Convert 2D arraylist to hashmap in Java

Question

I have a small question about converting 2d arraylist to hashmap in java. I have a dataset looks like this after reading as 2d arraylist:

Which first column stands for id and second column stands for the item. I would like to create frequent itemset using hashmap in java, the output should look like

I use these codes but I have some trouble on them:

HashMap<Integer, ArrayList<Integer>> map = new HashMap<Integer, ArrayList<Integer>>();
for(Integer elem : data){
        map.put(elem[1], elem[0]);
}

Where data is my 2d arraylist.

The error message said that

incompatible types: ArrayList<Integer> cannot be converted to Integer
    for(Integer elem : data){
                       ^

Any help will be appreciated!

GhostCat · Accepted Answer · 2016-10-18 19:32:20Z

1

You go like this:

List<List<Integer>> inputData = ...

Map<Integer, List<Integer>> dataAsMap = new HashMap<>();
for(List<Integer> row : data){
  Integer id = row.get(0);
  Integer item = row.get(1);
  List<Integer> rowInMap = dataAsMap.get(item);
  if (rowInMap == null) {
    rowInMap = new ArrayList<>(); 
    dataAsMap.put(item, rowInMap);
  }
  rowInMap.add(id);
}

Some notes:

You should only be using the interface types List, Map ... as types (you only specify the specific impl type such as HashMap when creating new objects!)
Your problem is: when using for-each on List of Lists (as you did) ... you don't get the individual "cells" Instead, you get Lists [ iterating lists of lists ... results in one list per iteration!]

So, what is left then is to fetch the elements of that inner List, and push them into the Map. The one other part to pay attention to: you want to create a Map of List objects. And those List objects need to be created as well!

( I didn't run the above through a compiler, so beware of typos, but in general it should be telling you what you need to know. If you don't get what the code is doing, I suggest adding println statements or running it in a debugger)

edited Oct 18, 2016 at 19:32

answered Oct 18, 2016 at 19:11

GhostCat

141k28 gold badges190 silver badges262 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

BigD Over a year ago

Thanks a lot! I really appreciate it!

ninja.coder · Accepted Answer · 2016-10-19 01:34:51Z

0

Here is one easy way of doing it:

Take Map<Integer, List<Integer>>
Iterate your Arraylist.
See if key is already present in the map. Retrieve the list and add the value to the list if key is present or else create a new list with the value.

Program:

class FooBar {
    public static void main (String[] args) throws Exception {
        int[][] data = {{0,1}, {0,2}, {1,2}, {1,3}};
        Map<Integer, List<Integer>> myMap = new HashMap<>();

        for(int i = 0; i < 4; i++) {
            List<Integer> values = myMap.containsKey(data[i][0]) ?
                                   myMap.get(data[i][0]) : new ArrayList<>();
            values.add(data[i][1]);
            myMap.put(data[i][0], values);
        }

        System.out.println(myMap);
    }
}

Output:

{0=[1, 2], 1=[2, 3]}

This is just to illustrate the basic method. You can obviously modify it to suit your needs. For example, you can have String instead of List<Integer> and choose to append the values to the String instead of adding it to the List.

EDIT:

Here is a sample program with List<List<Integer>> as input. Here I'm assuming the name of this list as input.

Program:

class FooBar {
    public static void main (String[] args) throws Exception {
        /* Input Data */
        List<List<Integer>> input = new ArrayList<>();
        input.add(new ArrayList<Integer>(){{add(0); add(1);}});
        input.add(new ArrayList<Integer>(){{add(0); add(2);}});
        input.add(new ArrayList<Integer>(){{add(1); add(2);}});
        input.add(new ArrayList<Integer>(){{add(1); add(3);}});

        Map<Integer, List<Integer>> myMap = new HashMap<>();
        for(int i = 0; i < input.size(); i++) {
            List<Integer> values = myMap.containsKey(input.get(i).get(0)) ?
                                   myMap.get(input.get(i).get(0)) : new ArrayList<>();
            values.add(input.get(i).get(1));
            myMap.put(input.get(i).get(0), values);
        }

        System.out.println(myMap);
    }
}

Output:

{0=[1, 2], 1=[2, 3]}

edited Oct 19, 2016 at 1:34

answered Oct 18, 2016 at 19:22

ninja.coder

9,6685 gold badges40 silver badges55 bronze badges

3 Comments

BigD Over a year ago

Thanks a lot! But what if the input data is arraylist?

ninja.coder Over a year ago

@lixx3013 If it is an ArrayList then you'll have to iterate it instead of int[][]. Have a look at the revised answer.

ninja.coder Over a year ago

@lixx3013 Please accept the answer if you think this resolved your query.

Collectives™ on Stack Overflow

Convert 2D arraylist to hashmap in Java

2 Answers 2

1 Comment

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related