2D Array Creation and Access: Revision Notes (Computer Science A 4.11)

Full explanation, figures and traced examples: Topic 4.11 study guide.

Recap

A 2D array is an array of arrays: the outer array holds one reference per row and each reference leads to a 1D row array. For the exam arr[row][col]: first index row, second index column. Only rectangular arrays are in scope. The size is fixed when the array is created.

int[][] grid = new int[3][4];            // 3 rows, 4 columns, all 0
int[][] m = { {1, 2, 3}, {4, 5, 6} };    // 2 rows, 3 columns
int rows = m.length;                     // 2
int cols = m[0].length;                  // 3
int x = m[1][2];                         // 6
m[0][1] = 20;                            // change one element
int[] row1 = m[1];                       // alias of the second row

Key relationships

Item	Rule
Number of rows	arr.length
Number of columns	arr[0].length (or arr[r].length)
Total elements	arr.length * arr[0].length
Valid row index	0 to arr.length - 1
Valid column index	0 to arr[0].length - 1
Defaults after new	0, 0.0, false, null by element type
Row	arr[r] is a 1D array and a reference (alias, not copy)
Bad index	ArrayIndexOutOfBoundsException
null element	calling a method on it throws NullPointerException

Assumptions

• The array is rectangular (all rows the same length).
• The array variable is not null.
• Indices are checked against the correct dimension: rows against arr.length, columns against arr[0].length.

A mini-example to rehearse

int[][] grid = new int[3][4]; grid[1][2] = 7; grid[2][0] = grid[1][2] + 1; leaves row 1 as 0 0 7 0 and row 2 as 8 0 0 0; grid.length is 3 and grid[0].length is 4. With int[][] m = { {1, 2, 3}, {4, 5, 6} }, m[1][2] + m[0][1] is 6 + 2 = 8. int[] row = m[1]; row[0] = 40; changes m[1][0] to 40, because the row is shared. m[2][0] and m[0][3] both throw ArrayIndexOutOfBoundsException.

Mistakes to avoid

1. Reversing row and column.
2. Using arr.length for the number of columns.
3. Expecting "" in a new String[][] (it is null).
4. Treating arr[r] as a copy of the row.
5. Using <= with length in an index check.
6. Forgetting that the size cannot change after creation.

How this topic is usually tested

• Give the value of an element, the number of rows or columns, or the total size.
• Trace assignments and write the final table.
• Spot the out-of-range index or the null element that causes an exception.
• Write short statements or methods that read or fill a cell or a row.

Quick self-check

1. new int[3][4]: how many rows and columns? 3 rows and 4 columns, 12 elements, all 0.
2. For { {1, 2, 3}, {4, 5, 6} }, what is m[0][2]? 3
3. What is m[1].length? 3
4. A new String[2][2] holds what in each cell? null
5. Which of m[2][0] and m[1][2] throws for the 2-by-3 array m? m[2][0] (row 2 does not exist).