Hi Mahmood,
> What is the actual difference between Property Lists and Linear Lists.
A linear list stores a bunch of "things", such as integers, strings,
symbols, other lists - whatever you want really.
For example:
myList = ["Picard", 50, ["Kirk", "Spock"]]
When you need to access one of the objects, you access it by it's position
in the list.
put myList[1]
-- "Picard"
put myList[3]
--["Kirk", "Spock"]
When the item stored in the list is another list, you can access it by first
accessing the number of the item in the list (which is a list) and then then
the number of the item in the list that you've just acccessed:
x = myList[3] --(that will be the list ["Kirk, "Spock"])
y = myList[2] --(that will be "Spock")
put y
--"Spock"
Or much more easily:
put myList[3][2]
--"Spock"
That's all very well, but when you write lst[2] and get the result of 50,
what does it really tell you? You have to know what the second position in a
list means.
But say you had a list like this:
property list:
IdList = [#name: "Picard", #age: 50, #kids: ["Kirk", "Spock"]
Each entry in the list is a "property", which is basically a variable that
holds a value, so that the value of #name is "Picard" etc.
put IdList[#name]
--50
You'd access it like this:
put IdList[#age]
--50
put IdList[#name]
--"Picard"
put IdList[#kids]
--["Kirk", "Spock"]
put IdList[#kids][1]
--["Kirk"]
You're list suddenly becomes a mini database, where you can store and access
records that actually mean something.
And you can turn it into a real database by making a "super list", such as
this:
Database = [[#name: "Picard", #age: 50, #kids: ["Kirk", "Spock"], [#name:
"Data", #age: 30, #kids: ["Spot"]]
If you analise this list you'll see it's a bunch of different lists:
first you have a linear list "Database", that holds two entries, each of
these is a property lists.
put Database[1]
--[#name: "Picard", #age: 50, #kids: ["Kirk", "Spock"]
put Database[2]
--[#name: "Data", #age: 30, #kids: ["Spot"]
Zooming in on one of these, you have a property list, with 3 properties
(variables) that hold values.
The first two are simple: the value of #name is "Picard"
The third one is another list, this time linear: ["Kirk", "Spock"]
So how do you access the entry "Kirk" in this super-list?
put Database[1][#kids][1]
--"Kirk"
Likewise,
put Database[2][#kids][[1]
--"Spot"
This makes lists, and especially property lists into a very powerful tools
for programming.
I just hope I didn't confuse you even more with all this :)
Regards,
Karina Steffens,
Lead Programmer
Martello Media Ltd.
4 Islington Avenue
Sandycove
Co. Dublin
Tel: +353 1 2844668
Fax: +353 1 2803195
http://www.martellomm.ie
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