How to convert a 1d array to 2d array?

Question

Say, I have a 1d array with 30 elements:

array1d[0] = 1  
array1d[1] = 2  
array1d[2] = 3  
.  
.  
.  
array1[29] = 30

How to convert the 1d array to 2d array?
Say 10x3?

array2d[0][0] = 1 array2d[0][1] =2 array2d[0][2] =3
.
.
.
array2d[9][0] = 28 array2d[9][1] =29 array2d[9][2] =30

Should I use a for loop?
But I cannot work it out.

Answer 1

int array2d[][] = new int[10][3];


for(int i=0; i<10;i++)
   for(int j=0;j<3;j++)
       array2d[i][j] = array1d[(j*10) + i]; 

Answer 2

Without writing any code for you...

• Think about how big your 2d array needs to be.
• Recognize that you'll need to loop over the contents of your source array to get each value into your destination array.

So it will look something like...

• Create a 2d array of appropriate size.
• Use a for loop to loop over your 1d array.
• Inside that for loop, you'll need to figure out where each value in the 1d array should go in the 2d array. Try using the mod function against your counter variable to "wrap around" the indices of the 2d array.

I'm being intentionally vague, seeing as this is homework. Try posting some code so we can see where you get stuck.

Answer 3

Here a generic function to convert from 1D -> 2D array:

public int[][] monoToBidi( final int[] array, final int rows, final int cols ) {
    if (array.length != (rows*cols))
        throw new IllegalArgumentException("Invalid array length");

    int[][] bidi = new int[rows][cols];
    for ( int i = 0; i < rows; i++ )
        System.arraycopy(array, (i*cols), bidi[i], 0, cols);

    return bidi;
}

If you want to do the contrary (2D -> 1D), here the function:

public int[] bidiToMono( final int[][] array ) {
    int rows = array.length, cols = array[0].length;
    int[] mono = new int[(rows*cols)];
    for ( int i = 0; i < rows; i++ )
        System.arraycopy(array[i], 0, mono, (i*cols), cols);    
        return mono;
}

Answer 4

public class Test{

    public static void main(String[] argv)
    {
        int x,y;
        for(int num =0; num<81;num++)
        {
            if((num % 9)>0)
            {
                x = num/9;
                y = num%9;

            }else
            {
                x = num/9;
                y = 0;
            }

            System.out.println("num ["+num+"]---["+x+","+y+"]");

        }
    }
}
/* Replace  9 by the size of single row of your 2D array */

Answer 5

You often will find the same problem: how to manipulate 2D array as 1D array. I wrote a generic class Grid, that lets access objects by index or by (x,y) .

See the following class and understand the idea behind it. :)

You could use the following class for data manipulation as 2D array or 1D array. Here is the code for that I wrote and use.

/**
 * Grid represents a 2 dimensional grid.
 *
 * @param <E> the type of elements in this grid
 */

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;

public class Grid<E>
{
    private int     size    ;
    private int     width   ;
    private int     height  ;
    private List<E> elements;

    public int getCapacity()
    {
        return getWidth() * getHeight();
    }

    /**
     * @return number of elements in grid. Null is also an element.
     */
    public int getSize()
    {
        return getElements().size();
    }

    /**
     * @param sideSize size of the grid side
     */
    public Grid(int sideSize)
    {
        this(sideSize,sideSize);
    }

    /**
     * @param width of the grid
     * @param height of the grid
     */
    public Grid(int width, int height)
    {
        this.width  = width ;
        this.height = height; 
        this.elements = new ArrayList<E>(
                Collections.nCopies(width*height, (E)null));
    }

    public int getHeight()
    {
        return height;
    }

    public int getWidth()
    {
        return width;
    }

    /**
     * @return all elements of the grid
     */
    public List<E> getElements()
    {
        return elements;
    }

    /**
     * @return iterator for a grid
     */
    public Iterator<E> iterator()
    {
        return getElements().iterator();
    }

    /**
     * Returns the element at position (x,y).
     *
     * @return the element at position (x,y)
     */
    public E get(int x, int y)
    {
        return getElements().get(
                idx(x,y));
    }

    /**
     * Returns the element at index idx.
     *
     * @return the element at given index
     */
    public E get(int idx)
    {
        return getElements().get(idx);
    }

    /**
     * Puts an element to the position idx
     *
     * @param element to be added
     *
     * @param x position x to add element to
     *
     * @param y position y to add element to
     */
    public void put(int x, int y, E element)
    {
        put(idx(x,y), element);
    }

    /**
     * Puts an element to the position idx
     *
     * @param element to be added
     *
     * @param idx to add element at
     */
    public void put(int idx, E element)
    {
        getElements().add(idx, element);
    }

    /**
     * Returns the x coordinate from the index.
     *
     * @return x coordinate of the index
     */
    public int x(int idx)
    {
        return idx % getHeight();
    }

    /**
     * Returns the y coordinate from the index.
     *
     * @return y coordinate of the index
     */
    public int y(int idx)
    {
        return (idx - idx % getHeight()) / getHeight();
    }

    /**
     * Returns index of element at (x,y).
     *
     * @return index of the coordinates
     */
    public int idx(int x, int y)
    {
        return y*getHeight() + x;
    }
}

Here is how to use the class (see a test example):

public class TestGrid
{
    public static final int     SIZE = 10;
    public static final Integer el1  = new Integer(2);
    public static final Integer el2  = new Integer(3);
    public static final Integer el3  = new Integer(3);

    public static void main(String[] args)
    {
        Grid<Integer> grid = new Grid<>(SIZE);
        assert grid.getCapacity() == SIZE*SIZE ;

        assert grid.idx(0,0) == 0 ;
        assert grid.idx(1,0) == 1 ;
        assert grid.idx(0,1) == 10;
        assert grid.idx(6,1) == 16; 
        assert grid.idx(9,9) == 99;

        grid.put(1, el1);
        assert grid.get(1) == el1 : grid.get(1);

        grid.put(0, 1, el2);
        assert grid.get(0,1) != el1 && el1 != el2 && grid.get(0,1) == el2;

        grid.put(15, el3);
        assert grid.get(5,1) == el3;
    }
}

Answer 6

int[] oneDArray = new int[arr.length*arr.length];
    //Flatten 2D array to 1D array...
    int s = 0;
    for(int i = 0; i < arr.length; i ++) 
          for(int j = 0; j < arr.length; j ++){                           
              oneDArray[s] = arr[i][j];
              s++;
          } 

Answer 7

package com.vikrant;

import java.util.Arrays;

public class TwoD {
    public static void main(String args[])
    {
        int a[][]=new int[4][3];
        int d[]={10,20,30,40,50,60,70,80,90,100,110,120};
        int count=0;
        for(int i=0;i<4;i++)
        {
            for(int j=0;j<3;j++)
            {
                if(count==d.length) break;
                a[i][j]=d[count];
                count++;
            }}
        int j=0;
        for (int i = 0; i<4;i++)
        {
            for(j=0;j<3;j++)
        System.out.println(a[i][j]);

        }
    }}

THIS DOES THE WORK

Answer 8

This is just a pseudo code. ROWS represent the length of the 2d array, and COLUMNS represent the length of the 1st element in the array

for(i=0; i<ROWS; i++)
   for(j=0; j<COLUMNS; j++)
       array2d[i][j] = array1d[ (i*COLUMNS) + j];

Answer 9

您不能将一维数组“转换”为二维数组，但声明数组时可以是多维数组。

int myArray2d[][] = new int[10][3]