📓4.11: 2D Array Creation & Access

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 4.11

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Two-Dimensional (2D) Arrays

We have only worked with one-dimensional arrays so far, which have a single row of elements. But in the real world, data is often represented in a two-dimensional table with rows and columns. Programming languages can also represent arrays this way with multiple dimensions. A two-dimensional (2D) array has rows and columns.

A 2D array in Java is actually an array of arrays!

Arrays in Java can store many items of the same type. You can even store items in two-dimensional (2D) arrays which are arrays that have both rows and columns. A row has horizontal elements. A column has vertical elements. In the picture below there are 3 rows of lockers and 6 columns.

image

Two dimensional arrays are especially useful when the data is naturally organized in rows and columns like in a spreadsheet, bingo, battleship, theater seats, classroom seats, or a picture. In battleship, letters map to the rows (A is the first row, B is the second row, and so on) and the column indices start with 1.

Array Storage

Many programming languages actually store two-dimensional array data in nested one-dimensional arrays. The typical way to do this is to store all the data for the first row, with each spot referring to a column, followed by all the data for the second row, and so on. This is called row-major order.

Some languages store all the data for the first column followed by all the data for the second column and so on. This called column-major order.

image

Java actually stores a two-dimensional array as an array of arrays. Each element of the outer array has a reference to each inner array.

The outer array can be thought of as the rows and the inner arrays as the columns.

image

The picture above shows a 2D array that has 3 rows and 7 columns. Notice that the array indices start at 0 and end at the length - 1.

Declaring 2D Arrays

To declare a 2D array, specify the type of elements that will be stored in the array, then double brackets ([][]) to show that it is a 2D array of that type, then at least one space, and then a name for the array.

int[][] ticketInfo;
String[][] seatingChart;

Note that the declarations above just name the variable and say what type of array it will reference. The declarations do not create the array. Arrays are objects in Java, so any variable that declares an array holds a reference to an object. If the array hasn’t been created yet and you try to print the value of the variable, it will print null (meaning it doesn’t reference any object yet).

To create an array use the new keyword, followed by a space, then the type, and then the number of rows in square brackets followed by the number of columns in square brackets, like this new int[numRows][numCols].

ticketInfo = new int[2][3];
seatingChart = new String[3][2];

The code above creates a 2D array with 2 rows and 3 columns named ticketInfo and a 2D array with 3 rows and 2 columns named seatingChart.

The total number of elements in a 2D array is the number of rows times the number of columns.

Set Value(s) in a 2D Array

When arrays are created, their contents are automatically initialized to 0 for numeric types, null for object references, and false for type boolean.

To explicitly set a value in a 2D array, you can use assignment statements with the name of the array followed by the row index in square brackets followed by the column index in square brackets, and then an = followed by a value:

// Initialize the 2D array
int[][] data = new int[2][3];
// Set value of the first item
data[0][0] = 15;

Initializer Lists for 2D Arrays

You can also initialize (set) the values for the array when you first create it. In this case you don’t need to specify the size of the array, it will be determined from the values you give.

The code below uses an initializer list to create a value-filled array called ticketInfo with 2 rows and 3 columns. It also creates an array called seatingInfo with 3 rows and 2 columns.

int[][] ticketInfo = { {25,20,25}, {25,20,25} };
String[][] seatingInfo = { {"Jamal", "Maria"}, {"Jake", "Suzy"}, {"Emma", "Luke"} };

💬 DISCUSS: What is the value at seatingInfo[2][1] after the code above executes?

Get a Value from a 2D Array

To get the value in a 2D array give the name of the array followed by the row and column indices in square brackets.

int[][] ticketInfo = { {25,20,25}, {25,20,25} };
String[][] seatingInfo = { {"Jamal", "Maria"}, {"Jake", "Suzy"}, {"Emma", "Luke"} };
int value = ticketInfo[1][0];
String name = seatingInfo[0][1];

The code above will get the int value at row index 1 and column index 0 from ticketInfo. It will also get the String value at row index 0 and column index 1 from seatingChart.

Row and Column length

Arrays know their length (how many elements they can store). The length is a public read-only attribute for array objects. The number of rows contained in a 2D array can be accessed through the array name and the length attribute: arrayName.length. The number of columns contained in a 2D array can be accessed through the length attribute of one of the rows, for example arrayName[0].length, which gives a single array that is a row of a 2D array using the 2D array name and a single set of square brackets containing the row index.

  ticketInfo.length // returns the number of rows
  ticketInfo[0].length // returns the number of columns

Note that length is an attribute and not a method, so you don’t add parentheses after length. However, if you use parentheses after length during the exam, you won’t lose any points. Since for the AP CSA exam all two-dimensional arrays are rectangular arrays (arrays that have the same number of columns in each row) you can just use the length of the first inner array as the number of columns as shown by ticketInfo[0].length.

  • The valid row index values for a 2D array are 0 through one less than the number of rows or the length of the array, inclusive.
  • The valid column index values for a 2D array are 0 through one less than the number of columns or the length of any given row of the array, inclusive.

For example, given a 2D array named values, the number of rows is values.length and the number of columns is values[0].length. Using an index value outside of these ranges will result in an ArrayIndexOutOfBoundsException.

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⭐️ Summary

  • (AP 4.11.A.1) A 2D array is stored as an array of arrays. Therefore, the way 2D arrays are created and indexed is similar to 1D array objects. The size of a 2D array is established at the time of creation and cannot be changed. 2D arrays can store either primitive data or object reference data. Nonrectangular 2D array objects (with varying column length for each row) are outside the scope of the AP Computer Science A course and exam.

  • 2D arrays are declared and created with the following syntax: datatype[][] variableName = new datatype[numberRows][numberCols];

  • (AP 4.11.A.2) When a 2D array is created using the keyword new, all of its elements are initialized to the default values for the element data type. The default value for int is 0, for double is 0.0, for boolean is false, and for a reference type is null.

  • (AP 4.11.A.3) The initializer list used to create and initialize a 2D array consists of initializer lists that represent 1D arrays; for example, int[][] arr2D = { {1, 2, 3}, {4, 5, 6} };.

  • (AP 4.11.A.4) The square brackets [row][col] are used to access and modify an element in a 2D array. For the purposes of the AP exam, when accessing the element at arr[first][second], the first index is used for rows, the second index is used for columns.

  • Row-major order refers to an ordering of 2D array elements where traversal occurs across each row, while column-major order traversal occurs down each column.

  • (AP 4.11.A.5) A single array that is a row of a 2D array can be accessed using the 2D array name and a single set of square brackets containing the row index.
  • (AP 4.11.A.6) The number of rows contained in a 2D array can be accessed through the length attribute. The valid row index values for a 2D array are 0 through one less than the number of rows or the length of the array, inclusive. The number of columns contained in a 2D array can be accessed through the length attribute of one of the rows. The valid column index values for a 2D array are 0 through one less than the number of columns or the length of any given row of the array, inclusive. For example, given a 2D array named values, the number of rows is values.length and the number of columns is values[0].length. Using an index value outside of these ranges will result in an ArrayIndexOutOfBoundsException`.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.