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Python for Beginners 2015.11.01

Conditional Loops¶

Introduction¶

Conditional loops are way to repeat something while a certain condition is satisfied, or True. If the condition is always satisfied (never becomes False), the loop can become infinite. If the condition starts off false, the code in the loop will never run! In Python conditional loops are defined with the while statement:

word = ''
sentence = ''
print('Please enter some words.')
print('Include a period (.) when you are finished.')
while '.' not in word:
    word = input('next word: ')
    sentence = word + ' ' + sentence
print()
print('Aha! You said:')
print(sentence)

We call this part of the code the ‘conditional’: '.' not in word

Whether the conditional return True or not determines whether the code inside the while loop runs.

Read the code above, and see if you can summarise in your head what it what it do (what its final output will be).

Then copy it into a file, say sentence.py and run it - see exactly what it does. Does that match up with what you thought?

Note

If you are using Python 2, you will need to replace input with raw_input to run the program correctly.

Turtle prison¶

Exercise¶

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

Let’s make a new version of forward(). One that will turn 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) - Returns the distance of the turtle from the origin (0, 0)
turtle.towards(0, 0) - Returns the angle to get back to origin (0, 0)
turtle.setheading(angle) - Directly sets the turtle’s direction

You could try playing with a turtle in the interpreter and using these functions to check exactly what they do, if you like.

Now you will need to implement the prison logic using these turtle functions, perhaps 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?

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