Logical operators

Introduction

Conditionals are a nice way to make decisions by asking if something equals True or not. But often one condition is not enough. We may want to take the opposite of our result. Or for instance if we want to make a decision upon turtle.xcor() and turtle.ycor() we have to combine them. This can be done with logical operators.

Negation of a statement

We can take the opposite value of a statement to use not its value. It is a logical operator:

x = False
if not x :
    print("conditon met")
else:
    print("condition not met")

Exercise

The turtle gives us a useful function to know if it is drawing or not: turtle.isdown(). This function returns True if the turtle is drawing. As we have seen earlier, the function turtle.penup() and turtle.pendown() toggle between writing while moving, or just moving without a trace.

Can we write a function that only goes forward if the pen is up?

Solution

def stealthed_forward(distance):
    if not turtle.isdown():
        turtle.forward(distance)

This and that or something else

Two easy to understand operators are and and or. They do exactly what they sound like: combine two statements in a way both have be true (and) or at least one of them has to be true (or):

if 1 < 2 and 4 > 2:
    print("condition met")

if 1 > 2 and 4 < 10:
    print("condition not met")

if 4 < 10 or 1 < 2:
    print("condition met")

You are not restricted to one logical operator. You can combine as may as you want.

Exercise

Create a function that accepts the argument angle and moves the turtle forward into that direction until either the vertical or horizontal distance to the center exceeds 100.

Solution

def move_to_square_border(angle):
    turtle.left(angle)
    x = turtle.xcor()
    y = turtle.ycor()
    while x > -100 and x < 100 and y > -100 and y < 100:
        turtle.forward(10)
        x = turtle.xcor()
        y = turtle.ycor()

Bonus

Can you use this to draw a star?