Nice but NIM


Document It

  • Watch the video…
  • Write an algorithm to solve the bridge riddle. You need to get everyone, in pairs, across in 17 minutes before the zombies get you.

Learn It

  • Recap - "Nim" is a two-player game played with sticks or stones. The objects are divided into piles.
  • The players alternate turns. On each player's turn they may remove any number of sticks from one of the piles, up to the number of sticks remaining in that pile; but they can only take from a single pile on a given turn.
  • The goal is to take the last stick. Whoever takes the last stick wins.
  • So the goal is to make the other person clear out all but one pile, so you can take all the sticks in that last pile. Etc.
  • Let's play a game starting from the game state 1, 4, 7. You can go first.
  • How many sticks are you going to take and from which pile?
    1. XX (1 sticks)
    2. XXXX (4 sticks)
    3. XXXXXXX (7 sticks)

Try It

Document It

  • Save your certificate/s to show how many levels you passed on the NIM game in 5 minutes.

Learn It: Strategy

  • There is a known winning strategy to playing 'Advanced NIM'. Let's use a simple example to illustrate how to use this strategy.
  • Let's say you have three piles with 1, 4 and 7 stones:
    • X
    • XXXX
  • To find out the optimal move, we first express the count of each pile as a binary number, using this table:
Decimal Binary
0 0000
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
10 1010
  • Next we perform a special operation on these binary numbers, by following the following process:
  • For each column of digits, if there are an even number of 1's, write a zero at the bottom of that column. If there are an odd number, write a one. This is called the Bitwise exclusive or operation.
Pile Sticks Binary Denary
1 X 0001 1
2 XXXX 0100 4
3 XXXXXXX 0111 7
  pile-sum: 0010 2
  • In the example above, the right-most column has a 1, then a 0, another 1. There are an even number of 1s (two of them), so we write 0 at the bottom of the column.
  • The next column (third from the left) has only one 1 in it; an odd number. We write a 1 at the bottom of that column.
  • The resulting 4-bit number (0010 in this example) is called the pile-sum.
  • To win, we want to get the pile-sum to total 0000 after our turn.
  • Looking over the numbers above, it looks like if we took the pile-sum away from the third pile, we'd changing a 7 (0111 in binary) to a 5 (0101 in binary). The pile sum would then be 0000.
  • On the other player's turn, it now doesn't matter which pile they take from, as long as you reduce the pile sum back to 0000 on your turn.
  • You can now continue this process until you win the game by taking the last stick.
  • If your opponent knows the strategy and leaves you with a pile sum of 0000, then you will lose, assuming perfect play on their part.

Code It

  • This is a lot of mental calculation. It'd be better if we wrote a program in Python to tell us our moves, so that we can always win!
  • To write this program, we'll need to take several steps.
    • We need the ability to convert the denary numbers into their binary equivalent.
    • This will need to ask for the three numbers and convert them into binary.
    • We will need to XOR these binary numbers to get the pile sum.
    • Finally, we should advise the player of what to do.
  • Now we've a general plan, let's get started…
  • Step 1: Make a function to convert denary to binary. A quick Google search tells us that Python has a built-in function to handle this.
  • Step 2: XOR function. Google to the rescue again; Python also has some built-in functions to do the XOR operation for us, by using the bin(a ^ b) function.
  • Our code now can be updated to include this new functionality…
  • Step 3: Once we know this information, we can calculate which pile to remove sticks from, and how many to take.
  • The Trinket below shows a solution for a 3-pile game. You can use this to complete up to level 4 on the online Nim game.

Research It

  • What is Exclusive Or (XOR)?
  • How do we use it in Computer Science?

Badge It Silver

  • Write a set of rules for someone who has never played '21' before so that they will always win. You may need to re-read the week 1 notes for this.
  • OR…
  • Use the helper program to win Nim level 2 on the online Nim game. Upload a screenshot of the victory screen as evidence.

Badge It Gold

  • Write a working two-player game of 21 in Scratch. Upload a screenshot of your code; the week one notes will help.
  • OR…
  • Modify the code above, so that you can complete level 5 and level 6 of the online Nim game. Upload a screenshot of the level 6 victory screen as evidence, and a seperate screenshot of your code.

Badge It Platinum

  • Implement a one-player version of '21' Nim in Scratch, so that you can play with a computer opponent. You may wish to look back at the last lesson's notes for hints.
  • The computer should randomly choose to play either a 1, 2 or 3.
  • Implement a strategy that means the computer will always win when it goes first.