Thread: Simple Adjacent List implementation

I'm starting to learn graphs and I want to implement via adjacent list so I thought I'd use an array of linked lists, specifically an ArrayList of LinkedList. I want to read in adjacenct vertices from a file, so why does compiler complain about: void type not allowed here

For my file, the first line is the number of vertices in graph, so that's size of the array (ArrayList), and -999 is indicator in file of when to stop reading into current array index and move to next one
e.g.
10
0 1 2 -999
1 2 3 5 -999
2 4 -999
4 3 -999
5 6 -999
6 8 -999
7 3 8 -999
8 10 -999
9 4 7 10 -999

public class Graphs_runner
{	
	public static void main(String[] args)
	{
		ArrayList< LinkedList<Integer> > adj_ls = new ArrayList< LinkedList<Integer> >();
		Scanner sc = new Scanner( System.in );
		int num_vertex = sc.nextInt();
 
		//now read adjacent vertices into each index
		for ( int i = 0; i < adj_ls.size(); ++i )
			while ( sc.hasNextInt() && sc.nextInt() != -999 )
				adj_ls.add(i, new LinkedList<Integer>().addLast( sc.nextInt() ));//line 62 in my source
	}
}

EDIT: I figured it out, it's b/c ArrayList::add() expects 2nd param to be Obj but LinkedList::addLast is void type

Originally Posted by IHeartProgramming

...I'm getting funny output...

For starters, how about adding some print statements to make the program show you exactly what it is adding to the Lists? Maybe something like

        for (int i = 0; i < num_vertex; ++i)
        {
            System.out.printf("Begin adding for i = %d\n", i);
            while (sc.hasNextInt() && sc.nextInt() != -999)
            {
                adj_ls.add(i, new LinkedList< Integer>()); 
 
                // For debugging: Use a temporary variable so that you can print it out
                int temp = sc.nextInt();
                System.out.printf("vertex %d: Adding %d to adj_ls\n", i, temp);
                adj_ls.get(i).addLast(temp);
            }
            // Make sure you got the correct number in each list
            System.out.printf("List for vertex %d has size = %d\n",
                    i, adj_ls.get(i).size());
 
        }

See how it goes? Make the debug print statements verbose enough to let you know exactly what the printed stuff means at each step.


Cheers!

Z