Conditionals (If Statements)

Conditionals (If Statements)#

You need conditionals whenever your program should behave differently based on a certain condition.

First Example#

For example, if you want to check whether a player has reached a certain score in your game, you can write:

if points > 100:
    print("You have won!")

General Syntax#

if <condition>:
    <code block>

Boolean Expressions#

A condition is an expression that evaluates to True or False. Such expressions are called boolean expressions.

The simplest boolean expressions are just True and False. But you usually create them using comparisons, for example:

10 < 100     # True
110 < 100    # False
x < 10       # True, if x is less than 10
"a" == "b"   # False
3 == 4       # False
"ab" == "ab" # True

These expressions can include variables and be more complex.

⚠️ Note: Use == for comparisons. A single = is used for assignments!

Comparison Operators#

Here are the most common comparison operators:

  • < : less than

  • <=: less than or equal

  • ==: equal

  • >=: greater than or equal

  • > : greater than

Code Blocks#

If you want to run multiple statements when a condition is true, group them in a code block using indentation:

if points > 100:
    print("You have won!")
    print("Congratulations")
print("The game is over")

The first two lines will only run if points > 100. The last line runs regardless.

Elif and Else#

Use elif (else-if) and else for alternative branches:

if points > 100:
    print("You have won!")
    print("Congratulations")
elif points > 50:
    print("You lost by a narrow margin")
else: 
    print("You have clearly lost")

General syntax:

if <condition>:
    <code>
elif <condition>:
    <code>
else:
    <code>

You can use as many elif branches as needed. else and elif are optional.

Complete Example#

A rectangle moves from right to left. When it reaches the left edge, it should reappear on the right.

Initial version:

from miniworlds import *

world = World(300, 200)
rect = Rectangle((280, 120), 20, 80)

@rect.register
def act(self):
    rect.x -= 1

world.run()

Now add the logic to reset its position:

from miniworlds import *

world = World(300, 200)
rect = Rectangle((280, 120), 20, 80)

@rect.register
def act(self):
    rect.x -= 1
    if rect.x == 0:
        rect.x = 280

world.run()

Another Example – Simple Flappy Bird#

Let’s program a basic version of Flappy Bird.

We’ll use a ball that falls down and jumps up on key press:

from miniworlds import *

world = World(300, 200)

rect = Rectangle((280, 120), 20, 80)
ball = Circle((20, 50), 20)
velocity = 1

@rect.register
def act(self):
    rect.x -= 1
    if rect.x == 0:
        rect.x = 280

@ball.register
def act(self):
    global velocity
    self.y += velocity
    if world.frame % 10 == 0:
        velocity += 1

world.run()

Add a key press to make the ball jump:

@ball.register
def on_key_down(self, key):
    global velocity
    velocity = -2

Collision Detection#

Now let’s detect if the ball hits the rectangle using a sensor:

@ball.register
def act(self):
    global velocity
    self.y += velocity
    if world.frame % 10 == 0:
        velocity += 1
    actor = self.detect_actor()
    if actor == rect:
        self.world.stop()

These lines detect a collision and stop the world:

actor = self.detect_actor()
if actor == rect:
    self.world.stop()

The final result is a simple Flappy Bird-like game.

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