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Hello I'm a low level comp sci student that's really struggling/unfamiliar with file i/o.

I'm attempting to read in a text file using buffered reader. I understand how to use a while loop to continue scanning until the end of the file is reached but how can I instruct my reader to read just one line and do something until the end of that one line is reached, then read the next line and do something until that line ends, etc?

basically my input text file is going to repeat every three lines. The text file represents nodes in a weighted directed graph.

The input text file would supposedly look like the following:

each node is represented by two lines of text. For example the on the very top line, the first 'S' is the name of the node, the second 'S' indicates that it's a start node, the third 'n' indicates that it's a regular node, not a goal node, which would be indicated by a 'g'.

On the second line are the two nodes connected to 'S' the first being 'B' with a weighted distance of 1, and the second being 'E' with a weighted distance of 2.

The third line is supposed to be blank, and the pattern is repeated. 

S S n                     
B 1 E 2            

B N n
C 2 F 3

C N n
D 2 GA 4

D N n
GA 1

E N n
B 1 F 3 H 6

F N n
I 3 GA 3 C 1

GA N g

H N n
I 2 GB 2 F 1

I N n
GA 2 GB 2

GB N g

My code is as follows:

public void actionPerformed(ActionEvent e) 
{

    if(e.getSource() == openButton)
    {
        returnVal = fileChooser.showOpenDialog(null);

        if(returnVal == JFileChooser.APPROVE_OPTION)
        {
            selected_file = fileChooser.getSelectedFile();

            String file_name = fileChooser.getSelectedFile().getName();
            file_name = file_name.substring(0, file_name.indexOf('.'));

            try
            {
                BufferedWriter buff_writer = null;
                File newFile = new File("."+file_name+"_sorted.txt");           

                boolean verify_creation = newFile.createNewFile();
                //if (verify_creation)
                //  System.out.println("file created successfully");
                //else
                //  System.out.println("file already present in specified location");

                file_reader1 = new BufferedReader(new FileReader(selected_file));
                file_reader2 = new BufferedReader(new FileReader(selected_file));

                FileWriter file_writer = new FileWriter(newFile.getAbsoluteFile());
                buff_writer = new BufferedWriter(file_writer);

                //find the number of nodes in the file
                while( (currentLine = file_reader1.readLine()) != null)
                {
                    k++;
                    //System.out.println("value of k: " + k);
                }


                nodeArray = new Node[k];

                while( (currentLine = file_reader2.readLine()) != null) 
                {   
                    //System.out.print(currentLine);


                        String[] var = currentLine.split(" ");


                        nodeArray[x] = new Node(var[0]);


                        if (var[1].equals('S') || var[1].equals('s'))
                            nodeArray[x].setType(NodeType.START);
                        else if (var[2].equals('g') || var[2].equals('G'))
                            nodeArray[x].setType(NodeType.GOAL);
                        else
                            nodeArray[x].setType(NodeType.NORMAL);

                        x++;

                }

            buff_writer.close();
            file_writer.close();

            }   
            catch (Exception e1)
            {
                e1.printStackTrace();
            }       
        }
    }

My node class is as follows:

import java.util.*;


enum NodeType 
{
    START, GOAL, NORMAL;
}

public class Node 
{

private String name;
private NodeType typeOfNode;
private final Map<Node, Integer> neighbors = new HashMap<>();


public Node(String name)
{
    this.name = name;
}

public void setType(NodeType type)
{
    typeOfNode = type;
}

public void addAdjacentNode(Node node, int distance)
{
    neighbors.put(node, distance);
}




public String toString()
{
    String output = "";

    output += "node name: " + name + ",\n";

    return output;
}

}

My other major problem is how to handle the second line in the repeating three line sequence. The second line gives all the adjacent nodes and their weighted distances from the node described on the first line. The problem is, I don't know how many adjacent nodes will exist for any given node. Technically there could be none, or maybe a large number.

A kind programmer here suggested that I use a hash map to record adjacent nodes but I'm not sure how to structure a line of code to account for an indeterminate number of such adjacencies

Note: this question is in reference to this earlier question I asked: how to create an adjacency matrix, using an input text file, to represent a directed weighted graph [java]?

if anyone could point me in the right direction I would be eternally grateful

Community
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Trixie the Cat
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2 Answers2

0

As far as the input problem goes, your while loop is treating every line it reads identically. You will have to add a variable to keep track of which line in the 3-line sequence you're dealing with.

David Choweller
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0

As for the adjacent nodes use an ArrayList, which is a dynamically sized array.

You need an ArrayList for each node, which stores information about that node's adjacent nodes.

So you will need an array containing (k divided by 3) ArrayLists.

David Choweller
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