AP Computer Science A College Board

2D Arrays

Understanding 2D Arrays

• An array is a data structure that stores a fixed-size sequential collection of elements of the same type.
• A 2D array, also known as a matrix, stores data in a grid format. It has rows and columns, similar to a table.
• In Java, a 2D array is an array of arrays. Each row of the 2D array is a 1D array.
• 2D arrays are useful for representing data structures such as spreadsheets, grids, and games boards.

Declaration and Initialization

• To declare a 2D array in Java, you use the syntax:

  type[][] arrayName;

• Initialization can take place at declaration, using nested curly braces

  {}

  . For example:

  int[][] matrix = {{1,2,3},{4,5,6},{7,8,9}};

• The size of the array can be defined during initialization as

  type arrayName[][] = new type[rows][cols];

Accessing Elements

• Elements of a 2D array can be accessed using their row index and column index. Remember, Java is zero-indexed, so indexing starts at 0.
• The syntax to access an element is:

  arrayName[row][column]

• The value can be changed in a similar way:

  arrayName[row][column] = newValue;

Traversing a 2D Array

• To traverse, or go through each element of a 2D array, we use nested loops. The outer loop iterates through the rows and the inner loop iterates through the columns.
• Sample syntax can be:

  for(int i = 0; i < rows; i++){
      for(int j = 0; j < cols; j++){
          System.out.print(arrayName[i][j] + " ");
      }
      System.out.println();
  }
  

2D Array Methods and Properties

• You can use the

  .length

  property to get the number of rows in the 2D array. Note that this does not give the total number of elements in the array.
• To get the number of columns, or the size of a particular row, we use

  arrayName[row].length

• Methods that can be used with arrays include

  Arrays.toString()

  and

  Arrays.deepToString()

  for printing,

  Arrays.sort()

  for sorting, and

  Arrays.equals()

  to check if two arrays are equal.