Arithmetic

Commands

These are the common basic commands in Logo.

• CS ClearScreen, clears the screen and resets the turtle back to its default position
• CT clear the Text window
• REPEAT repeats a set of instructions a number of times. The instructios are enclosed in square brackets. The common syntax for this is REPEAT 4 [FD 50 RT 90]
• TO to begin writing a procedure
• END denotes the end of a procedure
• IF allows you to include conditional instructions. For example: IF RANDOM 2=1 [FD 30]. The if command must include a statement that is either true or false, and then a list when will be carried out if the statement is true. Two lists can be given, the first being carried out if the statement is true and the second if it isn't, for example: IF RANDOM 2=1 [LT 45] [RT 45]

Common errors

Editing

Whenever you need to alter something you've typed in, this is called editing. Once words, numbers or any other instructions have been entered in the Work window, you can move back to them using the mouse to click where you need to be. You can use the Delete key or the backspace key (marked ) to remove numbers or letters. You can also scroll up and down the Work window using the scroll bar to its right to get to things you wrote a while ago. Remember that everything you type in is kept in the Work window until you delete it (by using clw "Work for example).

If you need to alter a procedure, then, after making your changes, you should move to the line that says simply End, click on this word and press Enter. This will register your edits with Win-Logo so that you can go on to use the edited procedure.

Errors

Functions

Geometry

TO triangle
REPEAT 3 [FD 30 RT 120]
END

The angle involved is not the 60 degrees given by each internal angle of the triangle, but instead the 120 degrees through which the turtle has to turn in drawing the triangle.

Heading

Information

You can get information about some aspects of your current Win-Logo work using the following primitives:

• BACKGROUNDBG tells you the background colour
• PENCOLOUR tells you the turtle's colour
• HEADING tells you the direction the turtle is pointing
• WHO tells you which turtles are active
• POS tells you the turtle's position
• XCOR tells you the turtle's x coordinate
• YCOR tells you the turtle's y coordinate

Interpret

1. Individual procedures. Move the cursor through the text in the Work window and press Enter on the word END at the end of each procedure required.
2. All procedures. Select all the text in the Work window, by Edit... Select All Then interpret them all by selecting Area... Interpret. (In Dos version, click at beginning then select Edit... Block Start, click at end and select Edit... Interpret or press F10.)

Lists

Loading

Loading files previously saved to disk can be achieved in two different ways. Suppose the file is called fred.log:

1. In the Work window. Type load "fred. This will load the contents of the file into memory and "interpret" them so that any procedures defined will be available for use. Use this method if you want to load files and run them immediately.

2. Using the Menus. Click on File... Load (Windows... Load in Dos version) and select fred.log from the list shown. This will load the file into the Work window. They have not been loaded into Win-Logo memory however, and so are not available at this moment to run. To load them into memory, your procedures need to be "interpreted". (See Interpret above for how to do this) Use this method if you want to load files and continue editing them.

Loops

Names

Variable names

TO SQUARE :side
REAPEAT 4 [FD :side RT 90]
END

The colon is used when introducing the name of a variable and when using the value of the variable.

Constant names

FD :size

Operations

Output

TO ADDUP :x :y
OP (:x + :y)
END

The result of applying ADDUP to two numbers is to add them up. However, if, having defined ADDUP, you enter ADDUP 5 6, you will not get 11 as logo's answer. Instead logo will say something like "I do not know what to do with 11". You have to tell logo what to do with that output, so something like this will work better:

TO ADDUP :x :y
PRINT OP (:x + :y)
END

Primitives

Printing

To print the graphics, click in the Graphics window and then select File... Print. To print the procedures, click in the Work window and then select File... Print.

Procedures

A procedure is a set of instructions beginning with TO and ending with END which logo can interpret and then carry out. Here is a simple procedure:

TO SHIFT
FD 10
END

Whenever SHIFT is entered, the turtle moves forward 10 steps.

Recursion

TO SQUARELOOP
RT 90
REPEAT 4 [FD 30 RT 90]
squareloop
END

This procedure will make the turtle turn right 90 degrees then draw a square, then turn 90 degrees and draw a square, and so on, and on, until the ESC key is pressed. The example shown is sometimes called "tail recursion", due to the loop being placed at the end of the procedure.

Saving

Saving files to disk can be achieved in two different ways. It is essential that you understand these differences. Suppose the file is to be saved as fred.log:

1. In the Work window. Type save "fred procedures. This will save all procedures (and only procedures) currently in memory into the file fred.log. Use this method if all you want to save are the procedures you've written and nothing else.

2. Using the Menus. Click on File... Save (Windows... Save in Dos version) and enter fred in the space next to Filename, overwriting *.log. This will save everything currently in the Work window - and nothing else! - in the file fred.log. Win-Logo will save an exact copy of the Work window contents.

Before you save your work, make sure you clearly understand what each method will achieve, and then choose the appropriate one.

Three dimensions

• CLEAN3Drestores the axis system and sets the viewpoint at (0,0,600)
• FORWARD3D move the turtle forward in three dimensions
• POS3D tells you the coordinates of the current turtle
• ROTATEX rotates the turtle about the x axis, similarly ROTATEY, ROTATEZ
• ROTATEXAXIS rotates the axis system about the x axis, similarly ROTATEYAXIS, ROTATEZAXIS
• SETPOS3D [ ] positions the turtle at the coordinates given
• SETVIEWPOINT [ ] resets the viewpoint at the given x and y coordinates and the given distance in the z direction.
• SETX positions the turtle at the x-coordinate given, similarly for SETY, SETZ
• XCOR tells you the x-coordinate of the current turtle, similarly YCOR, ZCOR

Turtle attributes

The turtle, the object that your logo instructions command, has two important attributes, its heading (ie., the direction it is pointing) and its position (as a pair of coordinates). Both of these are controllable, by using LT, RT, FD, BK or SETPOS.

The turtle may have other attributes too, like its colour, whether the pen is up or down (which determines whether or not the turtle will draw a line when it moves), its shape or its name.

Turtle commands

• FD forward
• BK back
• LT left
• RT right
• HT hide the turtle
• ST show the turtle
• PU pen up. When the turtle moves it will not draw.
• PD pen down. When the turtle moves it will draw.
• SETPOS setpos [] positions the turtle at the co-ordinates given. A line is drawn.

• SETX setx 50
• SETY sety 60
• SETHEADING sets the heading or bearing of the turtle, i.e., the direction the fd command will send it in.

• PENERASE After entering penerase, send the turtle back over an unwanted line, which will then be rubbed out. To switch off the eraser, type PD.
• SETSHAPE setshape 6 will change the turtle's shape to that defined by the number given.

Multiple turtles

• ACTIVATE selects which turtle you are currently commanding.
• ACTIVATEALL selects all 12 turtles to carry out subsequent commands.
• ASK temporarily selects a turtle or turtles to carry out a command, for example ASK 3 or
• ASK [2 3 4]
• EACH gives commands to each active turtle
• WHO returns as a list the numbers of the active turtles

Variables