Concept: Turtle Graphics Language & Interface Features

Starlogo allows you to create and manipulate large numbers of turtles, that move on a two-dimensional surface. The underlying language and environment features of the Turtle Graphics project are described below.

Language Features

• Turtle Creation: The operation crt number creates the number of turtles defined by the green number slider. Questions: How is it invoked? What if it is invoked twice? Three times? Where are the turtles? What are their colors?
• Turtle Movement: The operation fd step moves each turtle forward a number of steps. What happens when you chage the value of step (by moving the slider)? What happens when a turtle reaches the end of the screen?

• Turtle Properties:

• Direction: Every turtle has a heading in degrees (0 - 359). The operation rt angle turns the turtle right by a certain number of degrees. What happens to the pattern a turtle makes when you change the angle? How might you make a turtle turn left?
• Color: Turtle color can be changed with the setc command. (Note: the setc button in this program is a "forever" button, meaning it continues to change the turtle color; What would happen if it was not a "forever" button?)
  How could you set all of the turtles to the same color? What if you only wanted to use blue, green, and red?

• Turtle Trails: Each turtle has a pen that the turtle can put in the up or down position. You can control this using pu and pd. If a turtle's pen is down, it leaves a trail wherever it goes. What is the trail color? Does chaning the turtles' pen position change the pattern?
• Slider Variables: number, step, and angle are all slider variables. You can change these variables by moving the sliders. The turtles each "check" these variables before carrying out an instruction.
• Random Numbers: can be generated with the random function. For example, random 10 returns a random number between 0 and 9.

Buttons

• To add a button: Click the button icon on the toolbar, and then click on the location of the interface window where you want the button initially placed. A dialog box will come up. Type in a button name, and the instructions you want the turtle to follow when you press the button (for example, you might want your turtle to move fd 10 rt 45 fd 10 rt 45). You can either show the name of your button or the instructions.
  If you want the turtle to repeat your instructions forever, click on forever.
• To modify the contents of a button: Click the button icon on the toolbar; then click on the button you want to change. You can change the button name, instructions, show name/instructions, and forever.
• To delete a button: Click the button icon on the toolbar; then click on the button you want to delete. Press the delete key.
• To resize a button: Click and drag the mouse over the button so that the little square handles appear. To resize the button, click and drag on one of the handles until the button is the desired size.
• To Move a button: Click and drag the button to the desired location.