Author: Tim Cummings

Moon lander game in python

Floating point numbers, called floats in python, are numbers which have a decimal point and sometimes digits after the decimal point. They are different to integers which represent whole numbers and do not have a decimal point. You can display the type of values or variables using the type() function. The following example displays the type of several values. In python 3, the division operation / always returns a float.

>>> type(3)
<class 'int'>
>>> type(3.14)
<class 'float'>
>>> type(3.0)
<class 'float'>
>>> type(6/2)
<class 'float'>

Printing floats can be messy because python prints as many decimal places as it thinks it needs up to about 16 significant figures. However python provides several methods of formatting floats to improve output. We will look at the .format() method which can be applied to any string and can be used to format all data types. It produces a new string which can be printed or saved to a file or stored in a variable for later use or passed to another function. .format() uses {} placeholders in a string to insert values of variables in the new string. Inside these curly braces you can specify details such as which value from the arguments (before the :), how wide the string is (integer just after the :), how many decimal places to show after the decimal point (for example .3f includes 3 decimal places after the decimal point of a float. Hence, to include the value of altitude to 3 decimal places in a new string we use

'Altitude = {:.3f}'.format(altitude)

which returns a string 'Altitude=970.000'

A width can be provided immediately following the :. To format and print several values in the same string, provided in the correct order in format, we have the following example:

print("Altitude={:8.3f} Speed={:6.3f} Fuel={:8.3f} Impact={:6.3f} Previous burn={:6.3f}".format(altitude,speed,fuel,impact,burn))

which prints the following:

Altitude= 970.000 Speed=29.422 Fuel=1478.000 Impact=32.969 Previous burn=22.000

This exercise also demonstrates the benefits of comments in python code. Comments are not interpreted by python and do not affect the program at all. However, they make it much easier for other people, or even yourself a few weeks later, understand what the program is supposed to do. All comments start with a #. Here is a game to pilot the lunar landing module onto the moon. Read the comments to work out what you need to do and then give it a go. See if you can work out how it works.

# moonlander.py
# this game simulates a lunar landing module (LLM) landing on the moon
# you are the pilot and need to control how much fuel you burn in the
# retro rockets so that your descent speed slows to zero just as your
# altitude above the moon's surface reaches zero. If you impact the moon
# more than 5 m below the surface, or your speed on impact is
# greater than 5 m/s then you have considered to have crashed.
# Otherwise it is considered to be a 'good' landing.
# If you run out of fuel, LLM will accelerate towards moon by gravity.

# set up the initial parameters
speed = 30      # speed approaching the moon
fuel = 1500     # how much fuel is left
altitude = 1000 # altitude above moon
gravity = 1.622 # acceleration due to gravity
burn = 0        # initial rate of burning fuel in retrorockets

# while LLM is above the moon's surface,
# calculate flight data and take input from pilot
while altitude > 0:
    # calculate how long until LLM will impact moon at current speed (impact)
    if speed <= 0:
        impact = 1000
    else:
        impact = altitude / speed
    # display flight data
    print("Altitude={:8.3f} Speed={:6.3f} Fuel={:8.3f} Impact={:6.3f} Previous burn={:6.3f}".format(altitude,speed,fuel,impact,burn))
    # take input from pilot
    burn = float(input("Enter fuel to burn (0-50)?"))
    # ensure rate of fuel burning is within rocket's capability and doesn't exceed remaining fuel
    if burn < 0:
        burn = 0
    if burn > 50:
        burn = 50
    if burn > fuel:
        burn = fuel
    #calculate new flight data
    altitude -= speed
    speed += gravity - burn/10
    fuel -= burn
# loop has ended so we must have hit moon's surface
# display final flight data and assess whether it was a crash or a good landing
print("Altitude={:8.3f} Speed={:6.3f} Fuel={:8.3f} Last burn={:6.3f}".format(altitude,speed,fuel,burn))
if altitude <- 5 or speed > 5:
    print("You have crashed")
else:
    print("You have landed")
Categories: CoderDojo

Factors of a number using python

This exercise is to find all the factors of a given number. Remember that a factor divides evenly into a number without leaving a remainder. We can use the modulo operator (%) to check for any remainder. The program will check all integers from 2 up to but not including the original number. To loop through those numbers we will use the for loop combined with the range() function.

number = int(input("Enter the number?"))
for i in range(2,number):
    remainder = number % i
    quotient = number // i
    print(number,"=",i,"x",quotient,"+ remainder",remainder)
    if remainder == 0:
        print("remainder is zero so",i,"is a factor")
    else:
        print("remainder is not zero so",i,"is not a factor")

Using print() in this way shows clearly the flow of the program. However, two lines of code for each potential factor is a lot to read through. To reduce the number of lines of code, print() lets us specify a different ending than a new line. If we specify end=’ ‘ then print() will end the line with a space rather than a new line significantly reducing the number of lines of output.

