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symbol property question

RVERP

I declare 3 variables

a = 10
b= 20
test=list('a 'b)

now I do test->x = 99

now test = (a x 99 b)

 So my list has become 4 items long, containing symbols a,b and x, and a value 99
x is still unbound.

 what has happened when i did test->x = 99??

 

  • You kind of created a disembodied property list (DPL). It is not a correctly formatted list which should start with a nil and contain symbol/value pairs.

    list( nil 'a "valueForA" 'b 3.2  'x 99 )

    As for x, it is a symbol used as a key in your list. It has no value. In a properly formatted DPL, you would evaluate test->x and have returned the value 99

    I'm not sure what you will get with your malformed DPL.

    Ted

  • There is absolutely no problem with having a disembodied property list with something other than nil as the first element - in fact I have used this to allow me to store something useful in that place - to allow me to have assoc lists of dpls.

    The DPL will work fine - you can use test->x to retrieve the value. The fact that there is a strange dummy value at the end wouldn't be a problem. Not sensible, but not a problem either.

    Andrew.

  •  I remember using the first element in a DPL to hold a value but considered it a quirk that could be "fixed" in a later release. I never even thought of creating a DPL out of an existing list like that, it looks like a hard to find bug wating to byte at some later point.

    Ted

  •  Ok, thanks I lookes at the chapter in the manual and understand now.

     But what I was wondering, is there an reason not to make a DPL in a DPL, like this:

    masterlist->prop1->prop1.1 = something
    masterlist->prop1->prop1.2 = something  else
    masterlist->prop2->prop2.1 = something new

    or is this better:

    prop1list->prop1.1 = something
    prop1list->prop1.2 = something  else
    prop2list->prop2.1 = something new

  • No problem with having nested disembodied property lists. Note that you need to initialize the sub-property list - using ncons(nil) for example:

    masterlist=ncons(nil)
    masterlist->prop1=ncons(nil)
    masterlist->prop1->subprop1="hello"
    masterlist->prop1->subprop2="test"
    masterlist->prop2=ncons(nil)
    masterlist->prop2->subprop21=23

    Then if you look at the masterlist variable it contains:

    (nil prop2 (nil subprop21 23) prop1 (nil subprop2 "test" subprop1 "hello"))

    Regards,

    Andrew.

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