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Chapter 6: Property Lists

This chapter covers properties of Logo objects and property lists in general. You’ve been modifying properties in some way ever since you first typed FORWARD 100 back in Chapter 1 - Beginning Turtle Graphics. Properties are important for a full understanding of Logo objects and how to manipulate them. You’ll learn about property lists and create some of your own, understand about the built-in properties and how you manipulate them, and finally, you’ll add your own property to dress up your turtles. Before you begin, restart Logo so you have a clean workspace.

Property Lists in General

A property list is a collection of information attached to a Logo object. Each piece of information is a separate property that has its own property name as well as a property value. Together, the property name and the property value are called a property pair - you can’t have one without the other. A property list is like a huge multi-part variable - but it’s not a variable - it’s a special type of list. To see what property lists you have in your workspace, use the PRINTOUT PLISTS command (POPLS for short). Type:

There are no property lists in your workspace when you first start Logo. The built-in properties are not shown because they are not part of your workspace.

You create a property list with the PPROP command (it’s not short for anything, but it means ‘put a property’). PPROP takes three inputs: the name of the property list you want to create, the name of a property you want put into the property list, and the value to assign to the property name. That sounds like a lot but it’s really just the name of the property list and a property pair. Type:

This creates the property list with the name AL The property pair is the property name FULLNAME along with the value ALIBABA. Type POPLS to see your new property list:

If you want to see a particular property list instead of all of them, you can use the PLIST command. It needs the name of the property list as input and outputs the property list as one long list. Type:

The PPROP command is also used to assign a new value to a property. In other words, you use PPROP to modify an existing property value as well as create a new property list. Type:

And, the PPROP command is also used to add additional property pairs to a property list. Type:

Now, you have one property list called AL that has two values associated with it. One value is named FULLNAME and the other value is named CAPITAL. You can add as many property pairs as you want to the same property list. You’ll be adding more soon. If you had to type a PPROP command for every property pair, it would get tiring after a while. The PPROPS command is a short cut for entering more than one PPROP command at a time. It works in the same way as PPROP to create a new property list, modify existing property values, or add additional property pairs. However, it takes just two inputs: the name of the property list and a list of property pairs. Type:

Because the property pairs are enclosed in a list, you do not put the quotation marks in front of the property name or the property value

  • otherwise the quotation mark becomes part of the name or value and that would get very confusing.

Now, you have the beginning of a “database” of information about the states. Type:

Add the other states whose names begin with the letter ‘A.’ The capital of Arkansas has two words so you have to put Little Rock in as a list so it’s treated as one element of two words rather than two separate words. Type:

Even PPROPS would get tiring by the time you entered all fifty states. So, why not create a procedure to add states and capitals to the database? Enter this procedure:

Although the formal inputs FULLNAME and CAPITAL are the same words as the property names FULLNAME and CAPITAL, they are not the same thing. Logo understands the difference in the way they are being used.

Your NEW.STATE procedure is an abstraction for creating a new state “record” in the database. Use it to add the states whose names begin with the letter ‘C.’ Type:

(Note: After creating the record for Connecticut, the shortcut for CLEARTEXT will no longer work in this workspace.) Now, there are seven state records in your database. Of course, a collection of information is not much good if you can’t get the data you need when you want it. The GPROP command outputs a property value. It needs two inputs: the name of the property list and the name of the property whose value you want. Type:

Getting data would be much easier with a procedure for each of the properties. Define the procedures FULLNAME? and CAPITAL?:

Each procedure takes a state abbreviation as input and outputs the appropriate property value. The question mark at the end of the procedure names is a way to remind yourself that the procedure is asking a question and you expect it to return an answer. After your procedures are defined, type:

Of course, you’ll want to be able to modify properties if you make a typing mistake. Why not use the same procedure names but with a period at the end of the name? That way, one form of your command makes a statement while the other form asks a question. Define the procedures FULLNAME. and CAPITAL.:

Since these new commands use the PPROP command, they could be used to create a new property list. Type:

Although this works, it’s best to use commands for their intended purpose. Your NEW.STATE procedure makes sure that each new record has two properties. Of course, you can easily fix it now with the following:

Now is a good time to save your workspace so you don’t lose all the work you’ve done. Type:

You’re going to reload the workspace in a moment, so make sure you saved it before continuing.

