Building a linked list whose nodes data is the sum of nodes of other list

**AlaaAssaf** · May 1st, 2014, 11:58 AM

 
 
   public void add(int d){
   listNode l = new listNode (d, null);
     l.next = first;
     first= l;
 
       }
 
  public list Sum2List (list l1, list l2){
//variables
int sum; 
list l = new list ();
listNode p1= l1.first;
listNode p2= l2.first;
while (p1!=null){
   sum= p1.data + p2.data;
    l.add(sum);
    p1= p1.next;
    p2= p2.next;
 
}
return l;
 
}

but I have a problem in my first listNode where it ll be pointing to null, thus in the sum2List method the program checks the while condition into false and doesn't go through the loop,,
may u help me correct my code plz,,
thank u in advance;;
I appreciate giving me some of ur time ,,

**Norm** · May 1st, 2014, 12:05 PM

Can you make a small, complete program that compiles and executes for testing to see the problem?
Be sure to wrap all posted code in code tags.

**AlaaAssaf** · May 1st, 2014, 12:30 PM

Originally Posted by Norm

Can you make a small, complete program that compiles and executes for testing to see the problem?
Be sure to wrap all posted code in code tags.

I already did a complete program there are no errors ,, it is only that the first is pointing to null that is why it is not entering the loop

 
public class listNode {
      int           data;
  listNode next;
  listNode(int d, listNode n){
     data = d;
     next = n;
 
}
  public void printListElements(){
      System.out.print( data + ", ");
 
  }
}
//another file ,,list class
public class list {
   listNode first;
//add method 
 public void add(int d){
 listNode l= new listNode(d,null);
 l.next=first;
 first=l;
}
 //print method
 public void printList(){
     listNode current= first;
     System.out.println("List Elements are");
     while(current!=null){
         current.printListElements();
         current= current.next;
 
     }
 
 
 }
   public list Sum2List (list l1, list l2){
//variables
int sum; 
list l = new list ();
 
for( listNode p1= l1.first; p1!=null; p1=p1.next)
   for( listNode p2= l2.first; p2!=null; p2=p2.next){
    sum=p1.data+p2.data;
       l.add(sum);
}
return l;
     }
 
  public static void main(String[] args) {
list l1= new list();
l1.add(1);
l1.add(2);
l1.add(3);
l1.add(4);
l1.add(5);
l1.add(6);
l1.add(71);
l1.add(8);
list l2= new list();
l2.add(7);
l2.add(78);
l2.add(4);
l2.add(760);
l2.add(3);
l2.add(60);
l2.add(7);
l2.add(2);
list l=new list ();
l.Sum2List(l1,l2);
l1.printList();
l2.printList();
l.printList();
    }
}

**Norm** · May 1st, 2014, 12:45 PM

What does the program output when it executes? Please copy the full contents of the command prompt window and paste it here.

BTW The code does NOT follow standard java naming conventions. Class names should start with upper case letters.

The poor formatting of the code makes it hard to read and understand.
can you fix the formatting?

**AlaaAssaf** · May 1st, 2014, 01:19 PM

package q3;
//class for the nodes
public class ListNode {
 //variables the data and link to next node
  int  data;
  ListNode next;
  ListNode(int d, ListNode n){
     data = d;
     next = n;
 
}
//method to print data in each node  
  public void printListElements(){
      System.out.println( data + ", ");
 
  }
}
 
package q3;
//the class for building a list
public class List {
   ListNode first;
//add method 
public void add(int d){
   ListNode L = new ListNode (d, null);
   L.next = first;
   first= L;
    }//closing for add method
 //print method to print a list of nodes
 public void printList(int n){
     ListNode Current= first;
     System.out.println("List"+n+"'s elements are");
     while(Current!=null){
         Current.printListElements();
         Current= Current.next;  
     }//closing for while loop   
 }//closing for printList method
 
//sum2List method that takes two lists as an input,, add the data in nodes at the same position store result in sum   
public List Sum2List (List L1, List L2){
//variables
int sum; //instance variable that will hold the addition of two nodes
List L = new List(); //the list resulting of addition of two lists 
ListNode P1= L1.first;// p1 is the first listNode in list l1
ListNode P2= L2.first;// p2 is the first listNode in list l2
while (P1!=null){
   sum= P1.data + P2.data;
    L.add(sum);// using the add method to add node to list
    P1= P1.next;//move to next node in list l1
    P2= P2.next;//move to next node in list l2
 
}//closing for while loop
return L;
 
