Adding and deleting elements in ArrayList set as a value in a hashmap

Question

So I have a file that looks like this:

    1st 2nd nth
    e1, v1, 1
    e1, v3, 2
    e1, v4, 4
    e1, v5, 7
    e2, v1, 1
    ., ., .
    ., ., .
    ., ., .

where I want the first column to be the key of a hashmap (e1, or e2, or e3), and the value to be an ArrayList called "Ratings" that in I want the second column to have it's value (an int), inside the nth index of the arraylist.

Here is my code in it's entirety so far:

import java.util.*;
import java.io.*;

public class Setup
{
    public static void Setup(String[] args)
    {
        String user;
        int value, location;
        //Create a Hashmap that holds a string key and an ArrayList value
        HashMap<String, ArrayList<Integer>> userRatings = new HashMap<String, ArrayList<Integer>>();
        try
        {
            BufferedReader bufferReader = new BufferedReader(new FileReader("Student list.txt")); //read from file
            String line, sentence; //declare two string variables
            String[] sData; //declare a string array (store the contents here)
            line = bufferReader.readLine(); //Read the line
            while (line != null) //While there is a line, do this:
            {
                line = bufferReader.readLine();
                sData  = line.split(", "); //Into the string array, enter individual values in the line split by the ", " characters
                int iData[] = new int[sData.length]; //Create an int array the size of the string array
                user = sData[0];
                for (int i = 0; i <sData.length; i++) //fill the int array with the int-version of the string array
                {
                    iData[i] = Integer.parseInt(sData[i]); //pass the strings as integers into the integer array
                }
                value = iData[1];
                location = iData[2];
                if(!userRatings.containsKey(user)) //The user does not have ratings.
                {
                    ArrayList<Integer> ratings = new ArrayList<Integer>();
                    //                     ratings = userRatings.get(user);
                    for (int j = 0; j < 50; j++)
                    {
                        ratings.add(j);
                    }
                    System.out.println(user + " " + userRatings.get(user));

                }
                else //The user has ratings
                {
                    userRatings.get(user).add(location,value);
                    System.out.println(user + " " + userRatings.get(user));
                }
            }
            bufferReader.close();
        }    catch (FileNotFoundException e)
        {
            System.out.println("File does not exist or could not be found.");
        }
        catch (IOException e)
        {
            System.out.println("Can't read from file");
        }
        catch (NullPointerException e)
        {
        }
    }
}

I am having a problem with modifying the contents of the arraylist.

To sum up: Every string in the 1st column of the file would have it's own key in the hashmap (userList) The program will check if it has the key, if no key exists, it will create a new arraylist as the value for the key. The arrayList will be populated with 50 indexes, of which they will contain "0". After that, the arraylist will have new values added from the file in which the integer in the second column will be added at the corresponding value of the nth column.

How would I populate the arraylist, and how would I edit it so that if I want to add a new integer at the nth index of user e6, I could?

Bhesh Gurung · Accepted Answer · 2012-06-04 03:06:03Z

1

I think you can just read the nth value and use it to insert the value in the list.

e.g.

List<Integer> list = new ArrayList<Integer>();
//values values (0) will be inserted into the list
//after that for each value in the nth you do something as follows    
//i is the value in the nth column, if the least value for i is 1 otherwise just use i
list.add(i-1, value);

Also for referencing use, interface instead of implementing class as follows -

Map<String, List<Integer>> userRatings = new HashMap<String, List<Integer>>();
List<Integer> ratings = new ArrayList<Integer>();

To insert value into the nth index for user e6:

List<Integer> ratings = userRatings.get(user); 
if(ratings == null) { 
     ratings = new ArrayList<Integer>(); //it's not in the map yet
     //insert 50 0s here
     userRatings.put(user, ratings);
}
ratings.add(i, value); //i is the nth index and value is the rating

edited Jun 4, 2012 at 3:06

answered Jun 4, 2012 at 2:52

Bhesh Gurung

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1 Comment

JJJJohn KKKim Over a year ago

This is great, however, doing what mprabhat suggested was simple enough that I tried it, and had it working as I had imagined! Thanks for your answer though! You helped me grasp the concept a little better and gave me ideas for a different approach for later!

mprabhat · Accepted Answer · 2012-06-04 03:33:06Z

0

When given key is not present in your Map you have to add a new Key value pair

if(!userRatings.containsKey(user)) //The user does not have ratings.
{
   ArrayList<Integer> ratings = new ArrayList<Integer>();                   
   for (int j = 0; j < 50; j++) {
       ratings.add(j);
  }
  userRatings.put(user, ratings); // place new mapping 
  System.out.println(user + " " + userRatings.get(user));
} else //The user has ratings
{
    ArrayList<Integer> ratings  = userRatings.get(user);
    ratings.add(location,value); // Update your list with location and value
    System.out.println(user + " " + userRatings.get(user)); 
}

iData[i] = Integer.parseInt(sData[i]); This will not work given your file content it will throw NumberFormatException as v1 cannot be parsed to int.

Instead you can do something like this:

value = Integer.parseInt(sData[1].trim().substring(1));
location = Integer.parseInt(sData[2].trim());

To Compare values of two keys:

Approach 1

ArrayList<Integer> first = userRatings.get(e1);
ArrayList<Integer> second = userRatings.get(e2);

//Taking the smallest size will ensure that we don't get IndexOutOfBoundsException.

int length = first.size() < second.size() ? first.size() : second.size();

for(int iDx = 0; iDx < legth; iDx++){
   //compare content
}

Approach 2

ArrayList<Integer> first = userRatings.get(e1);
ArrayList<Integer> second = userRatings.get(e2);

Arrays.equals(first.toArray(), second.toArray());

edited Jun 4, 2012 at 3:33

answered Jun 4, 2012 at 2:52

mprabhat

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6 Comments

JJJJohn KKKim Over a year ago

Thank you @mprabhat! This is great! Pretty simple, I hadn't thought of it that way, but it makes sense! Take it out, edit it, and place it back. If I were to compare the values of two different keys, how would I approach that? i.e. I want to compare e1's arraylist with e2's arraylist.

mprabhat Over a year ago

If you want to compare two ArrayList then you will have to check loop and compare content one by one.

mprabhat Over a year ago

Another approach will be that you convert the two ArrayList to Array and then call Arrays.equals

JJJJohn KKKim Over a year ago

so for example, I would have to take the value of e1, and place it into an arraylist called compare1, and take the value of e2, and place it into arraylist compare2, and then compare? In my mind this makes sense, but I just want to check before I go about and try. I think it'll work.

mprabhat Over a year ago

since e1 and e2 are both arraylist no need to place it inside another arrayList,

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