ABC of arrays

[Richard Gaskin]

Graham Samuel wrote:

 > Well, Richard, I am always very willing to believe that I’ve
 > misunderstood something, but this is what happened (this material is
 > extracted from conversations I had on this list in the middle of
 > March).
 >
 > 1. I had an XML file - it was actually a .gpx file, which shows a
 > geographical route as a series of waypoints. The ‘top level’ of
 > this structure is ‘gpx’. I mean the first three lines are (watch
 > for wordwrap):
 >
 > <?xml version="1.0" encoding="UTF-8"?>
 > <gpx xmlns="http://www.topografix.com/GPX/1/1" creator="Geolives"
 > version="1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 > xsi:schemaLocation="http://www.topografix.com/GPX/1/1
 > http://www.topografix.com/GPX/1/1/gpx.xsd">
 >   <metadata />
 >   <trk>
 >
 > 2. I converted it to an array using the function ConvertXMLToArray,
 > by Trevor deVore - from Mike Doub’s Master Library.
 >
 > 3. I put the result into an array called ‘gGeneralArray’ - I can see
 > this in the IDE as a nested structure.
 >
 > 4. I executed
 >
 >   put the keys of gGeneralArray
 >
 > in the message box, and got the result
 >
 >  gpx
 >
 > 5. I discussed this on the use-list, and Mike Bonner suggested
 >
 >> try
 >> put the keys of gGeneralArray["gpx”]
 >> and you'll get the next level of keys.
 >
 > 6. I did that and got
 >
 > wpt[1]
 > trk
 > @attributes
 > wpt[4]
 > wpt[2]
 > metadata
 > wpt[3]
 >
 > I just did this all again to prove to myself that I hadn’t made an
 > obvious error.I am definitely only dealing with one array, so I can’t
 > explain this, and the LC dictionary certainly doesn’t help.
 >
 > Tell me where I went wrong!

You're using other people's code so you're off the hook. :)

There does appear to be a problem there, but I'm not sure it's with 
LiveCode, and even with the oddities in that example the "keys" function 
appears to be working as designed.

The "keys" should return a list of all keys, even when the value of any 
of them may be another array.

This is consistent with what you found:  your array gGeneralArray 
contain one element, and that element is another array.  You can also 
query the keys for the sub-array and obtain its keys, as you did.

Consider this much simpler example in which we have three keys, two of 
which have values that are not arrays and one of which has an array for 
its value:

on mouseUp
    put "1" into tA[1]
    put "2.1" into tA[2][1]
    put "2.2" into tA[2][2]
    put 3 into tA[3]
    put the keys of tA
end mouseUp

In my Message Box I get:

3
1
2

...showing that all keys are accounted for, even tA[2] where the value 
is another array.

So while the "keys" function seems to be fine here, your example above 
has other interesting factors that make it somewhat curious, no doubt 
contributing to the confusion here:

Nothing in the XML shown contains any of the elements shown in the 
sub-array keys listing, so it's not possible to see how it was parsed 
given what we have here.

Moreover, some of those sub-array keys appear to contain array notation 
in in the name itself, which may be valid but is unusual and potentially 
confusing.

Key names are pretty flexible, even more than the Dictionary entry 
recommends, allowing (IIRC) pretty much any character other than NULLs 
(and in v7 perhaps even NULLs are allowed).

So using "[" and "]" is as valid as part of a key name as any other 
characters, but stylistically the choice is confusing, and unlikely the 
actual name of the element as expressed in the source XML.

To figure out how you got the output you did I'd have to put together a 
test stack after gathering up the relevant handlers from Master Library, 
and given Trevor's characteristically excellent coding I'm sure the 
final analysis would make sense of that example.

But for the moment, I think the key lesson here (if you'll pardon the 
pun) is that sometimes stumbling across edge cases like this one can 
lead one to think features are more intimidating or confusing than they 
need to me.

Arrays in LiveCode are fairly straightforward name-value pairs, and 
there's not even very much we can do with them:  put, get, delete, and 
obtain the keys.  That's pretty much it.

If we stumble across an extremely uncommon case like the one you had the 
misfortune to discover above, it may seem much more complicated than it 
really is.

This is why I recommend experimentation on one's own to learn a specific 
feature, like arrays.  Get the basic operations down in an hour 
experimenting on a throwaway stack, and then even confusing edge cases 
like this one will become less daunting.

-- 
  Richard Gaskin
  Fourth World Systems
  Software Design and Development for the Desktop, Mobile, and the Web
  ____________________________________________________________________
  Ambassador at FourthWorld.com                http://www.FourthWorld.com