Unit 4 Cycle 1 Day 21: 2D Array Creation and Access
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2D Array Creation and Access
Section 4.9 — 2D Arrays
Key Concept
A 2D array in Java is an array of arrays. Declaration: int[][] grid = new int[3][4] creates 3 rows and 4 columns. Access uses two indices: grid[row][col]. The number of rows is grid.length and the number of columns in row r is grid[r].length. All elements are initialized to the type's default value (0 for int, null for objects). The AP exam frequently tests whether you correctly use row-first, column-second indexing.
Consider the following code segment.
What is printed?
Answer: (B) 7
grid[2] is the third row: {7, 8, 9}. grid[2][0] is the first element of that row: 7.
Why Not the Others?
(A) grid[0][2] = 3. The first index is row, second is column.
(C) grid[1][0] = 4. Row 1, column 0.
(D) grid[0][0] = 1. Row 0, column 0.
Common Mistake
2D arrays use [row][col] indexing. The first index selects the row, the second selects the column within that row.
AP Exam Tip
grid[r][c]: r = row, c = column. grid.length = number of rows. grid[0].length = number of columns.