Introduction¶

In this activity, you will program the micro:bit version of a famous classic game called Battleship. Battleship has been played since World War 1 with pencil and paper1. A plastic board game was released in 1967, and now, there are several electronic versions and apps! 2

Figure 1: Battleship board

Let's look at how this game works using the example board in the figure above.
In this example, each player uses their own 10x10 board, and each player's fleet includes 10 ships of different sizes (the grey rectangles). The figure shows the placement for one of the players: 4 ships of size 2, 3 ships of size 3, 2 ships of size 4, and 1 ship of size 6. Arrangement ships is of course hidden from the opponent. Once both players have placed their ships on their boards, they start guessing the position of (shooting at) their opponent's ships. In the example board, the crosses mark the shots of the opponent. Notice that some of these crosses did not hit any ships, and some of them did. The opponent has sunk the ship on squares 8A-8B. The ship on the squares 6J-7J-8J was hit twice. Another shot on 8J will sink it. The players keep a second board to mark the shots they have already tried. They record each hit and miss to decide which shot to fire next.

To program the Battleship into your micro:bits, you will use your networking knowledge. This game requires unicast and bidirectional communication, which you worked on in Unicast Communication: One to One and Two way unicast. If you program the variant in Exercises, you will use information from  Handling errors: Retransmissions and Handling errors: Acknowledgements. In summary, you will practice:

• The concept of unicast communication, two-way communication and retransmissions

• Sending and receiving messages

• Button inputs

• Display and its coordinates

• Variables and random numbers

• Arrays

• Loops

What you’ll need¶

``````2 micro:bits
1 teammate
``````

Designing the Battleship for Microbit {#sec:design}¶

How the game works¶

Let’s start with going over the different pieces we need to program the Battleship. In the section above, you saw an example of the game, with a 10x10 board.

Using the micro:bit display as a Battleship board: Since micro:bit has a 5x5 display, your battleship board needs to be smaller. This does not allow for many ships or big ones. So, your fleet will be 5 ships, each with a size just 1.

When you fire a shot, you will need to know if it was a hit or a miss. So, we need to reserve the top row to display hits and misses. If your opponent's micro:bit says you had a hit, your micro:bit will light the leftmost LED. If it was an unfortunate miss, your micro:bit will light the rightmost LED.

Since your micro:bit has a limited display, you won’t be able to show your tries and misses in the display. Maybe, that's a memory challenge that can be added to the game, or you can keep a track of these with paper like the children who played the game in earlier times?

Firing shots: To fire shots, you will use the buttons. You will select a row and a column number to fire a shot. Note that when LED’s coordinates are given as (x,y), x is column number, and y is the row number. For more information, see
https://microbit.org/guide/hardware/leds/.

Figure 2: Battleship in micro:bit

Button A will be used to select the column number and button B will be used to select the row number. Then to fire a shot to (2,3), you will need to press button A twice, and press button B three times, and press both buttons A and button B together. To check your understanding, discuss with your teammate you can send a shot to (0,4).

When you press both buttons to fire a shot, your program will send a message to your opponent’s micro:bit. So, for example, if you want to fire a shot at (4,4), you will send the coordinates (4,4). When your opponent’s micro:bit receives a shot, it will check whether it is a hit or a miss: It will send a message back with its radio saying either it is a “Hit” or a “Miss”.

When you receive a “Hit”, you will light up the LED on the left corner of the top row. When you receive a “Miss”, you will light up the LED on the right corner of the top row.

A sample game¶

Let’s see how things will look like in your micro:bits. At the beginning, you will have all your battleships placed in the lower 4 rows as in the figure below. The figure shows both players have 5 ships placed in the battle area.

Figure 3: Battleship game: Initial stage with randomly placed ships.

The attacker (on the left) presses button A three times, and button B once. Pressing both buttons at the same fires a shot, and sends a shot message over the radio for the position (3,1). There is a ship on this location, and so, this is a hit! In the figure below, the leftmost LED in the top row of the attacker’s micro:bit lights up. And, in the opponent’s display the LED in the position (3,1) gets turned off, because this ship was sunk.

Figure 4: Battleship game: Success! You hit a ship!

Let’s also look at a miss situation (see the figure below). In this case, nothing should change on the opponent’s board. But, in the attacker’s display, in the top row, the rightmpst LED lights up to show a miss.

Figure 5: Battleship game: An unfortunate miss!

Programming: Battleship¶

Battleship is a two-person game. Both players can run identical problems, or you can each program your own version, as long as you agree on the details of the radio messages. When writing a more complex program like this, you will find it easier if you split up into parts, and test each part as you write it. (This is a valuable skill as you learn more about programming!)

To help with this, we have split the program into four tasks: once you have completed the final task, you will be able to play with your teammate. If you find any errors (bugs) in your program, work with your teammate to fix them until you get the game play as described in the introduction.

