See Intro to 2D arrays before attempting the exercises below.
Increasingly complex manipulations of 2D arrays have been featured on the AP Computer Science A Exam. This exercise includes traversals, swaps, and other manipulations of 2 dimensional arrays.
The Java files below include skeleton code for each method and a JUnit 5 tester for each method. See Running JUnit 5 tests.
TwoDArrayPractice.java
TwoDArrayPracticeTest.java
rowSwap method
/**
* Swaps all values in the specified 2 rows of mat.
* @param mat the array
* @param rowAIndex the index of a row to be swapped
* @param rowBIndex the index of a row to be swapped
*/
public static void rowSwap(int[][] mat, int rowAIndex, int rowBIndex)
rowSwap example
mat (before call):
10 9 8 7
6 5 4 3
2 1 -1 0
rowSwap(mat, 1, 2);
mat (after call):
10 9 8 7
2 1 -1 0
6 5 4 3
colSwap method
/**
* Swaps all values in the specified 2 columns of mat.
* @param mat the array
* @param colAIndex the index of a column to be swapped
* @param colBIndex the index of a column to be swapped
*/
public static void colSwap(int[][] mat, int colAIndex, int colBIndex)
colSwap example
mat (before call):
10 9 8 7
6 5 4 3
2 1 -1 0
colSwap(mat, 1, 3);
mat (after call):
10 7 8 9
6 3 4 5
2 0 -1 1
fillRowMajor method
/**
* Returns an array with the specified number of rows and columns
* containing the characters from str in row-major order. If str.length()
* is greater than rows * cols, extra characters are ignored. If
* str.length() is less than rows * cols, the remaining elements in the
* returned array contain null.
*
* @param str the string to be placed in an array
* @param rows the number of rows in the array to be returned
* @param cols the number of columns in the array to be returned
* @return an array containing the characters from str in row-major order
*/
public static String[][] fillRowMajor(String str, int rows, int cols)
fillRowMajor example 1
String[][] result = fillRowMajor("stop whining", 3, 4);
result (after method call):
"s" "t" "o" "p"
" " "w" "h" "i"
"n" "i" "n" "g"
fillRowMajor example 2
String[][] result = fillRowMajor("stop complaining", 3, 4);
result (after method call):
"s" "t" "o" "p"
" " "c" "o" "m"
"p" "l" "a" "i"
fillRowMajor example 3
String[][] result = fillRowMajor("do more", 3, 4);
result (after method call):
"d" "o" " " "m"
"o" "r" "e" null
null null null null
fillColumnMajor method
/**
* Returns an array containing the elements of vals in column-major order.
*
* Precondition: vals.length == rows * cols
*
* @param vals the elements
* @param rows the number of rows in the array to be returned
* @param cols the number of columns in the array to be returned
* @return an array containing the elements of vals in column-major order
*/
public static int[][] fillColumnMajor(int[] vals, int rows, int cols)
fillColumnMajor example
int[] source = new int[] {10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, -1};
int[][] result = fillColumnMajor(source, 3, 4);
result (after method call):
10 7 4 1
9 6 3 0
8 5 2 -1
fillDownUp method
/**
* Returns an array with the specified number of rows and columns that
* contains the elements of vals in the order specified below. Elements
* from vals are placed in the array by moving down the first column,
* up the second column and so on.
*
* Precondition: vals.length == rows * cols
*
* @param vals the elements
* @param rows the number of rows in the array to be returned
* @param cols the number of columns in the array to be returned
* @return an array containing the elements from vals in the specified order
*/
public static int[][] fillDownUp(int[] vals, int rows, int cols)
fillDownUp example
int[] source = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
int[][] result = fillDownUp(source, 3, 4);
result (after method call):
1 6 7 12
2 5 8 11
3 4 9 10
grow method
/**
* Returns a larger array containing the same elements as the
* mat. The elements from mat are read in row-major order and
* appear in the new array in row-major order.
*
* Precondition: newRows > mat.length && newCols > mat[0].length
*
* @param mat the 2D array containing the original elements
* @param newRows the number of rows in the new array
* @param newCols the number of columns in the new array
* @return a larger array containing the elements from mat in row-major order
*/
public static int[][] grow(int[][] mat, int newRows, int newCols)
grow example
mat (before method call):
10 9 8 7
6 5 4 3
2 1 -1 0
int[][] result = grow(mat, 4, 5);
result (after method call):
10 9 8 7 6
5 4 3 2 1
-1 0 0 0 0
0 0 0 0 0
crop method
/**
* Returns a smaller array containing the elements in the specified
* range of the mat.
*
* @param mat the 2D array containing the original elements
* @param startRow the first row to be kept
* @param startCol the first column to be kept
* @param endRow the last row to be kept
* @param endCol the last column to be kept
* @return a smaller array containing the specified elements
*/
public static int[][] crop(int[][] mat,
int startRow, int startCol,
int endRow, int endCol)
crop example
mat (before method call):
10 9 8 7
6 5 4 3
2 1 -1 0
int[][] result = crop(mat, 0, 1, 1, 2);
result (after method call):
9 8
5 4
invert method
/**
* Returns an array containing the same elements as
* mat but with the rows and columns reversed.
*
* @param mat the array
* @return an array containing elements as described
*/
public static int[][] invert(int[][] mat)
invert example
mat (before method call);
10 9 8 7
6 5 4 3
2 1 -1 0
int[][] result = invert(mat);
result (after method call):
10 6 2
9 5 1
8 4 -1
7 3 0
consolidate method
/**
* Consolidates the non-null elements in mat such
* that they appear in row-major order, starting at
* the top left.
*/
public static void consolidate(String[][] mat)
consolidate example 1
mat (before method call):
"a" null "b" "c" null
null "d" null null null
"e" null null "f" null
consolidate(mat);
mat (after method call):
"a" "b" "c" "d" "e"
"f" null null null null
null null null null null
consolidate example 2
mat (before method call):
"a" null "b" "c" null
null "d" "e" null "f"
"g" "h" "i" "j" "k"
consolidate(mat);
mat (after method call):
"a" "b" "c" "d" "e"
"f" "g" "h" "i" "j"
"k" null null null null
Solution & comments
See the 2D array practice solutions or review them with AP CS Tutor Brandon Horn.
Comment on 2D array practice exercises