Conditional Loops

Introduction

Conditional loops are way to repeat something until a certain condition is met. If the condition is never met, the loop can become infinite. In Python conditional loops are defined with the while statement:

counter = 0
while counter < 10:
    print("in the loop")
    counter = counter + 1

Turtle prison

Exercise

The turtle has been up to its usual tricks again, robbing liqour stores and building up huge gambling debts. It’s time for turtle to be put into a box that it can’t get out of.

Let’s make a new version of forward(). One that turns the turtle around if it tries to go further than 100 from the origin. We’ll need a while loop, and some new turtle functions:

• turtle.distance(0,0) - Distance of the turtle from the origin
• turtle.towards(0,0) - The angle to get back to origin
• turtle.setheading(angle) - Directly set the turtle direction

Now you will need to implement the prison logic using these turtle functions, a while loop and a bit of conditional logic. It’s a bit of a stretch but keep at it! Don’t be afraid to talk it out with a coach or another student.

Solution

def forward(distance):
    while distance > 0:
        if turtle.distance(0,0) > 100:
            angle = turtle.towards(0,0)
            turtle.setheading(angle)
        turtle.forward(1)
        distance = distance - 1

Draw a spiral

Loops can be interrupted with the break statement. This is especially useful if you write an infinite loop, which is a loop where the conditional is always True.

Exercise

Write a while loop with a condition that is always True to draw a spiral. Interrupt the loop when the turtle reaches a certain distance from the center. Use the function turtle.distance(x, y) to get the turtle’s distance to the point defined by the coordinates x and y.

To do this you will need the turtle.xcor() and turtle.ycor() functions, which return the position of the turtle in X and Y axes respectively.

Note

To draw a spiral, the turtle has to rotate by a constant value and move forward by an increasing value.

Solution

def draw_spiral(radius):
    original_xcor = turtle.xcor()
    original_ycor = turtle.ycor()
    speed = 1
    while True:
        turtle.forward(speed)
        turtle.left(10)
        speed += 0.1
        if turtle.distance(original_xcor, original_ycor) > radius:
            break

Bonus

Can you make a conditional for this loop, so you don’t need the infinite loop while True or the break? Which version do you find easier to understand?