The following program ignores output when the number is not a factor, and displays all the factors on the same line, with a final message whether the original number is prime or composite. Instead of asking for the number to be entered, the program finds all factors for all numbers between 2 and 99 inclusive.

for number in range(2,100):
    print(number,'factors:', end=' ')
    prime=True
    for i in range(2,number):
        if number%i == 0:
            print(i, end=' ')
            prime=False
    if prime:
        print('prime')
    else:
        print('composite')
Categories: CoderDojo

Guessing number game in Python

This exercise is to write a program which asks the user to guess a number and the program will give hints to help the user guess the correct answer.

To display a prompt waiting for a user to enter some data, we use the input() function. The input() function provides the data as a string, which is a sequence of letters. To convert to an integer number we use the int() function.

To control the flow of the program based on the user’s answers, we use if and elif statements (elif is short for “else if”).

answer=15
guess=None
while guess != answer:
    guess=int(input("Guess a number between 0 and 50 ? "))
    if guess > answer:
        print("too high")
    elif guess < answer:
        print("too low")
print("Yes. Answer is ", answer)

Importing more functionality

Sometimes we want more functionality than is provided in the base python system. In our example we want a random number generator which is available in the random module (library). The program becomes a lot more interesting now, because we can’t see the answer by reading the source code.

import random
answer=random.randint(0,50)
guess=None
while guess != answer:
    guess=int(input("Guess a number between 0 and 50 ? "))
    if guess > answer:
        print("too high")
    elif guess < answer:
        print("too low")
print("Yes. Answer is ", answer)

In the previous countdown examples, we can put a timer to ensure the countdown happens one second at a time.

import time
x=10
while x>0:
    print(x)
    x-=1
    time.sleep(1)
print("blast off")
Categories: CoderDojo

Getting started with Python

There are many online resources for beginning in python. Here are a few

https://wiki.python.org/moin/BeginnersGuide/NonProgrammers – Python’s own suggestions for getting started
http://www.letslearnpython.com/learn/ – An online tutorial for kids used at PyCon each year
http://kata.coderdojo.com/wiki/Python_Path – Some international CoderDojo resources

At Kenmore CoderDojo Term 3 2017 we are going to be using Python 3. We will also be using Python with Minecraft. For full setup instructions, refer to https://www.triptera.com.au/wordpress/2017/06/02/coderdojo-minecraft-with-python-setup/

Python shell

At our first session, we installed Python 3.6.2 and used the IDLE development environment for running python in a shell.

Python can use operators +, -, *, /, //, % and ** to preform mathematical operations

>>> 5 + 3
8
>>> 5 - 3
2
>>> 5 * 3
15
>>> 5 / 3
1.666666666667
>>> 5 // 3
1
>>> 5 % 3
2
>>> 5 ** 3
125

+ addition
- subtraction
* multiplication
/ division (decimal)
// division (integer)
% modulo (remainder from division)
** to the power of

Variables

To store data in memory, create a name for the memory location, and assign it using the = sign. These memory locations are called variables because you can change the data in them. Type the variable name by itself to see the value stored in it. You can use variables in mathematical equations.

>>>speed=50
>>>duration=3
>>>speed
50
>>>speed * duration
150

Python program

When using the shell the commands are not saved. By putting the commands in a file they can be run over and over again without having to be retyped. In IDLE choose New File from the File menu and type the following

print(3)
print(2)
print(1)
print("blast off")

Save this file from the File menu. From the Run menu select Run module F5. Every time you run it you will see the same output in the shell window.

3
2
1
blast off

Using loops to save typing

Loops are sections of code which are run multiple times. while loops will keep looping until a specified condition is True. The indented lines indicate which part of the code is included in the loop.

x = 5
while x > 0:
    print(x)
    x -= 1
print("blast off")

The condition is x > 0 which is True for 10 loops and then becomes False when the value of x is zero.
The command x -= 1 is a shorthand for telling python to reduce the value of x by 1 every time it is run.
This program gives the following output every time it is run.

