[REBOL] Antwort: Antwort: Compiler for Rebol ? Re:(16)
From: joel:neely:fedex at: 28-Sep-2000 14:34
[Sharriff--Aina--med-iq--de] wrote:
> Thanks Jeff!! one question:
>
> the "local" value "k", what is it piroüose exactly , its not beineg
> breferenced at all
>
Hey, I did say I'd left something as an exercise! ;-)
> >> buildxref: func [/local xref k v] [
> [ xref: copy []
> [ foreach [k v] foo [
> [ append xref v/name
> [ append/only xref v
> [ ]
> [ xref
> [ ]
>
If the second argument of foreach is a list of words, then they take
consecutive values from the block that is the second argument.
>> foo: [1 "a" 2 "b" 3 "c" 4 "d"]
== [1 "a" 2 "b" 3 "c" 4 "d"]
>> foreach [n s] foo [print s]
a
b
c
d
>> foreach [n s] foo [print n]
1
2
3
4
>> foreach [n s] foo [print [n "->" s]]
1 -> a
2 -> b
3 -> c
4 -> d
>>
So, all I was saying in buildxref was "Walk through foo , treating
it as a collection of pairs. Call the left-hand element k and the
right-hand element v while looking at each pair. Now, here's what
to do with each pair..." It just happens that the simplified version
if cross-referencing I showed in that example didn't use the left-hand
element.
There are other ways to skin this cat. For example, using forskip as
illustrated by:
>> skipfoo: next foo
== ["a" 2 "b" 3 "c" 4 "d"]
>> forskip skipfoo 2 [print first skipfoo]
a
b
c
d
== false
>> skipfoo: foo
== [1 "a" 2 "b" 3 "c" 4 "d"]
>> forskip skipfoo 2 [print first skipfoo]
1
2
3
4
== false
>> skipfoo: foo
== [1 "a" 2 "b" 3 "c" 4 "d"]
>> forskip skipfoo 2 [print [first skipfoo "->" second skipfoo]]
1 -> a
2 -> b
3 -> c
4 -> d
== false
>>
Both of these approaches can be used with groupings of more then two
elements at a time. For instance, if I had a block containing ID#,
name, and phone number for a bunch of employees (I'll not bother to
make up sample data for this one), one could list all of the names
by saying either (*WARNING* *UNTESTED CODE FRAGMENT*)
foreach [emp-id emp-name emp-phone] emp-data-block [
print emp-name
]
or
emp-temp-block: next emp-data-block
forskip emp-temp-block 3 [print first emp-temp-block]
or (a TOTALLY UNSATISFACTORY solution, in my opinion)
emp-data-block: next emp-data-block
forskip emp-data-block 3 [print first emp-data-block]
emp-data-block: head emp-data-block
The first of these three (at least to my eye) seems to do the best
job of making clear that I'm dealing with emp-data-block as a
collection of triples, and making clear exactly what I'm doing with
(what parts of) each triple.
-jn-
PS: Since I've already blabbered on for this long, I might as well
include one solution to the open issue I raised in the earlier
post in this thread -- creating an index/crossreference to your
original block that lets me start with a name and get at both the
key and value block from the original data structure:
== false
>> foo: [
[ admin [
[ name "Sharriff Aina"
[ password "gong"
[ email [sharriff--aina--med-iq--de]
[ dept "Administration"
[ view "yes"
[ ]
[
[ sharriff [
[ name "Sharriff Aina"
[ password "jungle"
[ email [sharriff--aina--med-iq--de]
[ dept "Administration"
[ view "yes"
[ ]
[ ]
== [
admin [
name "Sharriff Aina"
password "gong"
email [sharriff--aina--med-iq--de]
dept "Admini...
>> buildxref: func [orig [block!] /local xref k v] [
[ xref: copy []
[ foreach [k v] orig [
[ append xref v/name
[ append/only xref reduce [k v]
[ ]
[ xref
[ ]
>> fum: buildxref foo
== ["Sharriff Aina" [
admin [
name "Sharriff Aina"
password "gong"
email sharriff.ain...
>> first select fum "Sharriff Aina"
== admin
This version uses both the key and value elements of each pair.
Of course, I can't let this drop without assigning more homework
(old teaching habits die hard ;-), so let's notice that the
name "Sharriff Aina" occurs more than once in your original data.
Wouldn't it be nice to have a version of buildxref that made a
crossreference structure that paired each name with a block of
key/value pairs, for all entries in the original data where the
name appeared?
--
; Joel Neely [joel--neely--fedex--com] 901-263-4460 38017/HKA/9677
REBOL [] print to-string debase decompress #{
789C0BCE0BAB4A7176CA48CAB53448740FABF474F3720BCC
B6F4F574CFC888342AC949CE74B50500E1710C0C24000000}
