8

I've been trying to figure out what exactly is happening here. I'm just trying to figure out what the 2 lines are doing that I've commented on below. I found this program that doesn't declare the full dimensions of the array (instead of new int[10][5]; it just decides to not declare it by saying 'new int[10][];' It's like the 2nd array length doesn't matter (changing it to 1 or 100 doesn't affect the output). 

int[][] tri = new int[10][];  //this lack of giving the size of the 2nd array is strange
 for (int r=0; r<tri.length; r++) {
 tri[r] = new int[r+1];   //I'm not sure what this line is doing really 
}
for (int r=0; r<tri.length; r++) {
 for (int a=0; a<tri[r].length; a++) {
     System.out.print(tri[r][a]);  
     }
 System.out.println();
 }
Tony DiNitto
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6 Answers6

12

The first line makes an array of int arrays. There are 10 slots for int arrays created.

The third line creates a new int array and puts it in one of the slots you made at first. The new int array has r+1 spaces for ints in it.

So, the int array in position 0 will have 1 slot for an int. The int array in position 1 will have 2 slots for an int. The overall shape will be:

[
    [0],
    [0,0],
    [0,0,0],
    ...,
    [0,0,0,0,0,0,0,0,0,0]
]

which is hinted at with the variable name tri (it looks like a triangle)

Riley Lark
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    [Sorry my last comment was incomplete and I can't delete it] So it seems like the first array length must always be declared, but after that the length of the array length can be created on the spot? (eg. new int[10][]; is ok since it has the first length declared). Also the line 'tri[r] = new int[r+1];' seems to create the 2nd part of the array, but is it possible to make this array bigger than the first (when I do it creates exceptions)? – Tony DiNitto Mar 28 '12 at 14:17
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    You have 11 arrays total. The first array is an array of the other 10 arrays. `new int[10][]` says "make me an array that can hold `int[]` objects, and make 10 slots for them." The third line of your snippet says "make an array of ints with `r+1` slots, and put the new array into the already-existing array at slot `r`." You can make any of the 11 arrays as big or small as you like. All 11 sizes are independent. – Riley Lark Mar 28 '12 at 14:27
5

All new int[10][] is declaring is an array of size 10, containing null arrays.

In the for loop, the null arrays are being instantiated into ever increasing array sizes.

Sheriff
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3

It's simply declaring an array of 10 arrays. The lengths of each of those "sub" arrays can all be different.

Oliver Charlesworth
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3

It makes more sense if you think of a multidimensional array as an array of arrays:

int [][] tri = new int[10][]; // This is an array of 10 arrays

tri[r] = new int[r+1]; // This is setting the r'th array to
                       // a new array of r+1 ints
trutheality
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0

It is better to use multidimensional collection in Java.

Syntax:

ArrayList <Object> x= new ArrayList <Object>();

Below is an application of multidimensional collection in Java.


import java.util.*; 

class MultidimensionalArrayList { 

    /*function for creating and returning 2D ArrayList*/
    static List create2DArrayList() 
    { 
        /*Declaring 2D ArrayList*/
        ArrayList<ArrayList<Integer> > x 
            = new ArrayList<ArrayList<Integer> >(); 

        /*one space allocated for 0th row*/
        x.add(new ArrayList<Integer>()); 

        /*Adding 3 to 0th row created above x(0, 0)*/
        x.get(0).add(0, 3); 

        /*Creating 1st row and adding values 
    (another way for adding values in 2D collections)*/
        x.add(new ArrayList<Integer>(Arrays.asList(3, 4, 6))); 

        /*Add 366 to 1st row 0th column x(1, 0)*/
        x.get(1).add(0, 366); 

        /*Add 576 to 1st row 4th column x(1, 4)*/
        x.get(1).add(4, 576); 

        /*Adding values to 2nd row*/
        x.add(2, new ArrayList<>(Arrays.asList(3, 84))); 

        /*Adding values to 3rd row*/
        x.add(new ArrayList<Integer>(Arrays.asList(83, 6684, 776))); 

        /*Adding values to 4th row*/
        x.add(new ArrayList<>(Arrays.asList(8))); 

        return x; 
    } 

    public static void main(String args[]) 
    { 
        System.out.println("2D ArrayList :"); 

        /*Printing 2D ArrayList*/
        System.out.println(create2DArrayList()); 
    } 
}
Sudam Yasodya
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0

it's not lacking, it's basically not setting a specific amount, it isn't required because it can have many fields

and the second line

tri[r] = new int[r+1];

is setting all the fields to not null

Travs
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