Getting the array length of a 2D array in Java

Question

I need to get the length of a 2D array for both the row and column. I’ve successfully done this, using the following code:

public class MyClass {

 public static void main(String args[])
    {
  int[][] test; 
  test = new int[5][10];

  int row = test.length;
  int col = test[0].length;

  System.out.println(row);
  System.out.println(col);
    }
}

This prints out 5, 10 as expected.

Now take a look at this line:

  int col = test[0].length;

Notice that I actually have to reference a particular row, in order to get the column length. To me, this seems incredibly ugly. Additionally, if the array was defined as:

test = new int[0][10];

Then the code would fail when trying to get the length. Is there a different (more intelligent) way to do this?

I found the following reference quite comprehensive, programiz.com/java-programming/multidimensional-array — M.Hossein Rahimi
– M.Hossein Rahimi, Commented Dec 3, 2019 at 12:18

NG. · Accepted Answer · 2010-10-22 19:21:06Z

241

Consider

public static void main(String[] args) {

    int[][] foo = new int[][] {
        new int[] { 1, 2, 3 },
        new int[] { 1, 2, 3, 4},
    };

    System.out.println(foo.length); //2
    System.out.println(foo[0].length); //3
    System.out.println(foo[1].length); //4
}

Column lengths differ per row. If you're backing some data by a fixed size 2D array, then provide getters to the fixed values in a wrapper class.

answered Oct 22, 2010 at 19:21

NG.

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8 Comments

user432209 Over a year ago

Unrelated question about your answer. What is the technique/method called where you put "{...};" after the object definition. As a new developer I keep seeing this more and more.

user432209 Over a year ago

Well, I understand that much :). I just thought there might be a specific name for the technique.

NG. Over a year ago

Not sure what the name of it is - object initialization? inline initialization? one of our Java Gurus will know

ILMTitan Over a year ago

Per JLS 10.6 and 15.10, the curly braces part is simply an Array Initializer, while the whole thing starting with new is an Array Creation Expression.

Erk Over a year ago

Also, if you accept a 2D array as input, e.g. in a constructor. You should check it and throw exceptions where applicable.

|

Ishtar · Accepted Answer · 2010-10-22 19:35:02Z

A 2D array is not a rectangular grid. Or maybe better, there is no such thing as a 2D array in Java.

import java.util.Arrays;

public class Main {
  public static void main(String args[]) {

    int[][] test; 
    test = new int[5][];//'2D array'
    for (int i=0;i<test.length;i++)
      test[i] = new int[i];

    System.out.println(Arrays.deepToString(test));

    Object[] test2; 
    test2 = new Object[5];//array of objects
    for (int i=0;i<test2.length;i++)
      test2[i] = new int[i];//array is a object too

    System.out.println(Arrays.deepToString(test2));
  }
}

Outputs

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

The arrays test and test2 are (more or less) the same.

AlDante · Accepted Answer · 2023-09-04 03:12:02Z

21

It was really hard to remember that

    int numberOfColumns = arr.length;
    int numberOfRows = arr[0].length;

Let's understand why this is so and how we can figure this out when we're given an array problem. From the below code we can see that rows = 4 and columns = 3:

    int[][] arr = { {1, 1, 1, 1}, 
            
                    {2, 2, 2, 2}, 
                    
                    {3, 3, 3, 3} };

arr has multiple arrays in it, and these arrays can be arranged in a vertical manner to get the number of rows. To get the number of columns, we need to access the first array and consider its length. In this case, we access [1, 1, 1, 1] and thus, the number of columns = 4. When you're given a problem where you can't see the array, you can visualize the array as a rectangle with n X m dimensions and conclude that we can get the number of columns by accessing the first array then its length. The other one (arr.length) is for the rows.

(Rows and columns corrected as per Evan's comment).

edited Sep 4, 2023 at 3:12

AlDante

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answered Jan 23, 2019 at 16:00

gocode

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2 Comments

Evan Rosica Over a year ago

You have this backwards. "In this case, we access [1, 1, 1, 1] and thus, the number of rows = 4." Actually, that means there are 4 columns.

Nitin Nanda Over a year ago

Correct @Evan Rosica

robert_x44 · Accepted Answer · 2010-10-22 19:21:15Z

5

Java allows you to create "ragged arrays" where each "row" has different lengths. If you know you have a square array, you can use your code modified to protect against an empty array like this:

if (row > 0) col = test[0].length;

answered Oct 22, 2010 at 19:21

robert_x44

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Comments

Thinchance · Accepted Answer · 2024-04-03 06:48:55Z

5

If you have this array:

String [][] example = {{"Please!", "Thanks"}, {"Hello!", "Hey", "Hi!"},
                       {"Why?", "Where?", "When?", "Who?"}, {"Yes!"}};

You can do this:

This gives you the number of rows:

example.length; = 4

Each row can be a different length, this gives you the number of columns:

example[0].length; = 2

example[1].length; = 3

example[2].length; = 4

example[3].length; = 1

Typing something like this will give you the length of the String inside the specific cell:

example[1][1].length(); = 3 //this cell has the word "Hey" which is 3 long.

edited Apr 3, 2024 at 6:48

Thinchance

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answered Oct 3, 2017 at 15:15

user8491933

Comments

kaliatech · Accepted Answer · 2010-10-22 19:23:36Z

3

There's not a cleaner way at the language level because not all multidimensional arrays are rectangular. Sometimes jagged (differing column lengths) arrays are necessary.

