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Section 16.1 Turtles as Objects

Strings are just one kind of object. Objects in programming are constructs that combine state - a collection of data (attributes) - with behaviors that can be done with/to that data.
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A string’s data is a list of characters like h e l l o t h e r e its data are the functions like length() or lower() or even the slice behavior [3:6] that allow us to work with that list of characters.
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While strings are immutable (they never change once created), most objects are mutable - they can mutate or change. In mutable objects, when we use the behavior of the object, it will make some kind of permanent changes.
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The turtles we learned about in Chapter 1 are an example of a mutable object. Each turtle object keeps track of the state of a turtle - its location and the direction it is facing. They have behaviors to do things like move forward() or turn left(). Asking a turtle to execute one of these behaviors, will change the location or direction of that turtle.
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We have seen the example below once before. It allows us to use the turtle library, creates a space for a turtle object to draw on, creates a turtle object and names it alex, asks alex to go forward by 150 pixels, asks alex to turn left 90 degrees, and asks alex to go forward 75 pixels.
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Checkpoint 16.1.1.

Which way does a turtle face when it is first created?
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  • North
  • The turtles in some of the examples faced north because of the setheading(90) instruction. Which way does alex move first?
  • South
  • Which way does alex first move in the example above? North is at the top of the page.
  • East
  • Turtles start off facing east which is toward the right side of the page.
  • West
  • Which way does the turtle first move in the example above? North is at the top of the page.
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Subsection 16.1.1 What does a left turn of 90 mean?

Turn instructions are always relative to the turtle’s current direction. The instruction alex.left(90) tells the turtle named alex to make a quarter circle turn to its left - or counter-clockwise. If alex is currently facing East, it would end up facing North. If it is currently facing North, it would end up facing West.
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Figure 16.1.2. The amount of turn for specified degrees for left and right turns. These diagrams assume the turtle is currently pointing East (to the right of your screen).
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Mixed up programs
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Checkpoint 16.1.3.

The following program uses a turtle to draw a capital L as shown below, but the lines are mixed up. The program should do all the necessary set-up: import the turtle module, get the space to draw on, and create the turtle. The turtle should turn to face south, draw a line that is 150 pixels long, then turn to face east, and draw a line that is 75 pixels long.
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Drag the needed blocks of statements from the left column to the right column and put them in the right order. The numbers listed are NOT in the correct order - do not assume block 1 comes first. In this problem there are extra blocks you will not use. You will only use 1a or 1b, not both. Same with 2a/2b, 4a/4b, etc…
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When you think you are done, click on Check to see if you are right.
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Checkpoint 16.1.4.

The following program uses a turtle to draw a checkmark as shown below but the lines are mixed up. The program should do all the necessary set-up: import the turtle module, get the space to draw on, and create the turtle. The turtle should turn to face southeast, draw a line that is 75 pixels long, then turn to face northeast, and draw a line that is 150 pixels long. We have added a compass to the picture to indicate the directions north, south, west, and east. Northeast is between North and East. Southeast is between South and East.
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Drag the needed blocks of statements from the left column to the right column and put them in the right order. The numbers listed are NOT in the correct order - do not assume block 1 comes first. Like last time, you will not use all the blocks. You will only use 1a or 1b, not both. Same with 2a/2b, 4a/4b, etc…
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When you think you are done, click on Check to see if you are right.
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