Lesson 3

Agenda:

• Functions with parameters
• • Introduction
  • A parameterized function for a variable size hexagon
  • A function of several parameters
• Conditional statements
• • Introduction
  • Examples
  • Giving Directions
  • “data munging”
• Conditional Loops
• • Introduction
  • Turtle prison
  • Draw a spiral

Introduction

So far we have accomplished predefined tasks, but in all honesty we were accomplishing no better than old-time music boxes following one set of instructions to the end. What makes programming so much more powerful are conditional statements. This is the ability totest_a variable against a value and act in one way if the condition is met by the variable or another way if not. They are also commonly called by programmers_if statements.

To know if a condition isTrue_of_False, we need a new type of data: the booleans. They allow logical operations. A logic statement or operation can be evaluated to beTrue_or_False. Our conditional statement can then be understood like this:

if (a condition evaluates to True) : then do these things only for ‘True’ else : otherwise do these things only for ‘False’ .

The condition can be anything that evaluates asTrue_or_False. Comparisons always returnTrue_or_False, for example==(equal to),>(greater than),<(less than.)

Theelsepart is optional. If you leave it off, nothing will happen if the conditional evaluates to ‘False’.

Examples

Here are some examples. You may want to read them over line-by-line and see what you think they do, or run them to be certain:

condition
=
True
if
condition
:
print
(
"condition met"
)
if
not
condition
:
print
(
"condition not met"
)
direction
=
-
30
if
direction
>
0
:
turtle
.
forward
(
direction
)
else
:
turtle
.
left
(
180
)
turtle
.
forward
(
-
direction
)

Giving Directions

Python turtles can be very good at following instructions. Let’s use theinput()function to ask the user for a direction to move the turtle. To keep things easy we will only accept two instructions: “left” and “right”.

Note

Using Python 2? Theinput()function is calledraw_input().

It’s much easier to define this as a function, like so:

def
move
():
direction
=
input
(
"Go left or right? "
)
if
direction
==
"left"
:
turtle
.
left
(
60
)
turtle
.
forward
(
50
)
if
direction
==
"right"
:
turtle
.
right
(
60
)
turtle
.
forward
(
50
)

Now whenever you usemove()you are prompted to chooseleftorright.

“data munging”

In this program, the turtle will only respond to exactlyleftorrightwith no variation. ThoughLeftorLEFTmight seem the same asleftto a human, that isn’t the case when programming. Python has a few utility methods to help with that. A string has the methods.strip(), which removes whitespace and.lower()which makes everything lower-case.

Here are some examples to print out the effects of.strip()and.lower():

my_variable
=
"       I Am Capitalised"
print
(
my_variable
)
my_stripped
=
my_variable
.
strip
()
print
(
my_stripped
)
my_lower
=
my_variable
.
lower
()
print
(
my_lower
)

Try addingdirection=direction.strip().lower()to themove()function, to see the effect. We often call this kind of code “data munging”, and it is very common.

Can you add some extra input choices to make the turtle draw other things? How abouthexagon?