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B7009 - What is the purpose of property lists? [Part 2] by - Joseph Ganci Hello again. I was tracking down what I thought might have been a bug
in Authorware, but as it turns out, it's not. It's just not as intuitive as
I would have thought. So now I have your answer, and also a warning to those
who may have thought as I did. Originally, I was going to suggest that you could set up a linear list
of property lists by using the following code: people := Array( [#name:""] , 10) However, the result of this is that all 10 property lists in the people
array would have the value "Joe" as the value of the name property. So what
gives? Well, a little background is needed here. Remember that lists can contain
gobs of data. I mean, GOBS of data. Normally, in Authorware, if you want to
copy the value of a NON-LIST variable into another variable, you would use the
approach: >newvar := oldvar If oldvar originally contained the value "Joe", then the value of newvar
will also become "Joe". Take a look at the following: oldvar := "Joe" After the above three lines are done, newvar will contain "Joe" and
oldvar will contain "Ivana". Note that you can make any changes you wish to
oldvar after you've set newvar to it, and the value of newvar doesn't change.
That's because the value of oldvar is COPIED into newvar. However, let's say you have created two lists, called mylist and yourlist.
Take a look at the following: mylist := ["Joe", "Ivana", "Frank", "Letha"] The result of the above is that yourlist and mylist will both contain
["Bill"]. The original value of mylist is destroyed. Why? It's because when you set yourlist := mylist you are not making a copy
of the contents of mylist into yourlist, but instead you are setting up yourlist
as a reference to mylist. Any changes to one will happen to another. When I teach Advanced Authorware classes, I use the following example
(feel free to borrow it if you like it). Let's say you're married and your spouse
and you just had your firstborn child, a girl. Now let's say you're partial
to naming the child "Melissa" and your spouse insists it must be "Susan."
You both are very hardheaded and the little girl grows up being called Melissa
by you and Susan by your spouse. She has two different names, but she is the
same person in either case. So, here's the analogy now: if Melissa falls and
scrapes her knee, is Susan's knee also scraped? Yes, of course, since she's
the same little girl. When Susan goes to school, so does Melissa. When Melissa
goes on her first date, so does Susan. Since lists can potentially contain so much data, it makes sense that
yourlist := mylist just create a reference, not a copy of the list. This makes
sense more often than you may think. If, for instance, you set up a subroutine
that manipulates a list in several ways, sorting it in a strange way that the
SortList function can't handle, for instance, then you can set up the subroutine
to use a list variable simply called "list". Whenever you have a list that needs
to run through the subroutine, you then just have to set "list" to the list
you want manipulated. So, if you have 20 lists in your file, and you want them
all to be able to run through the subroutine, you don't have to COPY the contents
of the list you want to run through the sub into the variable "list" and then
have to copy the contents back into your list after the subroutine is finished.
You need only set the variable list to the name of the variable you wish
run through the subroutine. No need for massive copies at the beginning and
end of the subroutine. So, how does one copy a list into another list? One uses the CopyList
function. yourlist := CopyList(mylist) makes a copy of mylist's values and places them into yourlist. This
means you can make changes to either one and it doesn't affect the other. So, getting back to the original question. Using the approach: people := Array( [#name:""], 10) is actually making 10 links to the same internal list. That's why making
a change to the name property in the first location in the list makes the same
change to all the locations in the list. Instead, you need to make copies in
each location of the list, like this: people := Array( [], 10) If you use the above approach, you'll see that "Joe" gets placed in
the first location and "Ivana" in the eighth location, and the other locations
behave, staying empty. Long-winded, but I hope it helps explain further the intricacies of
linear lists and property lists.
I followed your logic until here. But what four indices are
you referring to?
I mean that the Rect function returns a list variable that contains
four locations. If you set up a Rect function call as myrect := Rect(20, 100, 60, 300) then myrect[1] = 20 and myrect[2] = 100 and myrect[3] = 60 and myrect[4]
= 300. Those are the four indices, or locations, in the list. Add your review Back |