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
– Jon Skeet, Commented Feb 19, 2019 at 9:03
your points is a 5x5 matrix while the ids are in 4x6. Can you include the actual 2D array variables in your question? — molamk
– molamk, Commented Feb 19, 2019 at 9:03
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
– Tom, Commented Feb 19, 2019 at 9:04
Related: How to flatten 2D array to 1D array?. Search for more. — Anonymous
– Anonymous, Commented Feb 19, 2019 at 9:05
@JonSkeet Yes, are always laid in that ascending order. I'll try this and give you a feed-back. Thanks! — Johans Bormman
– Johans Bormman, Commented Feb 19, 2019 at 9:12

Alan · Accepted Answer · 2019-02-19 09:55:51Z

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, 2019 at 9:55

answered Feb 19, 2019 at 9:05

Alan

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4 Comments

Jon Skeet Over a year ago

Could you give more information about your edit? I don't see the point of using Math.floor here...

Alan Over a year ago

@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"

Jon Skeet Over a year ago

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. docs.oracle.com/javase/specs/jls/se8/html/…

Alan Over a year ago

i thought so. i just wasnt 100% sure if that was really the case

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