CS 145 Lecture 29 – Arrays in Two Dimensions

Dear students,

We’ve seen that we can create an array like this:

type[] id = new type[#];

That type can be anything, like int, String, double, boolean, BufferedImage, and so on. It can also be another array:

type[][] id = new type[#][];

Just as 1D arrays let us use one integer to map into a line of data, an 2D lets us use two integers to map into an area of data. We can do things like this:

String[][] teams = new String[nteams][3];

// Team A
teams[0][0] = "Jimmy Chonga";
teams[0][1] = "Jen Sing";
teams[0][2] = "Bill Dollar";

// Team B
teams[1][0] = "Bill Ding";
teams[1][1] = "Paula Ticks";
teams[1][2] = "Ann Ticks";

The beauty of 2D arrays is that we can organize data by two schema. Often these schema are spatial axes, but that is not a requirement.

We will explore 2D arrays by writing a simplified version of Battleship. But let’s ditch the war theme and trademark. We will call it Scatterpillar. It’s our job to find all the caterpillars scattered across a 2D Cartesian plane.

See you next class!

Sincerely,

P.S. Here’s the code we wrote together…

Teams.java

package lecture1116;

import java.util.Random;

public class Teams {
  public static void main(String[] args) {
    String[][] teams = new String[2][3];
    
    teams[0][0] = "Bill Ding";
    teams[0][1] = "Paula Ticks";
    teams[0][2] = "Will Powers";
    
    teams[1][0] = "Bill Dollar";
    teams[1][1] = "Jimmy Chonga";
    teams[1][2] = "Justin Thyme";
    
    faceOff(teams);
  }
  
  private static void faceOff(String[][] teams) {
    // pick a player from team 0
    // pick a player from team 1
    
    int nteams = teams.length;
    
    Random generator = new Random();
    int i = generator.nextInt(teams[0].length);
    String player0 = teams[0][i];
    
    i = generator.nextInt(teams[1].length);
    String player1 = teams[1][i];
    
    System.out.println(player0 + " vs. " + player1);
  }
}

Scatterpillar.java

package lecture1116;

import java.util.Random;
import java.util.Scanner;

public class Scatterpillar {
  public static void main(String[] args) {
    char[][] board = getNewBoard(5, 4);
    placeCaterpillar(board, 4);
    printBoard(board);

    Scanner in = new Scanner(System.in);

    while (hasUnhit(board)) {
      System.out.println();
      System.out.println("Where's that caterpillar? ");
      int c = in.nextInt();
      int r = in.nextInt();

      if (board[r][c] == 'o') {
        board[r][c] = '!';
        System.out.println("You find!!!!");
      } else {
        board[r][c] = 'x';
        System.out.println("Not yet. But you'll get it. Just try harder.");
      }
      System.out.println();

      printBoard(board);
    }
    
    System.out.println();
    System.out.println("Find all done!");
  }

  private static boolean hasUnhit(char[][] board) {
    for (int r = 0; r < board.length; ++r) {
      for (int c = 0; c < board[r].length; ++c) {
        if (board[r][c] == 'o') {
          return true;
        }
      }
    }
    return false;
  }

  private static void placeCaterpillar(char[][] board, int length) {
    Random gen = new Random();
    if (gen.nextBoolean()) { // vertical
      int c = gen.nextInt(board[0].length);
      int r = gen.nextInt(board.length - length + 1); // if height is 10 and
                                                      // length is 3, then range
                                                      // is [0, 7]
      for (int i = 0; i < length; ++i) {
        board[r + i][c] = 'o';
      }
    } else { // horizontal
      int r = gen.nextInt(board.length);
      int c = gen.nextInt(board[0].length - length + 1); // if width is 10 and
                                                         // length is 3, then
                                                         // range is [0, 7]
      for (int i = 0; i < length; ++i) {
        board[r][c + i] = 'o';
      }
    }
  }

  public static char[][] getNewBoard(int width, int height) {
    char[][] board = new char[height][width];

    for (int r = 0; r < height; ++r) {
      for (int c = 0; c < width; ++c) {
        board[r][c] = '.';
      }
    }

    return board;
  }

  public static void printBoard(char[][] board) {

    System.out.print(' ');
    for (int c = 0; c < board[0].length; ++c) {
      System.out.print(c);
    }
    System.out.println();

    for (int r = 0; r < board.length; ++r) {
      System.out.print(r);
      for (int c = 0; c < board[r].length; ++c) {
        if (board[r][c] == 'o') {
          System.out.print('.');
        } else {
          System.out.print(board[r][c]);
        }
      }
      System.out.println();
    }
  }
}

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