# 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 returns True or not determines whether the code inside the `while` loop runs. Of course, it repeats the check every time the loop is run again.

Read the code above, and see if you can summarise in your head what it should 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.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?