Objects

Create and manipulate objects.

Object commands create and destroy objects like turtles, or the objects defined in additional extensions to Terrapin Logo Pro. The built-in TELL list is often used to talk to a specific set of objects. For a description of commands around TELL, see the turtle commands. See also TELLALL, TELLODD, TELLEVEN, EACH, and WHO.

Once created, objects live forever unless you erase them with the ERASE command.

Reports a list of objects listening to commands.

.WHO

.WHO reports the list of currently active turtles, or other objects, as set by the TELL command. The output is always a list regardless of the number of objects that a previous TELL command has set. This is different to the WHO command.

See also ASK, SETTURTLES, TURTLES, and TELL.

TELL 0
WHO
Result: 0
.WHO
Result: [0]

Makes objects execute a list of commands.

ASK word runlist
ASK number runlist
ASK list runlist

ASK causes the turtle(s) or other objects named in its first argument to execute the commands in its second argument. ASK makes it possible to send commands to a turtle that is not currently active without making it one of the active turtles.

The first argument to ask can be either a single object name or a list of object names (like turtle numbers). The input may also be any other Logo object, like a robot, or a bitmap, or a list of these objects.

Logo sends the commands in the runlist to each object in the list; if one of these objects does not know the command, the execution proceeds silently. If any command outputs a value, the execution stops, and the value becomes the output of ASK. During the execution of ASK, the TELL list is set to the list that was handed to ASK as its first input; EACH, on the other hand, sets the TELL list to each of its inputs before executing the runlist.

You can also use ASK to ask a control to perform certain actions that are specific to the control (for that matter, we are talking about any callable properties at any object). For example, you can ASK a list box to APPEND an item instead of having to use the CPROP command. You cannot call callable properties inside a Logo procedure, though. Only ASK (and EACH) know how to call callable properties. See also the page about Object properties for such commands.

TELL 0
ASK 1 [ST]
ASK 1 [FD 100]
ASK 1 [HT]

Checks an value for a type.

CHECKTYPE value type

CHECKTYPE checks a Logo name for being an element of a certain type. Since a Logo name may be a turtle, or any other type, its properties vary from type to type. If the object given as its first input does not match the type given as the second input of CHECKTYPE, Logo throws a runtime error. A Logo name may have more than one type. The value 0 can, for example, be a number, or a turtle. CHECKTYPE can be used to check a value for being a procedure, a number, or simply anything that Logo knows.

CHECKTYPE 0 "TURTLE
CHECKTYPE "UNKNOWN "TURTLE

Creates an object.

DECLARE kind name

The DECLARE command allows you to create new Logo names, giving a new name a certain type. Logo uses this command internally to create objects of all kinds, like robots and turtles. When a new Logo name is declared, Logo creates the object according to its type. Note that object names must be unique, so creating a TURTLE named STAN and an ARRAY named STAN will not work.

Objects live forever until they are erased with the ERASE command.

Applies a runlist to every object in the TELL list.

EACH runlist

EACH causes each of the currently active objects on the TELL list to execute the commands contained in its input sequentially. Before executing the runlist, EACH sets the TELL list to each of the objects on the original TELL list. This allows each of several turtles to be given a variable input or to be addressed by WHO.

You can also use EACH to ask a control to perform certain actions that are specific to the control (for that matter, we are talking about any callable properties at any object). For example, you can use EACH to ask a list box to APPEND an item instead of having to use the CPROP command. You cannot call callable properties inside a Logo procedure, though. Only EACH (and ASK) know how to call callable properties. See also the page about Object properties for such commands.

See also ASK, SETTURTLES, TURTLES, and WHO.

TELLODD
HOME
EACH [PD SETH 30 * WHO FD 100]

Outputs a list of objects.

EVERY name

The EVERY command outputs a list of those objects whose type is given as input.

EVERY "PANEL
Result: [GRAPHICS LISTENER TOOLBOX FILES HELP EDITOR DEBUGGER]

Checks an object for being of a specific type.

