[REBOL] Re: Strange array behavior
From: joel:neely:fedex at: 21-May-2002 17:29
Hi, Geza,
Geza Lakner MD wrote:
> Hi REBOL-fellows!
>
> Why does not properly work this code segment:
>
> >> a: array/initial [3 2] [[]]
> == [[[] []] [[] []] [[] []]]
> >> append a/1/2 3
> == [3]
> >> a
> == [[[3] [3]] [[3] [3]] [[3] [3]]]
>
It's working properly, just not intuitively! ;-)
What's happening is that the initial value consists of a reference
to an (initially) empty series. All of the elements of the new
structure are initialized to that same reference, per the meaning
of the /INITIAL refinement. However, since they are all referring
to the same series, mutations of that series are visible through
all of those references.
> Hmmm, I' ve expected at querying 'a :
> == [[[] [3]] [[] []] [[] []]]
>
> Maybe the prototype initial value is a reference not a true value?
> OK, I tried it even this way:
> >> a: array/initial [3 2] copy [[]]
>
> Maybe that even 'copy has made only ONE copy and five references to it
> in array 'a ?
>
You got it!
> Now, if I want to manipulate arrays with "appendable" empty
> lists, I don't give initial values and change each occurrances of
> 'none to copy/deep [[]] and THEN append to the defined location.
>
> Is this the best practice to overcome the seeming limitation of
> 'array ?
>
How about this:
>> a: copy/deep array/initial [3 2] [[]]
== [[[] []] [[] []] [[] []]]
which uses ARRAY/INITIAL to make a "prototype" structure, then lets
COPY/DEEP make all of the blocks unique, after which:
>> append a/1/2 3
== [3]
>> a
== [[[] [3]] [[] []] [[] []]]
-jn-