Property values can be removed with the REMPROP command. It needs two inputs: the name of the property list and the name of the property to be removed. It removes the property pair identified by the property name. Type:

If you remove all of the property pairs in a property list, the property list itself is removed from the workspace. Type:

It is not an error to access a property list that does not exist! The PLIST command quietly outputs the empty list instead of an error message. The same is true for accessing a property name that does not exist. Type:

You can also get rid of a property list with the ERASE command. Type:

Now, reload your saved workspace to restore the property lists for Alabama and Alaska. Type:

There are many properties you could add to each state’s property list - state bird, state motto, state quarter description, and so on. It’s not necessary, however, for each “record” in your database to have the same properties. For example, not every state has a Major League Baseball team but you can add a BASEBALL property to the ones that do. Define the procedures BASEBALL? and BASEBALL.:

Now, you can add baseball teams to records and ask if a state has any baseball teams. Type:

You can find out which property lists have a particular property name with the GLIST command. It takes a property name as input and outputs a list of all the names of property lists that have the specified property name. Type:

If you want a total count instead of the actual list, then give the output of GLIST to the COUNT command. It counts the elements in its input and outputs the number. Type:

In database terminology, the output of GLIST is a selection set of records. The records are selected on the basis of matching a specified property name. You can use this selection set as the input to the FOREACH command to display information about each of the selected records. The first input to FOREACH must be a list. It also needs a list of instructions to be run for each of the items in the first list. FOREACH is like a special form of the REPEAT command except that it runs the instruction list for each item in the first list instead of a specific number of times. A special substitution parameter - a quoted question mark - is used in the run list. It takes the place of each item in the first list as the instruction list is run. A couple of examples will help illustrate this. Type:

In this example, the output of GLIST is the selection set [CA CO AZ] because only these three states have baseball teams. The run list is run three times - once for each of the elements in the selection set. The first time, CA is substituted for the quoted question mark so that FULLNAME? outputs the full name of California and the PRINT command prints it. Then, CO is used which gets the full name of Colorado. And finally, AZ gets the full name of Arizona.

The quoted question mark can be used more than one time if necessary. Type:

In this example, the output of GLIST is the selection set that includes all of your records because they all have a CAPITAL property. The run list is a bit more complicated because the quoted question mark appears two times. The parentheses around the PRINT statement are necessary because PRINT normally takes just one input; the parentheses tell it that something unusual is going on - in this case, there are two inputs to be printed.

If you create a separate property list for the state nicknames using the state abbreviation as the property name, you can match up the states with their nicknames in a way that is similar to a relational database. Create the NICKNAME property list with the following commands. You can add more nicknames later.

Create the NICKNAME? procedure:

Now, even though the nicknames are not part of the states’ property lists, you can still use the information from both property lists at the same time. Suppose you want to know the nickname of each state that has a baseball team. Here is a way to do it:

In this example, the selection set is the list of three states with baseball teams. Each of the state abbreviations is then used by NICKNAME? to print the nickname. Suppose you want the full name along with the nickname. Type:

If you continue adding new states and more properties, you’ll have a solid database on which to build projects about the states. Try using a map of the states and then create a property called XY that approximates the location of each capital. Create a trivia game about the states. Don’t forget to save your work. Type:

Create procedures to manipulate your properties. It’s much clearer than using the property list commands directly. Of course, you still need them inside the procedures. In fact, many of the built-in Logo commands are really an abstraction of property list manipulations. For example, PENUP picks up the turtle’s pen, but you probably didn’t know that behind the scenes, Logo was doing something like the following command for you:

Aren’t you glad that the abstract PU does the same thing? It’s so much more of a shortcut than you ever thought! Pretty sneaky, huh?

Property List Editors

You can edit property values with the EDP command. It takes the name of a property list as input and displays a window with a two-column table of property pairs. Type:

The Properties of CO window is displayed showing each property name and its value. Click in the text box of the value you want to modify and type a new value. Click on the OK button to accept the changes or click on the Cancel button if you change your mind. If you use the Logo text editor, your property lists are shown as PPROPS commands - one very long instruction to define the property list. That’s how property lists are saved in your workspace file so they can be rebuilt when you reload your workspace. It’s not the easiest way to view or edit property lists, but you can use it if you want.

