Property Lists of Objects

Property Lists of Objects

We begin with a discussion of property lists. Every Mathscript object may have a property list. In fact, each object has a name, and if we think of the object itself as the value of that name, then we see the origins of property lists. Each name is an atomic symbol and it has associated with it a list of properties. The most obvious property is its value (the object associated to the name). But we may create other properties of the name, and those other properties are located in the property list of the name.

The relevant functions and commands are:

Getproperty

Addproperty

Putproperty

Removeproperty

Whenever a property is assigned to a name, the property also has a name. We call that the Propname. The property itself is some object, such as a variable, or a list, and is designated the Prop.

Suppose we want to define a 'family tree' as a miniature database.

We have certain family members. Say William and Sarah are the parents of

John. John is married to Mary and they have children Liza and Tom.

Suppose William is 56 and Sarah is 50 years of age. John is 33 and

Mary is 34. Liza and Tom are twins and they are both 8 years old.

In MathScript, each of these names must be declared first as variables:

Make variable william

Make variable sarah

Make variable john

Make variable mary

Make variable liza

Make variable tom

And we have propnames: Father, Mother, Husband, Wife, Age, Children

Make variable father

Make variable mother

Make variable husband

Make variable wife

Make variable children

Make variable age

Now, we begin to create this database by entering:

putproperty william in father of john;

This creates for John a 'father' property. And it inserts William as the value of the father property of John.

To check this, we use the dual function getproperty.

Getproperty(john,father) returns as its value: William.

Now enter

putproperty sarah in mother of john;
putproperty mary in wife of john;
putproperty 33 in age of john;
putproperty list(liza,tom) in children of john;

This gives the relevant information about John.

Notice that the 'children' property of John is a list, and not just a variable. Suppose later that another boy, Ricky, is born to John and Mary. We might add Ricky to the list with the Command Addproperty in the following way.

Make variable Ricky
Addproperty Ricky to children of John.

Then the value of

Getproperty(John, Children)

would be: list(Ricky, Liza, Tom).

In general, the putproperty command overwrites any previous Prop, but the Addproperty command adds the item to the Prop if the Prop is a list, or has not yet been defined. For example, If you want to add John's hobbies: Tennis and chess to the database,

Make variable tennis
Make variable chess
Make variable hobbies

Addproperty tennis to hobbies of John
Addproperty chess to hobbies of John

would work. This is because the first Addproperty call creates a List as the Prop of Hobbies. And the second call adds Chess to that list.

Putproperty tennis in hobbies of John
Addproperty chess to hobbies of John

Would not work, because Putproperty makes the hobbies property a variable, not a list, and one cannot add to a variable.

Finally, suppose you wish to delete an item entered on the property list of an atom. Of course, a new Putproperty Command could replace it, but these lists consume memory, and it might be desirable at some time to release that memory to the system. Then

Removeproperty propname of John

has the effect of removing from the property list of John the property with the name propname.

Property List operators

1) Addproperty

Number of Arguments: 3

Argument types: First is arbitrary, second is a variable, third is an object name.

Command description:

Addproperty Prop to Proptype of target

or

Addproperty Prop Proptype target

has the effect of adjoining to the 'proptype' property of target the item Prop. The 'proptype' property must either be a list, or be undefined. If a proptype is undefined, its value is returned as FALSE (or the empty list). If the property Prop was already in the list, it is not added again. Target is the name of an object, and is not evaluated.

2) Getproperty

Number of Arguments: 2

Argument types: first is an object name, second is a variable

Function description:

Getproperty takes 2 arguments:

Getproperty(target, propname) returns the value of the property named propname on the property list of target. If there is no propname property, it returns FALSE, or list(). Target is the name of an object, and is not evaluated.

3) Putproperty

Number of Arguments: 3

Argument types: First is arbitrary, second is a variable, and third is an object name.

Command description:

Putproperty Prop in Proptype of target

or

Putproperty Prop Proptype target

has the effect of placing in the 'proptype' property of target the item Prop. The 'proptype' property is overwritten. Target is the name of an object, and is not evaluated.

Removeproperty

Number of Arguments: 2

Argument types: first is an object name, second is a variable.

Command description:

Removeproperty propname of target
or
Removeproperty propname target

removes the propname property from the property list of target. Target is the name of an object, and is not evaluated. The memory is freed.

If later you call

Getproperty(target, propname) the return value will be FALSE.