You could easy create your own class to abstract the functionality you need.

If you aren't limited to arrays, then perhaps some of the various collection classes would work as well, like a Multimap.

answered Oct 22, 2010 at 19:23

kaliatech

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Comments

asdfgghjkll · Accepted Answer · 2019-05-16 15:18:44Z

2

.length = number of rows / column length

[0].length = number of columns / row length

answered May 16, 2019 at 15:18

asdfgghjkll

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1 Comment

m02ph3u5 Over a year ago

Welcome to SO. You should add some more explanation to your answer.

Viral · Accepted Answer · 2022-01-18 08:23:00Z

1

Example Array 1:

int arr[][] = { { 1, 3, 1, 5 },
                 { 2, 2, 4, 1 },
                 { 5, 0, 2, 3 },
                 { 0, 6, 1, 2 } };

Example Array 2:

int arr[][] = { { 1, 3, 1 },
                 { 2, 2, 4 },
                 { 5, 0, 2 },
                 { 0, 6, 1 } };

Below function will work for any Symmetric and Asymmetric Array Matrix


 row_Count = arr.length
 column_Count = arr[0].length

edited Jan 18, 2022 at 8:23

Viral

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answered Jun 7, 2021 at 7:16

Ankush kumar

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Comments

abpatil · Accepted Answer · 2017-01-11 06:51:50Z

Try this following program for 2d array in java:

public class ArrayTwo2 {
    public static void main(String[] args) throws  IOException,NumberFormatException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int[][] a;
        int sum=0;
        a=new int[3][2];
        System.out.println("Enter array with 5 elements");
        for(int i=0;i<a.length;i++)
        {
            for(int j=0;j<a[0].length;j++)
            {
            a[i][j]=Integer.parseInt(br.readLine());
            }
        }
        for(int i=0;i<a.length;i++)
        {
            for(int j=0;j<a[0].length;j++)
            {
            System.out.print(a[i][j]+"  ");
            sum=sum+a[i][j];
            }
        System.out.println();   
        //System.out.println("Array Sum: "+sum);
        sum=0;
        }
    }
}

Feedforward · Accepted Answer · 2018-11-11 17:17:49Z

0

import java.util.Arrays;

public class Main {

    public static void main(String[] args) 
    {

        double[][] test = { {100}, {200}, {300}, {400}, {500}, {600}, {700}, {800}, {900}, {1000}};

        int [][] removeRow = { {0}, {1}, {3}, {4}, };

        double[][] newTest = new double[test.length - removeRow.length][test[0].length];

        for (int i = 0, j = 0, k = 0; i < test.length; i++) {
            if (j < removeRow.length) {
                if (i == removeRow[j][0]) {
                    j++;
                    continue;
                }
            }
            newTest[k][0] = test[i][0];
            k++;
        }

        System.out.println(Arrays.deepToString(newTest));   
    }
}

edited Nov 11, 2018 at 17:17

Feedforward

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answered Nov 11, 2018 at 16:54

ExceptionallyAverage

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1 Comment

ExceptionallyAverage Over a year ago

Will print [[300.0], [600.0], [700.0], [800.0], [900.0], [1000.0]] convert to ints to lose decimal point. This solution assumes your input array will always be of size ([x][1])

logbasex · Accepted Answer · 2020-05-08 15:06:46Z

0

With Java 8, allow you doing something more elegant like this:

int[][] foo = new int[][] {
        new int[] { 1, 2, 3 },
        new int[] { 1, 2, 3, 4},
    };

int length = Arrays.stream(array).max(Comparator.comparingInt(ArrayUtils::getLength)).get().length

answered May 8, 2020 at 15:06

logbasex

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Comments

Rohit Gupta · Accepted Answer · 2022-11-10 20:40:11Z

0

int rows=arr.length; //For knowing No of rows

int cols=arr[0].length; //For Knowing No of columns

Run This code... and Understand...

public class Store2darrays {   

    public static void main(String args[]) {
      int[]arr={1,2,3,4};
      System.out.println(arr.length);
      int[][] arr2d={{1,2,3,4},{5,6,7,8},{9,10,11,12}};
      System.out.println(arr2d.length);
      System.out.println(arr2d);
      System.out.println(arr2d[0]);
      System.out.println(arr2d[1]);
      System.out.println(arr2d.length);
      System.out.println(arr2d[0].length); }
}

edited Nov 10, 2022 at 20:40

Rohit Gupta

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answered Nov 6, 2022 at 18:05

Souhardya Gayen

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1 Comment

Community Over a year ago

As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

peter · Accepted Answer · 2017-04-20 13:00:46Z

-2

public class Array_2D {
int arr[][];
public Array_2D() {
    Random r=new Random(10);
     arr = new int[5][10];
     for(int i=0;i<5;i++)
     {
         for(int j=0;j<10;j++)
         {
             arr[i][j]=(int)r.nextInt(10);
         }
     }
 }
  public void display()
  {
         for(int i=0;i<5;i++)

         {
             for(int j=0;j<10;j++)
             {
                 System.out.print(arr[i][j]+" "); 
             }
             System.out.println("");
         }
   }
     public static void main(String[] args) {
     Array_2D s=new Array_2D();
     s.display();
   }  
  }

answered Apr 20, 2017 at 13:00

peter

1

1 Comment

alfonso.kim Over a year ago

this is not related to the question

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