Microbit Racing

Racing fleas is a fun pastime, especially when those fleas are actually microbit pixels and no harm is done to actual fleas. Let’s build a game where we can race 5 fleas across the screen and the first flea to reach the right hand edge wins.

On your marks…

We can represent a flea moving about on the micro:bit screen using a single pixel. Let’s create a list of two items that will represent the flea’s x and y position on the screen. Remember, the left column is 0 and the right column is number 4.


from microbit import *

flea = [0, 0]


Now we can race one flea across the screen.


from microbit import *

flea = [0, 0] # x, y

# start in left column.
display.set_pixel(flea[0], flea[1], 9)

for x in range(1, 4):
    # turn off current pixel
    display.set_pixel(flea[0], flea[1], 0)
    # move
    flea[0] += 1
    display.set_pixel(flea[0], flea[1], 9)
    # wait so we don't move too fast
    sleep(500)        


Randomness

I hope you will agree, that’s not very exciting because each time you run the game, the outcome is the same. What about if we made it different every time? We can use random (and remember to import it at the top) to mix things up a little bit.


from microbit import *
import random

flea = [0, 0]

display.set_pixel(flea[0], flea[1], 9)

# increase the opportunities to move because
# we don't move every time...
for x in range(1, 10):
    # 50 % chance
    if random.randint(0, 1) == 1:
        display.set_pixel(flea[0], flea[1], 0)
        flea[0] += 1
        display.set_pixel(flea[0], flea[1], 9)

    sleep(500)  


The random.randint function returns a random value between the two values given (0 and 1 in this case) each time it is called in the for loop and we use that value to see if we have been given a 1 value. If so, we can move the flea forward a little bit. Each flea has about the same opportunity to move, a 50% chance because there are only two values possible from randint in this code.

We increase the number of opportunities to move from 4 to 10 because we have the random chance where we may not move in 4 turns. There’s a problem with this code which you might have spotted.

There’s a problem with this code, though. Can you find it?

Going too far

In between moving the flea and actually drawing on the screen, we can make sure we don’t go beyond the fourth column by using the min function.


from microbit import *
import random

flea = [0, 0]

display.set_pixel(flea[0], flea[1], 9)

# increase the opportunities to move because
# we don't move every time...
for x in range(1, 10):
    # 50 % chance
    if random.randint(0, 1) == 1:
        display.set_pixel(flea[0], flea[1], 0)
        flea[0] += 1
        flea[0] = min(flea[0], 4)
        display.set_pixel(flea[0], flea[1], 9)

    sleep(500)  


Now we have one flea racing with a different outcome every time, we can make preparations to race a lot of fleas.

We can do this by creating a collection of fleas and adding our single flea to it. That way we can write the code to work on the collection and don’t have to worry about how many fleas (within reason) we actually create later.

So after we created a single flea, let’s create a collection and add the flea to it.


from microbit import *
import random

flea = [0, 0]
all_fleas = [ flea ]


Then we can expand our initial code to draw the fleas at their starting points.



for flea in all_fleas:
  display.set_pixel(flea[0], flea[1], 9)


And change the main movement and drawing code to use a collection:


# race !
for x in range(1, 10):
    for flea in all_fleas:
      # 50 % chance
      if random.randint(0, 1) == 1:
          display.set_pixel(flea[0], flea[1], 0)
          flea[0] += 1
          flea[0] = min(flea[0], 4)
          display.set_pixel(flea[0], flea[1], 9)

    sleep(500) 


Be careful to keep the sleep in line with the for loop so that the half-second delay is only done once around the loop. Now that we have things set up, we can add more fleas. Let’s given them more descriptive names:


alice = [0, 0]
bob = [0, 1]
carol = [0, 2]
dave = [0, 3]
eleanor = [0, 4]

all_fleas = [ alice, bob, carol, dave, eleanor ]


That’s the only change we need to have 5 fleas, all racing in their own lane (y coordinate).

Cheating

So that’s a nice racing game which should have a different result each time. Each flea has a 50% chance or moving each time around the for loop, with random giving everyone an equal chance of winning, on average.

But what if we wanted to be a bit sneaky and cheat at the race? Could we make sure that one or two fleas would have a better chance than the others?

The critical piece of code is the call to random.randint that always has the same chance for each flea. If we manipulated the range of values available we could change the odds for each competitor. A flea which had a 1 in 4 (25%) chance of advancing would probably not win against a flea which had a 1 in 2 (50%) chance of advancing.

We can use the flea object to store a chance value as well as the x and y coordinates.


alice   = [0,  0,  1] # x, y, chance
bob     = [0,  1,  2]
carol   = [0,  2,  3]
dave    = [0,  3,  1]
eleanor = [0,  4, 10]


Now we can change the random code to use the new chance value:


if random.randint(0, flea[2]) == 1:
      display.set_pixel(flea[0], flea[1], 0)
      flea[0] += 1
      flea[0] = min(flea[0], 4)
      display.set_pixel(flea[0], flea[1], 9)


If you run the code again, you should see that alice and dave have the best chance of winning while poor eleanor has the worst (but not impossible) chance of winning with only 1 chance in 10 of moving for each turn.

Finish


from microbit import *
import random

# name     x,  y,  chance
alice   = [0,  0,  1]
bob     = [0,  1,  2]
carol   = [0,  2,  3]
dave    = [0,  3,  1]
eleanor = [0,  4, 10]

all_fleas = [ alice, bob, carol, dave, eleanor ]

# line up at start
for flea in fleas:
    display.set_pixel(flea[0], flea[1], 9)

# race !
for x in range(1, 10):
    for flea in all_fleas:
        # will this flea move this turn?
       if random.randint(0, flea[2]) == 1:
            display.set_pixel(flea[0], flea[1], 0)
            flea[0] = min(flea[0] + 1, 4)
            display.set_pixel(flea[0], flea[1], 9)

    sleep(500)