The EDP command always displays your property lists in the two-column window format. The property lists for turtles are a different story. The format you see for turtle properties depends on the setting of the Extended Property Lists option in the Preferences dialog of the Edit menu.

Click on the Preferences menu and select Operation… to display the Operation Preferences dialog.

In the middle, you’ll see a checkbox labeled Extended Property Lists. By default, the box is not checked. Make sure it is not checked now. Click OK to close the Preferences dialog.

Move the mouse pointer over the turtle and right-click (on a PC) or Ctrl-click (on a Mac). This action displays the Turtle Center for TURTLE.0 dialog - a special property editor for turtles. (The turtle name in the title depends on which turtle you select.) The Turtle Center allows you to modify some of the turtle properties by clicking on the controls or entering new values in the text boxes. You should be familiar with most of the properties and how they work as well as the commands that you’ve used to modify them. If you change a property value and then click on the OK button, the changes will take effect in the same way as if you had typed in the commands in the Listener. For example, removing the check mark in the Show Turtle checkbox will cause the turtle to hide after you click the OK button.

The textbox labeled Run when clicked is for the turtle’s RUN property. The commands you put in the RUN property are run in the same way as if you had typed them in the Listener when you click on the turtle. They will apply to all of the active turtles. If you want the commands to apply to a specific turtle, then consider using the ASK command.

Be careful with the checkbox in the lower left corner of the Turtle Center for TURTLE.0 dialog - the one labeled Tell all active turtles. If you put a checkmark in that box, then the changes you make to the turtle properties are applied to all of the active turtles. Click on Cancel to close the Turtle Center for TURTLE.0 without making any changes.

Now, go back to the Preferences dialog and put a checkmark in the Extended Property Lists option and click the OK button. Move the mouse pointer over the turtle and right-click (on a PC) or Ctrl-click (on a Mac).

The property list for the turtle is now displayed in the same two-column format as your own property lists. There are more properties shown in the extended property list than in the Turtle Center dialog. Most of them are still familiar - except the SIZE and RUN2 properties.

The turtle’s size is normally 31 by 31 dots. The SETTURTLESIZE or SETTSIZE command changes the turtle size by scaling both the width and the height of the turtle. For example, SETTSIZE 2 is the same as setting the SIZE property to 62 by 62. The width and height of a turtle do not have to be the same but the SETTURTLESIZE command keeps them the same. You can have fun stretching your turtle - make it short and wide or tall and narrow. Try using a size of 15 by 300 and then STAMP a secret message.

It’s hard to read until you look at it at a very shallow angle. Some fonts work better than others. Rotating a stretched turtle will probably not look like what you wanted - try using the arrow shape. The RUN2 property works in a similar way to the RUN property. However, the commands are run when you double click on the turtle. You can put commands in either the 'RUN or the RUN2 property but not both.

Click on the Turtle Center button in the button bar. This is another property editor but it is not affected by the Extended Property Lists option in the Preferences dialog. It looks very much like the dialog for a specific turtle - except the Turtle name field is not available for change.

Be careful with the checkbox labeled Tell all active turtles - it is checked when you call up the dialog. If you remove the checkmark, then the changes you make will apply to the first turtle in the WHO list - usually that’s turtle number 0 but turtle names could change that.

The property list editors are convenient while you’re exploring. Most of the time, you will want to make sure that Extended Property Lists is not checked. This will give you quick access to the most common properties. In a procedure, however, you have to use either the built-in Logo commands that modify the properties or use the property list commands. They’re not so bad, really.

Your Own Turtle Properties

You can add properties to any property list. It’s a convenient way to keep track of information, especially for lots of turtles. Suppose you want to dress up a turtle and put on a fashion show? First, you have to buy clothes for the turtle and put them in a wardrobe. Then, you can begin the show and watch as the turtle tries on each outfit.