5
4
3
2
1
blast off

Only one line needs to be changed to start the countdown from a different number.

Categories: CoderDojo

Python in Minecraft 9 – railway track

1 Comment

To add railway track we need to have some normal rail and some powered rail. Next to every section of powered rail we need to provide a redstone torch.

First we define the items which are not defined in mcpi.block yet

    RAIL = block.Block(66)
    RAIL_POWERED = block.Block(27)
    TORCH_REDSTONE = block.Block(76)

We are already placing a plain torch every 4 blocks. Now we want to place a redstone torch every 4 blocks but offset by 2. Next to the redstone torch we want two pieces of powered rail, at positions 1 and 2. The rest of the rail is normal rail. This code goes in the second loop where the full tunnel profile is created.

    if x % 4 == 2:
        mc.setBlock(x,y+1,z+1,TORCH_REDSTONE.id,5)
    if x % 4 == 1 or x % 4 == 2:
        mc.setBlock(x,y+1,z,RAIL_POWERED)
    else:
        mc.setBlock(x,y+1,z,RAIL)

Here is a screenshot showing the railway track in the tunnel.

Here is the full code.

import mcpi.minecraft as minecraft
import mcpi.block as block
mc=minecraft.Minecraft.create()
XMIN = -400 # x value at one end of tunnel.
XMAX = -200 # x value at other end of tunnel. Must be greater than XMIN
GROUND = 72 # y position of tunnel at each end
FLOOR = 10  # minimum value of y at bottom of tunnel
ZPOS = -244 # z position for full length of tunnel
TAIL = 10   # length of horizontal at each end of tunnel

# Define blocks currently missing from mcpi.block
STAIRS_STONE = block.Block(109)
RAIL = block.Block(66)
RAIL_POWERED = block.Block(27)
TORCH_REDSTONE = block.Block(76)
 
# constants for tunnelvertical function
kx = (XMAX + XMIN) / 2
ky = GROUND + TAIL - (XMAX - XMIN) / 2
 
# tunnelvertical returns a vertical position of the tunnel
# for each value of the x position (eastposition)
def tunnelvertical(eastposition):
    y = abs(eastposition - kx) + ky
    if y < FLOOR:
        return FLOOR
    if y > GROUND: 
        return GROUND
    return y
 
# set the z coordinate for the tunnel which doesn't change for the full length    
z = ZPOS
 
# Initial loop to create a route made of solid glass with a stone base 
for x in range(XMIN,XMAX+1):
    y = tunnelvertical(x)
    mc.setBlocks(x,y,z-2,x,y+6,z+2,block.GLASS)
    mc.setBlocks(x,y,z-1,x,y,z+1,block.STONE)
 
# Initialise previous value of y so can determine if stairs going up or down
yprev = tunnelvertical(XMIN)
# Second loop to convert solid glass into a tunnel
for x in range(XMIN+1,XMAX):
    y = tunnelvertical(x)
    # replace centre glass with air to make it a tunnel
    mc.setBlocks(x,y+1,z-1,x,y+5,z+1,block.AIR)
    # place a torch every 4 positions to light the tunnel
    # variation 5 = torch facing up
    # every 4 blocks
    # pos 0: rail and normal torch
    # pos 1: powered rail
    # pos 2: powered rail and redstone torch
    # pos 3: rail 
    if x % 4 == 0:
        mc.setBlock(x,y+1,z+1,block.TORCH.id,5)
    if x % 4 == 2:
        mc.setBlock(x,y+1,z+1,TORCH_REDSTONE.id,5)
    if x % 4 == 1 or x % 4 == 2:
        mc.setBlock(x,y+1,z,RAIL_POWERED)
    else:
        mc.setBlock(x,y+1,z,RAIL)
    # check if stairs are going up or down 
    if y > yprev:
        # variation 0 = stairs ascending to the east
        mc.setBlock(x,y,z-1,STAIRS_STONE.id,0)
    if y < yprev:
        # variation 1 = stairs ascending to the west
        mc.setBlock(x-1,yprev,z-1,STAIRS_STONE.id,1)
    yprev=y
Categories: CoderDojo