Hi I am having trouble trying to get my add and remove function to work.
We had to create an ADT List using array's.
It keeps giving my program errors
any help would be great, I think my problem might be my second part of the add function
where it will add in between 2 arrays

// ****************************************************
// Reference-based implementation of ADT list using arrays.
// ****************************************************
public class List {
  // reference to linked list of items

  public static final int MAX_LIST = 20;
  public static final int NULL = -1;
 
  private ListItem item[] = new ListItem[MAX_LIST];  // data
  private int      next[] = new int[MAX_LIST];       // pointer to next item

  private int head;     // pointer to front of list
  private int free;     // pointer to front of free list
  private int numItems; // number of items in list

// Constructor must initialize used list to empty and free list to
// all available nodes.

  public List()
  {
    int index;
    for (index = 0; index < MAX_LIST-1; index++)
      next[index] = index + 1;
    
    next[MAX_LIST-1] = NULL;

    numItems = 0;
    head = NULL;
    free = 0;
  }  // end default constructor

  public void removeAll() {   // reinitialize all nodes to free
    int index;
    for (index = 0; index < MAX_LIST-1; index++)
      next[index] = index + 1;
    
    next[MAX_LIST-1] = NULL;

    numItems = 0;
    head = NULL;
    free = 0;
  } // end removeAll

  public boolean isEmpty() {
      return numItems == 0;
  }  // end isEmpty

  public int size() {
      return numItems;
  }  // end size

  private int find(int index) {
  // --------------------------------------------------
  // Locates a specified node in a linked list.
  // Precondition: index is the number of the desired
  // node. Assumes that 1 <= index <= numItems
  // Postcondition: Returns a reference to the desired
  // node.
  // --------------------------------------------------
       int curr = head;
        for (int skip = 1; skip < index; skip++)
        {
          curr = next[curr];
        }
        return curr;
      }
 
  public ListItem get(int index)
  {
   if (index >= 1 && index <= numItems)
   {
     int curr = find(index);
     ListItem dataItem =item[curr];
     return dataItem;
   }
   else
   {
     System.out.println("List index out of bounds on get");
     return null;
   }
 } // end get


  public void add(int index, ListItem newItem)
  {
    if (index >= 1 && index <= numItems+1)
    {
     if (index == 1)
     {
          item[free]=newItem;
          int free2 = next[free];
          
          next[free]=head;
          head=free;
          free = free2;
      }
      else
      {
        int curr=free;
        int free2=next[free];
        int prev = find(index-1);
        int prev2=next[prev];
        
        next[prev]=curr;
        next[curr]=prev2;
        free=free2;
     }
     numItems++;
    }
    else
    {
      System.out.println("List index out of bounds on add");
    }
  }  

 
  public void remove(int index)
  {
    if (index >= 1 && index <= numItems)
    {
      if (index == 1)
      {
        int nextHead=next[head];
        next[head]=free;
        free=head;
        head=nextHead;
      }
      else
      {
        int prev = find(index-1);
        int curr=find(index);
        int getNext=find(index+1);
        next[prev]=getNext;
        next[curr]=free;
        free=curr;
      }
      numItems--;    
    } // end if
    else
    {
      System.out.println("List index out of bounds on remove");
    }
  }
}// end remove

For your add (should be called insert?) method:

1. You have to first figure out what index they want to add the item at. If the index they want to add the item at is the empty, then just insert it and you're done.
2. If the index is not empty, and your array is large enough to add an item, then you have to move every element after that index down. You have to start at the last filled index and move it down first. Then move the one next to it into it's place, and do this until you can add the item you want to add. You have to start at the very end so that you don't overwrite any items.
3. If you do not have an array large enough, you have to create a new array with enough space, then copy the elements over in the correct spots.

I hope that explanation helps. If you need an example, I can try to give you one, but it's hard to draw it.

iampandiyan: this question was asked five years ago. chances are, that if the OP hasn't found the answer yet, he wasn't looking anymore.

This article has been dead for over six months. Start a new discussion instead.