
     void insert(list *head, char incident[20], int year)
    {
     list event_between;
     event_between.event[20] = incident[20];
     event_between.year = year;
    
     //The pointer in event1 should point to the reference of event_between and relink the linked list
     event1.next =&event_between;
     event_between.next = &event2;
     event2.next=NULL;
    
     cout<<"The new list has the following entries: "<<endl;
     while(head!=NULL)
     {
     cout<<head->event<<' '<<head->year;
     cout<<endl;
     head=head->next;
     }
    }
    Could you also please give me some pointers how on to reduce the size of this program?
    I'm only beginning C++, thanks for all helps

  2. #2
    ATH0 quzah's Avatar
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    The whole point of linked lists is to have dynamic data. None of yours are dynamic. Furthermore:
    Code:
     void insert(list *head, char incident[20], int year)
    {
     list event_between;
    That is a local variable that is destroyed when your function returns.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
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    Quote Originally Posted by Hybodus View Post
    Could you also please give me some pointers how on to reduce the size of this program?
    I'm only beginning C++, thanks for all helps
    Well, considering this is C++ and not C, you could simplify your program by using the STL and drop all the C stuff. Additionally, there is a std::list which will make life much easier.
    Code:
    #include <iostream>
    /*use the STL*/
    #include <string>
    #include <list>
    #include <iterator>
    
    struct node{
    	std::string name;
    	int year;
    };
    void addNode(std::list<node>&,node);
    
    int main(void){
    
    	std::list<node>myList; //create list
    	std::list<node>::iterator list_it; //iterator for printing list
    	node input; //insertion node
    
    	
    	std::cout<<"Enter event name and year: ";
    	std::cin>>input.name>>input.year;
    	while(!std::cin.eof()){ //Terminate input with EOF
    		addNode(myList, input);
    		std::cout<<"Enter event name and year: ";
    		std::cin>>input.name>>input.year;
    	}
    
    	//print output
    	list_it = myList.begin();
    	while(list_it != myList.end()){
    		std::cout<<"Event: "<<list_it->name<<" Year: "<<list_it->year<<std::endl;
    		++list_it;
    	}
    	
    	return(0);
    }
    void addNode(std::list<node>& myList, node mynode){
    
    	std::list<node>::iterator list_it = myList.begin();
    	
    	while(list_it!=myList.end()){ //walk list and insert based on year
    		if(mynode.year < list_it->year)
    			break;
    		++list_it;
    	}
    
    	myList.insert(list_it,mynode);
    }
    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

  4. #4
    Registered User MacNilly's Avatar
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    I suspect that everyone posting questions about linked lists is a student of some sort, and telling them to use the STL is probably not going to help much. Most instructors won't allow it, and (more importantly) how would they ever learn how it works if they don't code it themselves at least once?

    Anyways, to the OP:

    Your list structure is correct but here's a few points:

    Code:
    struct list{
        char event[20];
        int year;
        list *next;
    };
    
    list *head = NULL; // the head of an empty list
    
    void insert(list*, char incident[20], int);
    
    int main()
    {
     // This is how you make a new node in the list:
     head = new list;
     strncpy(list->event, "World War II", 20);
     list->year = 1914;
     list->next = NULL; // remember to mark the end of the list
     return 0;
    }

  5. #5
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    Quote Originally Posted by MacNilly View Post
    I suspect that everyone posting questions about linked lists is a student of some sort, and telling them to use the STL is probably not going to help much. Most instructors won't allow it, and (more importantly) how would they ever learn how it works if they don't code it themselves at least once?
    My response was to his question about making his program shorter, which I showed him how to do, using the STL. If there are specific requirements per his homework then he should post them; I am not responsible for his homework.

    Quote Originally Posted by MacNilly View Post
    Anyways, to the OP:

    Your list structure is correct but here's a few points:

    Code:
    struct list{
        char event[20];
        int year;
        list *next;
    };
    
    list *head = NULL; // the head of an empty list
    
    void insert(list*, char incident[20], int);
    
    int main()
    {
     // This is how you make a new node in the list:
     head = new list;
     strncpy(list->event, "World War II", 20);
     list->year = 1914;
     list->next = NULL; // remember to mark the end of the list
     return 0;
    }
    1. Using global variables is a bad practice and not required for this problem. The anchor of the linked list should be within main.
    2. It is not ok to use char arrays vice std::strings in C++, this is extremely poor practice and should never be suggested.
    3. In order to make a new node, he should use a function, but not one that you have suggested since your function is broke.

    Honestly, it would have been much easier to just rework my example without the STL list container if that is what you are so concerned with. For the future if you are going to correct someone, ensure you are in fact correct.
    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

  6. #6
    hqt
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    I suggest this link: Linked List Basics and other topic on this site. This site help me very very so much about Pointer and Linked List. Shortest & Greatest, two words I give for this document.

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