Concept: Branching

Concept: Branching#

You need branches whenever the flow of your program should depend on certain conditions.

First Example#

For example, in a game you might want to check if a certain number of points has been reached:

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

General Syntax#

This is the general syntax of a conditional statement:

if <condition>:
    <code block>

Boolean Expressions#

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

The simplest ones are True and False. More useful expressions are based on comparisons:

10 < 100        # True
110 < 100       # False
x < 10          # Depends on x
"a" == "b"      # False
3 == 4          # False
"ab" == "ab"    # True

You can build arbitrarily complex expressions involving variables.

Warning

⚠️ Attention: For comparisons use double equals (==) – not a single equals (=), which is used for assignment!

Comparison Operators#

You can use the following comparison operators:

  • < : less than

  • <= : less than or equal

  • ==: equal

  • >=: greater than or equal

  • > : greater than

Code Blocks#

If you want to execute multiple lines of code when the condition is met, you use code blocks, i.e., indented lines below the if statement.

Example:

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

The last line is always executed. The indented lines only run if points > 100.

elif and else#

With elif and else, you can build alternatives:

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")

Syntax:#

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

You can skip elif or else, and use multiple elifs if needed.

Detailed Example – Moving a Rectangle#

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

Version 1:

import miniworlds

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

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

world.run()

Now let’s reset the position:

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

Another Example – A Simple Flappy Bird#

We want a ball to move up when a key is pressed, and fall down otherwise (gravity).

import miniworlds

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

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

world.run()

The ball falls, and gets faster due to gravity. When a key is pressed, it jumps up.

Collisions#

You can also check whether two objects touch each other using sensor methods like detect_actor():

@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()

This checks if the ball collides with the rectangle. If so, the game ends.

Final Result – Flappy Bird Prototype#

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