IS.A object type

IS.A checks a Logo name for being an object of a certain type. Since a Logo name may be a a turtle, or any of the other object types, its properties vary from type to type. If the object given as the first input of IS.A matches the type given as the second input of IS.A, IS.A returns TRUE; otherwise IS.A returns FALSE. IS.A is handy for checking whether a Logo name has a certain built-in property. A Logo name may have more than one type. The value 0 can, for example, be a number, or a turtle. Logo knows several types, and the list keeps growing. Buttons, sliders, and other controls have their own type, but all controls are also CONTROLs. In addition, controls, turtles, and bitmaps are also WIDGETs.

IS.A 0 "TURTLE
Result: TRUE
IS.A 0 "WINDOW
Result: FALSE

Creates a new Logo object.

NEW kind

The NEW command allows you to create new Logo objects of a certain type. The output of this command is the name of the object, which may be assigned to a Logo variable for later use. The NEW command is an abbreviation for (DECLARE :kind).

Defines a list of objects listening to commands.

TELL number
TELL object
TELL list

TELL activates the objects that respond to turtle commands. Objects may be turtles, or other moveable objects like robots. A single number or names as an argument to TELL activates that single turtle or object. A list of numbers or names activates all the objects named in the list. Use TELLALL 0 (TURTLES - 1) to activate all available turtles. TELLEVEN and TELLODD also activate a range of turtles.

Before turtles may be activated, they must be defined by the SETTURTLES command. The default number of turtles available when Logo starts is 16. After startup or after execution of a SETTURTLES command, turtle 0 becomes the active turtle.

See also ASK, EACH, TURTLES, and WHO.

TELL 0
ST PD FD 50
TELL 1
ST PD SETH 45 FD 50
TELL 0
HOME

Tells a range of turtle numbers.

TELLALL first last

TELLALL activates a range of turtles to respond to turtle commands. TELLALL takes two numbers as arguments. The first number is the number of the first turtle in the range to be activated. The second number is the last turtle in the range to be activated. Use TELL to activate a single turtle or a list of turtles. Additional commands that activate a range of turtles are TELLEVEN and TELLODD. EVERY “TURTLE reports a list of all available turtles.

TELLALL 1 4
WHO
Result: [1 2 3 4]

Tells all turtles with even numbers.

TELLEVEN

TELLEVEN activates all the even numbered available turtles. Before turtles may be activated, they must be defined by the SETTURTLES command. The default number of turtles available when Logo starts is 16. See also TELLODD, TELLALL, and WHO.

TELLEVEN
WHO
Result: [0 2 4 6 8 10 12 14]

Tells all turtles with odd numbers.

TELLODD

TELLODD activates all the odd numbered available turtles. Before turtles may be activated, they must be defined by the SETTURTLES command. The default number of turtles available when Logo starts is 16. See also TELLEVEN, TELLALL, and WHO.

TELLODD
WHO
Result: [1 3 5 7 9 11 13 15]

Reports the type of a name.

TYPEOF name

The TYPEOF command returns the type of the Logo name supplied as input. Logo knows several types, and the list keeps growing. Buttons, sliders, and other controls have their own type, but all controls are also CONTROLs. In addition, controls, turtles, and bitmaps are also WIDGETs.

TYPEOF 1.234
Result: NUMBER
TYPEOF "HELLO
Result: WORD
; Note that object or widget names take precedence!
TYPEOF 0
Result: TURTLE

Reports a list of objects listening to commands.

WHO

WHO reports the list of currently active turtles, or other objects, as set by the TELL command. The output is either a list if there is more than one object listening, or a word, if only one object is listening. If you always need a list of objects regardless of the number of objects listening, use the .WHO command.

See also ASK, SETTURTLES, TURTLES, and TELL.

TELLALL 0 3
WHO
Result: [0 1 2 3]
TELL 0
WHO
Result: 0