CODEWITHSUNDEEP
javaarrays2d
Radovan Kočík
Radovan Kočík

Reputation: 71

How to start iterating an 2d arraylist from 2nd row for example?

So I have created a 2D field that is filled with identifiers by group of threes.

Is there a way to start iterating for example from 4th row? if yes how is it done? Here is an image of 2d field.

enter image description here

I tried for each loop but it seems it won't work the right way.

enter image description here

This is the code, where I can't figure out, how to get 4th row for example.

Upvotes: 0

Views: 753

Answers (6)

Pankaj Malik
Pankaj Malik

Reputation: 1

If you only want to get a particular row you can change the outer loop to basic for loop as already mentioned. as shown:-

    private Integer[][] comparedProducts = new Integer[][]{
            {1,2,3},
            {4,5,6},
            {7,8,9},
            {10,11,12}
    };

    for(int i=3;i<size;i++){
      for(int a:comparedProducts[i]){
        //DO work
      }
    }

Also, you can can use ArrayList instead of array and then you can use the subList method of ArrayList.

    List<List<Integer>> comparedProducts = new ArrayList<>();
    public void initialiseList(){
        comparedProducts.add(Arrays.asList(1,2,3));
        comparedProducts.add(Arrays.asList(4,5,6));
        comparedProducts.add(Arrays.asList(7,8,9));
    }


  for(List comparedProduct:comparedProducts.subList(3,comparedProducts.size()){
    // One more foreach loop for each element
    // Or do some Work.
  }

ArrayList has a lot of really helpful methods. So if it's not much change use ArrayList.

Upvotes: 0

Gavin
Gavin

Reputation: 1767

In this situation you have two choices. Traditional you can use a ‘for' loop something like this:

for (int i=0, i<10, i++) {
  *commands here*
}

int i=0 is your initial condition here you could replace 0 with the index you wish to start from. Remember array indexes start at 0 so the 4th element would be index 3

i<10 controls how many times you wish to loop, in this case 10 times.

i++ is how you wish to increment your index value

Of course you could use a while but for loops are generally better in this case. I would suggest looking them up as they have some useful features I haven’t covered here.

Your second choice is to use streams:

Arrays.toStream(myArray)
   .drop(4)   // drop the first four elements
   .other processing

Streams can be less “efficient” in some cases but I find them more readable.

Upvotes: 1

Paolo Mossini
Paolo Mossini

Reputation: 1098

I think this will work 

int mat[][] = { {10, 20, 30, 40, 50, 60, 70, 80, 90},
                {15, 25, 35, 45},
                {27, 29, 37, 48},
                {32, 33, 39, 50, 51, 89},
              };


    for(int i=3; i<mat.length; i++) {
        for(int j=0; j<mat[i].length; j++) {
            System.out.println("Values at arr["+i+"]["+j+"] is "+mat[i][j]);
        }
    }

Upvotes: 2

m-2127
m-2127

Reputation: 161

Yes you can do it. Start the iteration by specifying the initial loop to 3 if you want to start it from the 4th row. for(int i=3 ; i < (maximum number of rows initiated at the beginning of the array); i++)

In general the code can be as follows

for(i=the row which you want to start from; i < maximum number of rows initiated at the beginning of the array; i++){ for(j=0; j<maximum number of columns initiated at the beginning of the array; j++) { System.out.println(arr[i][j]); } }

Upvotes: 1

Adel
Adel

Reputation: 91

sorry your image is missing, but I'd suggest something like this, pick the index in row loop

int startRow = 2;
for (int i = startRow; i < board.length; i++) {
        for (int j = 0; j < board[i].length; j++) {
            board[i][j] = i + j;
        }
    }

Upvotes: 3

piy26
piy26

Reputation: 1592

Instead of using foreach use the classical for loop with starting index as 3.

for(i=3; i<row; i++)
   {
       for(j=0; j<col; j++)
       {
           arr[i][j] = scan.nextInt();
       }
   }

Upvotes: 3

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