Or, suppose you want to disguise a turtle and send it on a secret mission? First, you have to issue the disguises and put them in a backpack. Then, you can start the mission and watch as the turtle changes its identity.

The turtle’s property list is a good place to put the wardrobe or the backpack. And, what’s good for the one turtle is good for the many turtles you might want to include later on in your fashion show or on your secret mission. Since each turtle has its own property list, then each turtle can have its own wardrobe or backpack. There won’t be any confusion - even with lots of turtles - because property is privately owned.

So, which one do you want to do - the fashion show or the secret mission? It doesn’t really matter because they are the same thing! It just depends on how you think about it. Sure, the idea is different, but the concept is the same. Pick the one you want, try it out, and then try the other one. You’ll see that only the names have been changed to protect the ideas - in a moment, the concept will become your own.

A Turtle Fashion Show

Buying clothes for a turtle is just a matter of selecting turtle shapes and putting their names in a list. The wardrobe is the name of a property and the clothes are the property value. Here is a procedure for buying clothes and putting them away:

To buy clothes for turtle number 0, type:

The fashion show is really just the turtle setting its shape to an outfit in the wardrobe, waiting a while to show it off, and then doing the same thing for each of the outfits. Here is a procedure for the fashion show:

You could use this procedure directly in the Listener. However, the WARDROBE input has to be the list of clothes and you wouldn’t want to type it again. Furthermore, you could use the LAUNCH command to make the fashion show run in the background so you can continue playing around. But then, when you want to stop the show, you have to remember the process identification number that is output by the LAUNCH command so you can give it to the HALT command. Otherwise, you’ll have to press the Red Traffic Light button which stops everything!

No problem - define the following procedure to begin the show. The PPROP command is one long line that does a lot of things. It creates a new turtle property called ID that will contain the process identification number that is output by LAUNCH. The LAUNCH command runs the list of instructions built by LIST which usually takes just two inputs. The parentheses are required because the list will contain three elements: the word FASHION.SHOW, the turtle number, and the list of clothes from the wardrobe.

You can begin the fashion show for turtle number 0 with the following instruction:

Since the show is running in the background, you can do other commands while its running. For example, start another fashion show with turtle number 1:

The following procedure can be used to stop a turtle’s fashion show. The process identification number needed for the HALT command is the value of the turtle’s property called ID.

To stop the shows, type:

Save your workspace so you can keep this example. You may not always want a fashion show, but the concepts of animating a turtle are worth having around for other ideas.

A Turtle Secret Mission

Issuing disguises for a turtle is just a matter of selecting turtle shapes and putting their names in a list. The backpack is the name of a property and the disguises are the property value. Here is a procedure for issuing disguises and packing them away:

To issue disguises for turtle number 0, type:

The secret mission is really just the turtle setting its shape to a disguise from the backpack, waiting a very short time, and then doing the same thing for each of the disguises. Here is a procedure for the secret mission:

You could use this procedure directly in the Listener. However, the :BACKPACK input has to be the list of disguises and you wouldn’t want to type it again. Furthermore, you could use the LAUNCH command to make the secret mission run in the background so you can continue playing around. But then, when you want to terminate (or end) the mission, you have to remember the process identification number that is output by the LAUNCH command so you can give it to the HALT command. Otherwise, you’ll have to press the Red Traffic Light button which stops everything!

No problem - define the following procedure to begin the secret mission. The PPROP command is one long line that does a lot of things. It creates a new turtle property called ID that will contain the process identification number that is output by LAUNCH. The LAUNCH command runs the list of instructions built by LIST which usually takes just two inputs. The parentheses are required because the list will contain three elements: the word SECRET.MISSION, the turtle number, and the list of disguises from the backpack.

You can begin the secret mission for turtle number 0 with the following instruction:

Since the mission is running in the background, you can do other commands while its running. For example, start another secret mission with turtle number 1:

The following procedure can be used to stop a turtle’s secret mission. The process identification number needed for the HALT command is the value of the turtle’s property called ID.

To stop the missions, type:

Save your workspace so you can keep this example. You may not always need to send a turtle on a secret mission, but the concepts of animating a turtle are worth having around for other ideas.

Chapter 6: Property Lists
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