}//closing for method sum2List
}//closing for class
package q3;
//main class
public class Q3 {
 
     public static void main(String[] args) {
List l1= new List();//l1 is the first list
l1.add(1);
l1.add(2);
l1.add(3);
l1.add(4);
l1.add(5);
l1.add(6);
l1.add(71);
l1.add(8);
List l2= new List();//l2 is the second list
l2.add(7);
l2.add(78);
l2.add(4);
l2.add(760);
l2.add(3);
l2.add(60);
l2.add(7);
l2.add(2);
List l=new List ();// the list that will hold the sum
l.Sum2List(l1,l2);//use sum2List to sum l1 and l2 
l1.printList(1);//print list1
l2.printList(2);//print list2 
l.printList(3);// print resulting list
 
 
     }
}

I tried to enhance the code formatting,, hope this could help,, thank u for ur help
now the point of the code lets say l1 was 8 -> 71 -> 6 ->5 -> 4 -> 3 -> 2 -> 1
and l2 was 2 -> 7 -> 60 ->3 -> 760 -> 4 -> 78 -> 7
then the resulting list should be 10 -> 78 -> 66 ->8 -> 764 -> 7 -> 80 -> 8

**Norm** · May 1st, 2014, 01:24 PM

What does the program output when it executes?
Please copy the full contents of the command prompt window and paste it here.

A suggestion for the initial testing: Use short lists (say with only 2 items) to make the debugging easier.

Try debugging the code by adding some println() statements that print out the values of the variables as the code executes so you can see what the computer sees when the code is executed.

The formatting is still poor. Logically nested statements (within {}s) should be indented. Too many statements start in the first column.

**AlaaAssaf** · May 1st, 2014, 01:40 PM

 
 
package q3;
 
 
public class ListNode {
 //variables the data and link to next node
  int  data;
  ListNode next;
 
  //constructor
  ListNode(int d, ListNode n){
     data = d;
     next = n;
}
//method to print data in each node  
  public void printListElements(){System.out.print( data+ "->");}
}
 
package q3;
//the class for building a list
public class List {
   ListNode first;
 
   //add method 
   public void add(int d){
        ListNode L = new ListNode (d, null);
        L.next = first;
        first= L;
    }//closing tag for add method
 
    //print method to print a list of nodes
    public void printList(int n){
           ListNode Current= first;
           System.out.println("\n"+"List"+n+"'s elements are");
      while(Current!=null){
       Current.printListElements();
       Current= Current.next;  
       }//closing tag for while loop   
    }//closing tag for printList method
 
//sum2List method that takes two lists as an input,, add the data in nodes at the same position store result in sum   
   public List Sum2List (List L1, List L2){
      //variables
      int sum; //instance variable that will hold the addition of two nodes
      List L = new List(); //the list resulting of addition of two lists 
      ListNode P1= L1.first;// p1 is the first listNode in list l1
      ListNode P2= L2.first;// p2 is the first listNode in list l2
     while (P1!=null){
         sum= P1.data + P2.data;
         L.add(sum);// using the add method to add node to list
         P1= P1.next;//move to next node in list l1
         P2= P2.next;//move to next node in list l2
      }
    return L;
    }
}
 
 
package q3;
//main class
public class Q3 {
 
     public static void main(String[] args) {
List l1= new List();//l1 is the first list
        l1.add(1);
        l1.add(2);
        l1.add(3);
 
List l2= new List();//l2 is the second list
       l2.add(7);
       l2.add(78);
       l2.add(4);
 
List l=new List ();// the list that will hold the sum
 
l.Sum2List(l1,l2);//use sum2List to sum l1 and l2 
l1.printList(1);//print list1
l2.printList(2);//print list2 
l.printList(3);// print resulting list
 
 
     }
}

now the output it is giving me is:
run:

List1's elements are
3->2->1->
List2's elements are
4->78 ->7->
List3's elements are
BUILD SUCCESSFUL (total time: 0 seconds)
Hope now it is more obvious

**Norm** · May 1st, 2014, 01:50 PM

The formatting is a little better but there are still statements that are not indented.

It looks like nothing is printed by printList() for the list: l.
Try debugging the sum2List() method by adding some println statements that print out the changes to the variables as the code executes to see why nothing is printed by the printList() method.

