How to get an element from a 2D array without looping the entire array?

Question

I have a simple class:

public class XYPoint {
    public int x, y;
}

And a 2D array that contains arrays of Point objects:

(-2,  2)(-1,  2)(0,  2)(1,  2)(2,  2)
(-2,  1)(-1,  1)(0,  1)(1,  1)(2,  1)
(-2,  0)(-1,  0)(0,  0)(1,  0)(2,  0)
(-2, -1)(-1, -1)(0, -1)(1, -1)(2, -1)
(-2, -2)(-1, -2)(0, -2)(1, -2)(2, -2)

The correspond ids are:

 0  1  2  3  4 
 5  6  7  8  9
10 11 12 13 14
15 16 17 18 19
20 21 22 23 24

Let's say I want to get the element at id 0, then it will return -2, 2. If I want to get from 6, it will return -1, 1 and so on.

Is there any way I can get an element without looping the entire array?

Are the IDs *always* laid out that way? If so, can you just use `array[id / 6][id % 6]`? — Jon Skeet, Feb 19 '19 at 09:03
your points is a 5x5 matrix while the ids are in 4x6. Can you include the actual 2D array variables in your question? — molamk, Feb 19 '19 at 09:03
That's a simple math problem, nothing more. You know the amount of "columns", so you also know when to shift to a new row when the id is larger than that amount. You really haven't come up with a solution? — Tom, Feb 19 '19 at 09:04
Related: [How to flatten 2D array to 1D array?](https://stackoverflow.com/questions/2569279/how-to-flatten-2d-array-to-1d-array). Search for more. — Ole V.V., Feb 19 '19 at 09:05
@JonSkeet Yes, are always laid in that ascending order. I'll try this and give you a feed-back. Thanks! — Johans Bormman, Feb 19 '19 at 09:12
@molamk Sorry, my mistake. Just updated my question. It's a 5x5 array. — Johans Bormman, Feb 19 '19 at 09:15
@JohansBormman: When I used `id / 6` and `id % 6`, you had a different layout in the question. If each row only contains 5 elements rather than 6, then you want `id / 5` and `id % 5` instead. — Jon Skeet, Feb 19 '19 at 09:37
@JonSkeet Replacing `array[id / 6][id % 6]` with `array[id / 5][id % 5]`, did the trick. Thank you so much! — Johans Bormman, Feb 19 '19 at 09:38
@JonSkeet Please add it as an answer so I can accept it. You literally saved my day! — Johans Bormman, Feb 19 '19 at 09:38
@JonSkeet Oh, I see. I'll accept Alan's answer. Thank you so much again! — Johans Bormman, Feb 19 '19 at 09:40

Alan · Accepted Answer · 2019-02-19T09:55:51.927

1

you essentially imagine a 1d array, structured in a 2d manner. with that in mind you can map the 1d coordinate (in your case 7 for instance) onto 2d like this:

[1d/width][1d mod width]

1d being your "id" and width being the 2d array length

1d/width results in the corresponding "row" of your id, and 1d mod width in the corresponding "column"

edited Feb 19 '19 at 09:55

answered Feb 19 '19 at 09:05

Alan

949
8
26

Could you give more information about your edit? I don't see the point of using `Math.floor` here... – Jon Skeet Feb 19 '19 at 09:40
@JonSkeet because with id = 14, you would do 14/5 = 2,8. You can see from his post, that 14 belongs into the row at index 2. so you would want to floor the result of the division, to result in your required index. i was not sure wether that happens automatically or not so i added a math.floor "to be on the safe side" – Alan Feb 19 '19 at 09:46
1

There's no need for that - you're performing integer division, so the result would just be 2. Java's integer division rounds towards zero. https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.17.2 – Jon Skeet Feb 19 '19 at 09:54
i thought so. i just wasnt 100% sure if that was really the case – Alan Feb 19 '19 at 09:55

How to get an element from a 2D array without looping the entire array?

1 Answers1