How to iterate through two dimensional ArrayList using iterator?

Question

I would like to iterate through two dimensional ArrayList which includes String objects using iterator. I also would like to iterate in a way that let me choose whether I want to iterate horizontally(row) first or vertically(column) by using a boolean value. How can I implement this in java?

What I've tried so far.

public class IterateThis implements Iterator<String>{
ArrayList<ArrayList<String>> array;

public IterateThis(){
    array = new ArrayList<ArrayList<String>>();
    array.add(new ArrayList<String>());
    array.add(new ArrayList<String>());
    array.add(new ArrayList<String>());
    array.get(0).add("1");
    array.get(0).add("2");
    array.get(0).add("2");
    array.get(1).add("4");
    array.get(1).add("5");
    array.get(1).add("6");
}

Iterator<String> it = array.iterator(); //This gives me an error...why?

I don't know how I can implement the boolean value though.

What type of error you find the there? Please mention the Error here. and "array" is the list of ArrayList<String>. so when you create a Iterator. it should be same type of list. — Janny
– Janny, Commented Feb 24, 2014 at 12:54
"This gives me an error...why?" Because array.iterator() is not an Iterator<String> but an Iterator<List<String>>. — m0skit0
– m0skit0, Commented Feb 24, 2014 at 12:56
since your class implements Iterator, you have to implement the iterate method for class IterateThis, and put the logic of 2-dimensional iteration in that method. — weidi
– weidi, Commented Feb 24, 2014 at 13:02

GameDroids · Accepted Answer · 2014-02-24 13:05:25Z

3

Maybe you need to implement two versions, with a boolean that decides which loop to use:

public void iterate(boolean horizantalFirst){

    if(horizontalFirst){
        for(int i=0; i<array.size(); i++){              // first iterate through the "outer list"
            for(int j=0; j<array.get(i).size(); j++){   // then iterate through all the "inner lists"
                 array.get(i).get(j)="1";
            }
        }
    }else{ 
        int j=0;                            // index to iterate through the "inner lists"
        for(; j<array.get(j).size(); j++){   //dangerous, you need to be sure that there is a j-th element in array
            for(int i=0; i<array.size(); i++){  // iterate here through the outer list, by always working on the j-th element                
                array.get(i).get(j)="1";
            }
        }
    }
}

answered Feb 24, 2014 at 13:05

GameDroids

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3 Comments

GameDroids Over a year ago

sorry, I just realized I didn't use any Iterator

Leo Over a year ago

I prefer this way, anyway ;-)

Zip Over a year ago

We had to use Iterator for this task at school. So for loopis no option for me.

Aman Agnihotri · Accepted Answer · 2014-02-24 14:33:07Z

2

Why not try this:

import java.util.ArrayList;

public class Iteration
{
  private ArrayList<ArrayList<String>> array;

  public Iteration()
  {
    array = new ArrayList<>();

    array.add(new ArrayList<String>());
    array.get(0).add("000");
    array.get(0).add("001");
    array.get(0).add("010");

    array.add(new ArrayList<String>());
    array.get(1).add("100");
    array.get(1).add("101");
    array.get(1).add("110");
    array.get(1).add("111");

    iterateRowWise();
    System.out.println("\n\n");

    iterateColumnWise();
  }

  public void iterateRowWise()
  {
    // This uses iterator behind the scene.
    for (ArrayList<String> row : array)
    {
      for (String element : row)
      {
        System.out.print(element + " ");
      }
      System.out.println();
    }
  }

  public void iterateColumnWise()
  {
    int arraySize = array.size();
    int maxColumns = getMaximumListSize();
    for (int c = 0; c < maxColumns; c++)
    {
      for (int r = 0; r < arraySize; r++)
      {
        ArrayList<String> rowList = array.get(r);
        if (c < rowList.size())
        {
          System.out.print(rowList.get(c) + " ");
        }
      }
      System.out.println();
    }
  }

  private int getMaximumListSize()
  {
    int maxListSize = 0;
    for (ArrayList<String> rowList : array)
    {
      if (maxListSize < rowList.size())
        maxListSize = rowList.size();
    }

    return maxListSize;
  }

  public static void main(String[] args)
  {
    new Iteration();
  }
}

The iterateRowWise() method iterates using the iterator, but it does so behind the scene.
The iterateColumnWise() method doesn't use iterator, but its safe to use.

edited Feb 24, 2014 at 14:33

answered Feb 24, 2014 at 13:15

Aman Agnihotri

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2 Comments

sakura Over a year ago

rowwise is simple, suggest columnwise!!

Aman Agnihotri Over a year ago

Added iterateColumnWise() method. ;)

sakura · Accepted Answer · 2014-02-24 14:29:35Z

Row-wise iteration is simple as shown in the @Awfully Awesome answer.

Tried a columnwise iteration with assumption that List will always have m cross n elements where m=n

public static void IterateThis() {
    ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>();
    array.add(new ArrayList<String>());
    array.add(new ArrayList<String>());

    array.get(0).add("1");
    array.get(0).add("2");
    array.get(0).add("2");
    array.get(1).add("4");
    array.get(1).add("5");
    array.get(1).add("6");

    Iterator<ArrayList<String>> it = array.iterator();

    int topLevelIteratorResetCounter = 0;
    int noOfIteratorNextRequired = 1;

    int size = array.size();

    while (it.hasNext()) {

        ArrayList<String> strList = it.next();
        if (noOfIteratorNextRequired > strList.size())
            break;
        Iterator<String> itString = strList.iterator();
        int numtimes = 0;
        String str = null;
        while (numtimes != noOfIteratorNextRequired) {
            str = itString.next();
            numtimes++;
        }
        System.out.println(str);
        numtimes = 0;
        topLevelIteratorResetCounter++;
        if (topLevelIteratorResetCounter == size) { //as column count is equal to column size
            it = array.iterator();  //reset the iterator
            noOfIteratorNextRequired++;
            topLevelIteratorResetCounter = 0;
        }
    }
}

The answer uses Iterator.

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