Introduction to Two-Dimensional Arrays
Sometimes data makes more sense if it is represented in a two-dimensional(2D) grid, instead of in a single one-dimensional list. Consider the following examples of data being stored in a 2D grid:
- data in a spreadsheet with rows and columns
- a seating chart
- table of
x and y values for a graph
How Data is Stored?
An array of arrays can be used to represent a table of information. This is called a two-dimentional (2D) array.
Each value stored in an array is called an element. The size of the array is set when it is declared and created. This size does not change. So if more elements are needed a new array would need to be created, the current elements copied over, and the new elements added.
Each element in a 2D array has a row and column value. Row values start with 0 and end at the number of rows - 1. Column values also start at 0 and end at the number of columns - 1. When we draw out these diagrams, we traditionally label the rows down the left side, starting at 0 on the top, and the columns along the top, starting with 0 for the left most column, similar to the way rows and columns are layed out in a spreadsheet program.
Constructing a 2D Array
When constructing a 2D array object, we must specify the type of data that is stored. A 2D array can store either primitive or reference data.
<type of data>[][] nameOfArray = new <type of data>[<number of rows>][<number of columns>];
An example of a seating chart could be:
String[][] firstClass = new String[5][6];
Initial Values of an Array or 2D Array
When we create an array, some initial values are stored in the array.
null - if the type of values are reference, null is stored in the elements to indicate that there is no object assigned to these elements. 0 is assigned to the elements if the type is int.0.0 is assigned to the elements if the type is double.\000 is assigned to the elements if the type is char.false is assigned to the elements if the type is boolean.
Your Turn
Let's try it in the Java Playground.
- Predict the values stored in
testArray.
- Run the code to determine if your prediction is correct. In the Java Playground, it will show you what the line of code does even though no output statement is written.
- Change the type of the array to each of the following, predict the code, and run the code to check your prediction.
Stringdoublecharboolean
Constructing a 2D Array with Initial Values
We can also create a 2D array and provide all the initial values. One way we can do this is by using an initializer list. This is good for when you have a shorter list of known values for an array. e values are listed in braces ({ }) and the array is assigned this list. Remember, a 2D array is an array made up of arrays, so your initializer list, will be comprised of initializer lists for each row of data. Th
<type of data> [][] nameOfArray = { {<values for row 0>}, {<values for row 1>}, {<values for row 2>} };
NOTE: Not all rows have to have the same number of columns. For example:
int[][] numTimeAttended = { {3, 4}, {1, 7, 10}, {1}};
Produces the 2D Array:
3 4
1 7 10
1
Your Turn
Let's try it in the Java Playground.
- Predict the values stored in
gradeBook.
- Run the code to determine if your prediction is correct. In the Java Playground, it will show you what the line of code does even though no output statement is written.
Accessing Elements of a 2D Array
To access the elements of an array, use square brackets ([ ][ ]) and include the index value for the row and column in the square brackets.
<type of data>[][] name2DArray = { {<list of elements>}, {<list of elements>}, ... , {<list of elements>}};
<type of data> variable = name2DArray[<row index>][<col index>];
If we want to access the first element in row 0 and column 1, it would look as follows:
int[][] gradeBook = { {80, 70, 90, 95}, {86, 73, 92, 92}, {83, 79, 90, 100} };
int grade = gradeBook[0][1];
Your Turn
Let's try it in the Java Playground.
- Predict the output.
- Run the code to determine if your prediction is correct.
Determine the Number of Rows and Columns in a 2D Array
You can access the constant length to access the number of rows in the 2D array. To access the number of columns for each roow, you can use the constant lengthon an individual row.
int[][] gradeBook = { {80, 70, 90, 95}, {86, 73, 92, 92}, {83, 79, 90, 100} };
int numRows = gradeBook.length;
int numCols = gradeBook[0].length;
Your Turn
Let's try it in the Java Playground.
- Predict the output.
- Run the code to determine if your prediction is correct.
- Add a new variable
lastGrade and assign it the value in the last row and last column.
- Run the code to test.
Resources
To come