lesson5.md

HSSP Introductory Python Programming through Games

This is a repository that holds the code for the Summer 2018 intro to python programming for MIT's HSSP program.

This code was developed for Python (version 2.7).

Running the game templates and game example relies on the Pygame library (version 1.9.3) which can be downloaded here.

If you are looking to program without having to install Python on your computer, we recommend using the www.learnpython.org which is an interactive tutorial that covers many of the same topics as we will in the course.

Instructions for August 4th class (for loops and lists)

Today we will talk about lists and for loops in python.

###Lists First we'll talk about lists. This is a new type of variable that can hold multiple things (called elements). We can make an empty list using brackets:

mylist = []

We can make a list that has elements, separated by commas:

mylist = ["item1","item2"] #this list has two strings inside it

We can add things to our list:

mylist = []
mylist.append("item")
print(mylist) #list has one object, "item"

We can check how many things are inside our list:

mylist = []
mylist.append("item1")
print(len(mylist)) #len(mylist) is 1
mylist.append("item2")
print(mylist) #list has two object, an item1 and item2
print(len(mylist)) #len(mylist) is 2

We can check if something is in our list:

mylist = []
mylist.append("item1")
if "item1" in mylist:
   print("List contains item1")
if "item2" in mylist:
   print("List contains item2")

Only "List contains item1" will be printed because we didn't add item2 to our lists.

Lists can have items from any variable type: ints, floats, bools, strings, or lists!

myintlist = [1,2,3]
myfloatlist = [1.0,1.1,1.2]
myboollist = [True,True,False]
mystringlist = ["item1","item2"]
mylistlist = [[1,2,3],[4,5,6]]

We can get specific elements from a list, by indexing. In python the first element in the list starts at zero:

mylist = ["milk","juice","eggs"]
print(mylist[0]) #will print the first element, milk
print(mylist[1]) #will print the second element, juice
print(mylist[2]) #will print the third element, eggs

One very useful feature is to create a set of numbers (maybe you want a list of the numbers 1 through 10, we can get this using the range command:

numbers1to10 = range(1,11)
print(numbers1to10) # this is a list of the numbers 1 through 10

Notice to get numbers 1 thorough 10, we actually have to tell the range we want the numbers starting at 1 and stopping before 11.

###For loops We are adding for loops to our toolbox, in addition to while loops and if statements.

for loops iterate over a list, with a specific synatax:

mylist = ["milk","juice","eggs"]
for item in mylist:
    print(item) 

We write the keyword for, then a variable where we will store the elements the the keyword in, then our list that we want to iterate over.

Coding instructions

Drag your lesson4 folder from your flashdrive to the desktop.

In the terminal type:

cd ~/Desktop/lesson4

python game.py

You should see the code to control a player moving. This is a basic game loop, with a player that you can control with the left and right arrow keys. If this doesn't work, it may be that pygame isn't installed:

Type into the terminal:

sudo apt-get install python-pygame

and respond yes to any prompts.

But now try moving to the left. Eventually our character moves off the screen. How can we fix this? We attempted this last week.

Before moving on to this week's task, make sure your player can't move to the left or the right all the way off the screen. Open the code with:

gvim game.py

Remember our player's position can be checked with:

player.left
player.right
player.top
player.botton

Ok now we will add falling objects to create an avoider-style game. Open your game.py code:

gvim game.py

After the line:

# Define the player x,y,width,height
player = pygame.Rect(300,400,60,60)

Add a new line:

all_objects = [] #empty objects list

Now directly below that line will add an object to the list:

pos_x = 100 #x screen position
pos_y = -10 #y screen position make the circle off screen
obj = pygame.draw.circle((screen),(0,0,0),(pos_x,pos_y),20,0)
all_objects.append(obj) 

Now we actually have to draw these objects in our game loop, after the line:

    pygame.draw.rect(screen, (0,0,0), player) #draw player

add the new lines:

    for obj in all_objects:
    	obj.y += 3 # object is falling
	if obj.y >= height: #check if object off the screen
            obj.y = -10
        obj = pygame.draw.circle((screen), (0, 0, 0),
				 (obj.x, obj.y),
				 20,1)

If you run the code, you should see a circle fall from the top of the screen. On your own, try adding a few more circles by appending them to the all_objects. Try varying the x and y positions.

Now nothing happens currently if your player touches the circle. To make the player die if he hits a circle, we can add to our object for loop, under the line:

     for obj in all_objects:

add:

    #Test collisions with the bubbles
    if obj.colliderect(player):
       all_objects.remove(obj)
       pygame.quit()
       sys.exit()

You've made your first game!