Concept: Branches
Contents
Concept: Branches¶
You need branches whenever you want to check conditions and the program flow should depend on them.
First example¶
For example, if you want to check whether a certain score has been reached in your game, this can be done with the following statement
if points > 100:
print("You have won!")
Boolean expressions¶
A condition is an expression that can have the value True or False.
Such expressions are called boolean expressions.
The simplest boolean expressions are True and False. You can usually get more expressions with Compare, e.g.:
10 < 100 # True
110 < 100 # False
x < 10 # True, if x < 10
"a" == "b" # False
3 == 4 # False
"ab" == "ab" # True
The expressions can be arbitrarily complicated and contain variables.
Warning
Warning: In comparisons, always use a double equal sign instead of a single equal sign
Comparisons¶
You can use the following comparisons:
<: Less than<=: Less than or equal to==: Equal>=: Greater than or equal to>Greater than
Code blocks¶
If you want to execute several statements depending on the condition, this can be done with the help of code blocks. Code blocks are always indented the same distance and all statements that are indented accordingly will be
Example:
if points > 100:
print("You have won!")
print("Congratulations")
print("The game is over")
Regardless of the score, the last line of code is executed in any case. However, the two indented lines are only executed if the score is greater than 100.
Elif and Else¶
With elif and else you can insert alternatives. This goes for example like this:
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)
The general syntax is:
if <condition>:
<code block>
elif <condition>:
<code block>
else <Condition>:
<code block>
Both elif and else can be omitted. Multiple elif blocks are also possible.
Detailed example¶
A rectangle is to move from right to left. When it reaches the left side, it should reappear on the far right.
The first variant looks like this:
from miniworldmaker import *
board = Board(300, 200)
rect = Rectangle((280,120), 20, 80)
@rect.register
def act(self):
rect.x -= 1
board.run()
The crucial part is still missing.
This can be formulated like this:
If the x-coordinate reaches the value 0, set the rectangle to the right again.
This can be translated directly into Python:
from miniworldmaker import *
board = Board(300, 200)
rect = Rectangle((280,120), 20, 80)
@rect.register
def act(self):
rect.x -= 1
if rect.x == 0:
rect.x = 280
board.run()
Another example - A simple Flappy Bird¶
We want to program a kind of (simple) Flappy-Bird.
Our main character should be a ball, which moves upwards when the key is pressed. We can realize this as follows:
from miniworldmaker import *
board = Board(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 board.frame % 10 == 0:
velocity += 1
board.run()
The ball falls and falls faster and faster.
In the line:
if board.frame % 10 == 0:
velocity += 1
will increase the speed at which the ball falls. In the first step the ball should be able to move upwards when a key is pressed.
from miniworldmaker import *
board = Board(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 board.frame % 10 == 0:
velocity += 1
@ball.register
def on_key_down(self, key):
global velocity
velocity = -2
board.run()
Colissions¶
Now we want to compare not only simple positions, but also the position of objects in relation to each other.
For this we can use various sensor methods.
This goes like this, for example:
from miniworldmaker import *
board = Board(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 board.frame % 10 == 0:
velocity += 1
token = self.sensing_token()
if token == rect:
self.board.stop()
@ball.register
def on_key_down(self, key):
global velocity
velocity = -2
board.run()
The logic is in the following lines:
token = self.sensing_token()
if token == rect:
self.board.stop()
The first line checks with a sensor, which token was found at the own position (and returns the first found token). Then the so found token is compared with the rect. If these are the same objects, then the game is stopped.