12. Two Dimensional Arrays

Fundamentals

int[][] data = new int[30][60];
int rows = data.length;
int cols = data[0].length;

import java.util.Arrays;
int[][] given = {{1,2},{3,4},{5,6}};
System.out.println(Arrays.deepToString(given));

Introduction

Array 2D intro and practice slides.

Exercises

1. Write the makeBoard function that creates a matrix of ones surrounded by a moat of zeros.

   public static int[][] makeBoard(int rows, int cols);
   
   int[][] result = makeBoard(5,7);
   System.out.println(Arrays.deepToString(result); 
   /*
   0 0 0 0 0 0 0
   0 1 1 1 1 1 0
   0 1 1 1 1 1 0
   0 1 1 1 1 1 0
   0 0 0 0 0 0 0
   */

2. Make boxSplit, which takes in a 1D array of integers of even length and splits it into two rows (in “row major order”).

   public static int[][] boxSplit(int[] data);
   
   int[] data = {10,20,40,80,120,160};
   int[][] result = boxSplit(data); 
   /* result == {{10,20,40},
                 {80,120,160}} */

3. Make the boxSplitV, which takes in a one dimensional array of integers of even length and splits it into two rows, but distributes the values in “column major order”.

   public static int[][] boxSplitV(int[] data);
   
   int[] data = {10,20,40,80,120,160};
   int[][] result = boxSplitV(data); 
   /* result == {{10,40,120},
                 {20,80,160}} */

4. The colMax function takes in a (nonempty, rectangular) 2D array of data and returns a 1D array containing the maximum value from each column of the array.

   public static int[] colMax(int[][] data);
   
   int[][] data = {{-50, 100, 40},
                   {-30,   0, 90},
                   {-40,  95, 30}};
   int[] result = colMax(data);
   System.out.println(Arrays.toString(result));
   // {-30,100,90}