### Stage 6: Dodge those pipes

So far, we’ve got a bird that flaps and a pipe that displays but the pipe doesn’t move. That’s no fun. In this step we’re going to make the pipes move and detect if the bird has crashed into them.

First, we need to keep track of how many times the main game loop has repeated. We’ll call this the frame number. We’ll start at 0 and every time we refresh the screen we’ll increase the frame number.

• Add a lineÂ frame = 0Â near the top, under your other global variables (y, speed and score)
• Under the line that says while True:Â , add a line that says frame += 1

frame += 1Â is shorthand for, and means exactly the same as frame = frame + 1

They both mean “increase the value stored in the variable called frame by one” or “increment frame”

There’s no point keeping track of the frame number unless we use that number for something helpful. Take a look at the highlighted lines in the code below to see what we’ll use it for:

```# Flappy bird Stage 6: Dodge those pipes
# https://blog.withcode.uk/2016/05/flappy-bird-microbit-python-tutorial-for-beginners
from microbit import *
import random

# Game constants
DELAY = 20                      # ms between each frame
FRAMES_PER_WALL_SHIFT = 20      # number of frames between each time a wall moves a pixel to the left
FRAMES_PER_NEW_WALL = 100       # number of frames between each new wall
FRAMES_PER_SCORE = 50           # number of frames between score rising by 1

# Global variables
y = 50
speed = 0
score = 0
frame = 0

# Make an image that represents a pipe to dodge
def make_pipe():
i = Image("00003:00003:00003:00003:00003")
gap = random.randint(0,3)   # random wall position
i.set_pixel(4, gap, 0)      # blast a hole in the pipe
i.set_pixel(4, gap+1, 0)
return i

# create first pipe
i = make_pipe()

# Game loop
while True:
frame += 1

# show pipe
display.show(i)

# flap if button a was pressed
if button_a.was_pressed():
speed = -8

# show score if button b was pressed
if button_b.was_pressed():
display.scroll("Score:" + str(score))

# accelerate down to terminal velocity
speed += 1
if speed > 2:
speed = 2

# move bird, but not off the edge
y += speed
if y > 99:
y = 99
if y < 0:
y = 0

# draw bird
led_y = int(y / 20)
display.set_pixel(1, led_y, 9)

# move wall left
if(frame % FRAMES_PER_WALL_SHIFT == 0):
i = i.shift_left(1)

# create new wall
if(frame % FRAMES_PER_NEW_WALL == 0):
i = make_pipe()

# increase score
if(frame % FRAMES_PER_SCORE == 0):
score += 1

# wait 20ms
sleep(20)```

Firstly, lines 8-12 define some constants. Constants are similar to variables in that they store data but unlike variables, the values don’t change as your code runs.

Constants are really useful for giving values or data a name to make your code easier for other programmers to understand. They also make it easier to tweak and change how your code works.

The constants we’ve defined are DELAYÂ , FRAMES_PER_WALL_SHIFTÂ , FRAMES_PER_NEW_WALL Â andFRAMES_PER_SCOREÂ . See the comments after each one in the code for a description of what they do.

Notice how in python, constants are written in capital letters and variables are written in lowercase. Both use underscores to separate words. Your code wont crashÂ if you don’t stick to these rules but it’s one of many rules (called conventions) that python programmers are encouraged to follow (see PEP8 for more info if you’re feeling really geeky)

Try changing the values of these constants

Try changing the value of these constants on lines 9-12 and see how each affects the game.

Lines 62-72 check the current frame number to see if the program should shift the wall left, create a new wall or increase the score depending on the value you set for the constants FRAMES_PER_WALL_SHIFTÂ , FRAMES_PER_NEW_WALLÂ  and FRAMES_PER_SCORE Â respectively.

They do this using the % operator (called the modulo operator). If you want to understand how this works, you’re best off learning to search on websites like stackoverflow.com for a suitable answer to questions like that.

We’re almost there now. The only bit that’s missing is to detect if the bird crashes into a pipe: to stop the game and display the score. Can you do that without cheating and going on to the next page? Go on…