Task 1: Setting up the game¶

Description: This part needs to take place before the game starts. You will place 5 ships on your board. Think about randomly placing 5 points in the battle area, which is a 4 x 5 matrix. Answer the following questions:

• How will you represent the battle area in your program as a data structure?

• How will you select random coordinates (column_number, row_number) for 5 ships, where column_number is between 0 and 5, and row_number is between 1 and 5?

• How will you represent the information that there is a ship at each of these coordinates?

You will also set up your radio and packet configuration to send unicast messages.

Instruction: Create the necessary data structures and variables that represent the ships in the battle area. Set up your radio and packets for unicast communication. Test whether your program displays 5 ships randomly placed on the lower 4 rows of the display, like in the example figures.

Description: When button A is pressed, it indicates the column_number for a shot. So, you need to count how many times this button was pressed to get the column_number. When button B is pressed, it defines the row_number for the same shot. Again, count the number of times to get the row_number.

Important: If you do not press either button A or button B, the column_number=0 and row_number=0. A shot with row_number=0 is a wasted shot because there cannot be any ships on that row. Also, make sure if either button is pressed more than four times, it shoud start counting again from 0. In other words, the button counters should increment with each button press like this: 0, 1, 2, 3, 4, 0, 1, 2, 3, 4.

Pressing both buttons together will send column_number and row_number over the radio to your opponent. Decide how to send this message in a packet, and agree on this with your teammate if you are writing separate programs.

Instruction: Program the button presses for A, B, and A+B. The program piece for buttons A+B will send a radio message.

Test the correctness of your code. Add a little test code so that when you fire, as well as sending a radio message, it also lights up the LED at column_number and row_number. Use this to check that aiming is working correctly. This is just test code, so remove it once you're confident that it works.

Description: When you receive a shot, you will check whether you have a ship on the (column_number, row_number) of the shot. If you have a ship there, then it was hit and sunk. You will send a “Hit” message to your opponent, and remove the ship from the display. If it is a miss, send a “Miss” message too your opponent.

Instruction: Depending on how the packet was formatted, decode (column_number, row_number) from the received packet. If you have a ship on (column_number, row_number), it is a hit: Turn off the LED in that position. If you have a separate data structure as a variable to represent your ships, update that too. Send your opponent a “Hit” message. If it is a miss, send a “Miss” message to your opponent.

Task 4: Receiving the shot result: “Hit” or “Miss”¶

Description: Turn on LEDs in top row depending on the result. If it is a “Hit”, check if you reached 5 hits. Then you won! Display a smile!

Instruction: If you receive a “Hit”, light the leftmost LED of the top row (the LED in (0,0) position). Update the count of your hits, and if you reached 5, display a smile! If the result was a “Miss”, light the rightmost LED of the top row (the LED in (4,0) position).

Test your program(s) with your opponent. To start with, it'll be easier if you can see each other's screens. You might find it helpful to put in some test code, like you did for the previous task. For example, you could print out "hit" or "miss" when you receive and decode a shot. You might even find it helpful to print out the coordinates of the show when you receive the packet.

Extended Activity¶

Battleship game has many variations. See the Wikipedia site in Resources to read about the variations.

Exercise 1

One variation allows players to keep it secret that a ship has been sunk. So, their opponent has to take further shots to confirm that an area is clear. This is like having a packet loss! Remember how you dealt with packet losses in Handling errors: Retransmissions and Handling errors: Acknowledgements. How would you apply those concepts to this case? Discuss possible solutions with your friend. Then, program and test your new solution.

Exercise 2

Imagine a variant when it takes 3 hits to sink a ship instead of 1 hit. How would your program change? Do you need to make changes on the sender side or the receiver side? How similar is this to using default retransmissions in Handling errors: Retransmissions?

Problems¶

Figure 6: Battleship game: A random battle area

Problem 1: The figure above shows randomly placed ships in a battle area. Which coordinates do you need to send to hit all the ships?

Figure 7: Battleship game: Two players

Problem 2: The figure above shows randomly placed ships in the battle areas of two micro:bits. Table below lists all the shots that are fired from the micro:bit 1 (left/red micro:bit) and micro:bit 2 (right/yellow micro:bit). Who wins?

Rounds Micro:bit 1 Micro:bit2 Result
1 (3,1) (2,1)
2 (0,3) (0,1)
3 (1,1) (3,2)
4 (4,1) (3,3)
5 (0,3) (4,3)
6 (2,2) (0,3)
7 (3,2) (1,4)

If you wanted to play this game with another opponent, what do you need to change in your program?

1. Battleship in Wikipedia: https://en.wikipedia.org/wiki/Battleship_(game)

2. Online Battleship game 1: https://battleship-game.org and Online Battleship game 2: http://www.mathplayground.com/battleship.html