How to break an ArrayList into groups?

Question

I am trying to implement this method:

public ArrayList<ArrayList> groupWords(ArrayList<String> scrambledWords, int groupNumber);

The method takes an ArrayList of Strings and a number that represents the number of words in each group as parameters and then returns an ArrayList made of ArrayLists that contain groups of words according to the groupNumber parameter. For example, there is an ArrayList of 20 strings and I want to group that ArrayList into groups of 5 so I call the method like this:

ArrayList<ArrayList> groupedWords = groupWords(ArrayList, 5);

I am pretty sure that I need to have a for loop with another for loop nested inside, but I am not sure how to implement it.

How do I implement this method?

Mark Peters · Accepted Answer · 2016-03-13 18:15:57Z

3

With Guava:

List<List<String>> groupedWords = Lists.partition(words, 5);

answered Mar 13, 2016 at 18:15

Mark Peters

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Comments

Garuno · Accepted Answer · 2016-03-13 18:38:53Z

Something like this should work:

ArrayList<ArrayList<String>> grouped = new ArrayList<>(); for(int i = 0; i < words.size(); i++) { int index = i/groupSize; if(grouped.size()-1 < index) grouped.add(new ArrayList<>()); grouped.get(index).add(words.get(i)); }

I haven't tested this code but basically I'm using the fact that integer division is always rounding to the next lowest Integer. Example: 4/5=0.8 and is rounded to 0.

It's throwing an IndexOutOfBoundsException for grouped at the grouped.get(i).add(words.get(i)) line

nits.kk · Accepted Answer · 2016-03-13 18:47:58Z

public ArrayList<ArrayList> groupWords(ArrayList<String> scrambledWords, int groupNumber){ int arraySize = scrambledWords.size(); int count = 0; ArrayList<ArrayList> result = new ArrayList<>(); ArrayList<String> subResult = new ArrayList<>(); for(int i = 0 ; i < arraySize; i++){ if(count == groupNumber){ count = 0; result.add(subResult); subResult = new ArrayList<>(); } subResult.add(scrambledWords.get(i)); count++; } return result; }

This is simple Java Collections Soultion.

Suggestion : As a return type you should use ArrayList<ArrayList<String>>, and this should be the type for result also.

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