**AlaaAssaf** · May 1st, 2014, 02:02 PM

 
 
package q3;
 
 
public class ListNode {
 //variables the data and link to next node
  int  data;
  ListNode next;
 
  //constructor
  ListNode(int d, ListNode n){
     data = d;
     next = n;
}
//method to print data in each node  
  public void printListElements(){System.out.print( data+ "->");}
}
 
package q3;
//the class for building a list
public class List {
   ListNode first;
 
   //add method 
   public void add(int d){
        ListNode L = new ListNode (d, null);
        L.next = first;
        first= L;
    }//closing tag for add method
 
    //print method to print a list of nodes
    public void printList(int n){
             ListNode Current= first;
             System.out.println("\n"+"List"+n+"'s elements are");
       while(Current!=null){
          Current.printListElements();
          Current= Current.next;  
       }//closing tag for while loop   
    }//closing tag for printList method
 
//sum2List method that takes two lists as an input,, add the data in nodes at the same position store result in sum   
   public List Sum2List (List L1, List L2){
      //variables
           int sum=0; //instance variable that will hold the addition of two nodes
                  System.out.println("This is the original value of sum "+sum);
          List L = new List(); //the list resulting of addition of two lists 
          ListNode P1= L1.first;// p1 is the first listNode in list l1
          ListNode P2= L2.first;// p2 is the first listNode in list l2
        while (P1!=null){
             sum= P1.data + P2.data;
                  System.out.print("This is the sum "+sum + "  ");
             L.add(sum);// using the add method to add node to list
             P1= P1.next;//move to next node in list l1
             P2= P2.next;//move to next node in list l2
        } 
    return L;
    }
}
 
 
 
package q3;
//main class
public class Q3 {
 
     public static void main(String[] args) {
List l1= new List();//l1 is the first list
        l1.add(1);
        l1.add(2);
        l1.add(3);
 
List l2= new List();//l2 is the second list
       l2.add(7);
       l2.add(78);
       l2.add(4);
 
List l=new List ();// the list that will hold the sum
 
l.Sum2List(l1,l2);//use sum2List to sum l1 and l2 
l1.printList(1);//print list1
l2.printList(2);//print list2 
l.printList(3);// print resulting list
 
 
     }
}

The output after adding the print statements:
run:
This is the original value of sum 0
This is the sum 7 This is the sum 80 This is the sum 8
List1's elements are
3->2->1->
List2's elements are
4->78 ->7->
List3's elements are
BUILD SUCCESSFUL (total time: 0 seconds)

**Norm** · May 1st, 2014, 02:05 PM

Did you find the problem by looking at the output for the println statements you added?
Does the sum2List() method work correctly? Print out the list it creates: L at the end of the method to see if it is correctly computed.

**AlaaAssaf** · May 1st, 2014, 02:17 PM

run:
This is the original value of sum 0
This is the sum 7 This is the sum 80 This is the sum 8
List1's elements are
3->2->1->
List2's elements are
4->78 ->7->
List3's elements are
BUILD SUCCESSFUL (total time: 0 seconds)

That means it is adding them but I m wondering why it is not printing the list, i guess there is a problem with add method though I used the same method to build up list l1 and l2 and they are printed

**Norm** · May 1st, 2014, 02:26 PM

What happens to the list: L that is returned by the sum2List() method?
Does the caller of the method save it?

Did you print the list: L at the end of the sum2List() method so you see what is in it?

**AlaaAssaf** · May 1st, 2014, 02:43 PM

OKAAAY I should have called the printList method in the sum2List itself,,
Thank u very much ,,
u helped me point out my misconception

**Norm** · May 1st, 2014, 02:53 PM

I should have called the printList method in the sum2List itself

That is not the solution. printing the list inside of the method was for debugging. The list needs to be printed in the main() method.

**AlaaAssaf** · May 1st, 2014, 03:12 PM

Oops then nw I have two problems
1 the printing issue (it printed when I called the method within the sum2List()method,, but it is not printing when i call it in the main method )
2 reversing the order of the resulting list L
what do u recommend me to do,,

**Norm** · May 1st, 2014, 03:30 PM

it is not printing when i call it in the main method )

Where is the list with the sums in it? The sum2List() creates a list and returns it. What happens to that list?

There is confusion in the comments describing what the code does:

   List L = new List(); //the list resulting of addition of two lists 
  ...
  return L;   //??? What happens to L???
  ....
  List l = new List ();// the list that will hold the sum  ??? See sum2List()
  //  Following needs to save returned value???
  l.Sum2List(l1,l2);//use sum2List to sum l1 and l2  <<<< Where is the list?

Two choices:
1) have sum2List() add the new items to the list object that the method is in.
2) save the List that is returned by sum2List() in a new List variable.

**AlaaAssaf** · May 1st, 2014, 04:30 PM

i took the second option ,, and IT WORKED ,,

Thank u very much for ur time

**Norm** · May 1st, 2014, 04:35 PM

That raises the question about why create a new instance of the List class just to call its sum2List() method.
That method is in all instances of the List class. So it could be called with the variables: l1 or l2. There is not need for l.

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