Creating List in a loop

Question

Is there any way to create a loop which will be creating new List

static Map<Integer, List> dataMap = new HashMap<>();
public static void dec_hex() {

        // Map<Integer, Integer> dataMap = new HashMap<>();
        // dataMap.put(0, 0);
        List<Integer> ls2 = new ArrayList<Integer>();
        List<Integer> ls3 = new ArrayList<Integer>();
        List<Integer> ls4 = new ArrayList<Integer>();
        List<Integer> ls5 = new ArrayList<Integer>();
        List<Integer> ls6 = new ArrayList<Integer>();
        List<Integer> ls7 = new ArrayList<Integer>();
        List<Integer> ls8 = new ArrayList<Integer>();
        List<Integer> ls9 = new ArrayList<Integer>();
        List<Integer> ls10 = new ArrayList<Integer>();

        System.out.println("Liczby");
        for (int i = 0; i < 201; i++) {
            String hex = Integer.toHexString(i);
            System.out.println("DEC: " + i + " HEX: " + hex);
            //for (int a = 2; a<11 ; a++){
            if (i % 2 == 0) {
                ls2.add(i);
                //myList.add(i);
                dataMap.put(2, ls2);
            }
            if (i % 3 == 0) {
                ls3.add(i);
                //myList.add(i);
                dataMap.put(3, ls3);
            }

            if (i % 4 == 0) {
                ls4.add(i);
                //myList.add(i);
                dataMap.put(4, ls4);
            }
            if (i % 5 == 0) {
                ls5.add(i);
                //myList.add(i);
                dataMap.put(5, ls5);
            }
            if (i % 6 == 0) {
                ls6.add(i);
                //myList.add(i);
                dataMap.put(6, ls6);
            }
            if (i % 7 == 0) {
                ls7.add(i);
                //myList.add(i);
                dataMap.put(7, ls7);
            }
            if (i % 8 == 0) {
                ls8.add(i);
            //  myList.add(i);
                dataMap.put(8, ls8);
            }
            if (i % 9 == 0) {
                ls9.add(i);
                //myList.add(i);
                dataMap.put(9, ls9);
            }
            if (i % 10 == 0) {
                ls10.add(i);
                //myList.add(i);
                dataMap.put(10, ls10);
            }
        }

        //String list = myList.toString();
        //System.out.println(list);
        dataMap.toString();
        System.out.println(dataMap);

    }

Is there any way to create a loop to check if number is divisible? If I'm going to use this

    for (int a = 2; a<11 ; a++){
 lsa.add(i); // lets say we've got one list
 dataMap.put(a, ls);
}

The program won't work correctly cause the numbers will reapeat, cause I'm using the same lsa array.

Anjana · Accepted Answer · 2017-06-05 13:19:49Z

4

You can create a List of Lists.

int max = 10;
List<List<String>> listOfLists = new ArrayList<>();
for(int i=0; i<max; i++) {
             //create a new list and add it to listOfLists
    List<String> myList = new ArrayList<>();
    listOfLists.add(myList);
}

Hope this helps. :)

edited Jun 5, 2017 at 13:19

answered Jun 5, 2017 at 13:07

Anjana

8937 silver badges22 bronze badges

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

2 Comments

Jens Over a year ago

Thanks for providing an answer! But could you please change the variable max to a number (like 10 for example) or define a variable max and describe what the loop is doing? The reason: Although a lot of people on Stack Overflow know Java, some users might be writing their very first lines of code. Those people could be confused what max is and what is happening in the loop.

QBrute Over a year ago

Additionally, listOfLists has to be of type List<List<String>>

JB Nizet · Accepted Answer · 2017-06-05 13:11:23Z

1

Yes, of course. That's what a Map is for: you give it a key, and it gives you back the value.

    Map<Integer, List<Integer>> dataMap = new HashMap<>();
    for (int i = 0; i < 10; i++) {
        dataMap.put(i, new ArrayList<>());
    }       

    for (int i = 0; i < 201; i++) {
        int key = i % 10;
        List<Integer> list = dataMap.get(key);
        list.add(i);
    }

But since your keys are integers from 0 to 9, you could also just use a List<List<Integer>>, instead of the map.

answered Jun 5, 2017 at 13:11

JB Nizet

694k94 gold badges1.3k silver badges1.3k bronze badges

1 Comment

Samuel Owino Over a year ago

Exactly my point, using a HashMap complicates a rather simple problem set

nafas · Accepted Answer · 2017-06-05 13:36:33Z

0

you can shorten your by checking if the map contains a key, if not create a new list and add your data :

for (int i = 1; i < 201; i++) {

   for(int j=2;j<11;j++){

      if(i%j==0){

         if(dataMap.containsKey(j){

           dataMap.get(j).add(i);

         }else{

           List<Integer> list = new ArrayList<Integer>();
           list.add(i);
           dataMap.put(j,list);

        }

      }

   }
}

edited Jun 5, 2017 at 13:36

answered Jun 5, 2017 at 13:15

nafas

5,4533 gold badges33 silver badges59 bronze badges

7 Comments

Samuel Owino Over a year ago

Why do you need a map for this, its a fairly simple operation

nafas Over a year ago

@samuelowino the OP uses map in his code. beside it looks easier with map anyways.

Piotr Szczepanik Over a year ago

Why key = 1%10 ?

nafas Over a year ago

@PiotrSzczepanik its i not '1' mate

Piotr Szczepanik Over a year ago

my mistake, but still. Why :D ?

|

Collectives™ on Stack Overflow

Creating List in a loop

3 Answers 3

2 Comments

1 Comment

7 Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

7 Comments

Related