Lesson 4

Agenda:

• Logical operators
• • Introduction
  • Negation of a statement
  • This and that or something else
• Where to go from here
• • Learning Python
  • What to do with Python

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 uponturtle.xcor()andturtle.ycor()we have to combine them. This can be done with logical operators.

Negation of a statement

If we want something to be_False_we can usenot. It is a logical operator:

x
=
False
if
not
x
:
print
(
"condition 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 functionturtle.penup()andturtle.pendown()toggle between drawing 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
)

Hide

This and that or something else

Two easy to understand operators areandandor. They do exactly what they sound like::

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

Earlier we put the turtle in a circular prison. This time let’s make it a box. If the turtle goes more than 100 in the X_or_Y axis then we turn the turtle back around to the center.

Solution

def
forward
(
distance
):
while
distance
>
0
:
if
(
turtle
.
xcor
()
>
100
or
turtle
.
xcor
()
<
-
100
or
turtle
.
ycor
()
>
100
or
turtle
.
ycor
()
<
-
100
):
turtle
.
setheading
(
turtle
.
towards
(
0
,
0
))
turtle
.
forward
(
1
)
distance
=
distance
-
1