Re: [xsl] for vs. for-each

["Dimitre Novatchev dnovatchev@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Finally a good progress has been made on one of the proposals for XPath 4:
Retrieving a sequence of items from a given sequence based on a sequence of
indexes
 (https://github.com/qt4cg/qtspecs/issues/50)

With this proposal:
https://github.com/qt4cg/qtspecs/issues/50#issuecomment-799228627 ,
Michael Kay defines a concrete operator syntax and explores and evolves the
main idea to its logical boundaries.

I find the result to match exactly what I had wished for.

Thank you, Dr. Kay!

On Sun, Mar 14, 2021 at 12:48 PM Dimitre Novatchev <dnovatchev@xxxxxxxxx>
wrote:

>
>
> On Sun, Mar 14, 2021 at 11:55 AM Michael Kay mike@xxxxxxxxxxxx <
> xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
>>  For 4.0, however, I would like to see better ways of accessing items in
>> a sequence by position, and there has been much discussion about how best
>> to achiieve this.
>>
>
> Yes, we already do have the index-of() function, but we need a reverse to
> this:
>
> from-indexes($vSeq, $vIndexes)
>
> and this would produce a sequence with items each of which is the item of
> $vSeq at position the value of the $index-value in $vIndexes, when
> $index-value iterates over $vIndexes, ot more strictly:
>
>      for $ind in $vIndexes
>        return $vSeq[$ind]
>
> Why reverse of index-of() ?
>
> Because, for any $x in $vSeq it is true that
>
>      from-indexes($vSeq, index-of($vSeq, $x))
>
> is a sequence containing all $x items from $vSeq.
>
> Or:
>
>      $x eq distinct-values( from-indexes($vSeq, index-of($vSeq, $x)) )
>
> Or if we had sets in XPath, then:
>
>      set {$x} === set { from-indexes($vSeq, index-of($vSeq, $x)) }
>
> And this can also be written as:
>
>      set {$x}  ===  set { $vSeq => from-indexes( $vSeq => index-of($x)) }
>
>
> There is even this proposal for an operator notation for the
> from-indexes() function, but individual preferences at present seem to vary
> too much in order to choose such an operator:
>
> https://github.com/qt4cg/qtspecs/issues/50
>
>
> Thanks,
> Dimitre
>
>
>
>
>
>
>>
>> Michael Kay
>> Saxonica
>>
>> > On 14 Mar 2021, at 18:04, Michael MC<ller-Hillebrand mmh@xxxxxxxxx <
>> xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > Folks,
>> >
>> > Given a variable with a sequence of values
>> >
>> > <xsl:variable name="values" select="(1,2,3,4,5)" as="xs:double+"/>
>> >
>> > these are three methods to report its content
>> >
>> > <xsl:sequence select="for $i in 1 to count($values) return
>> $values[$i]"/>
>> >
>> > <xsl:for-each select="1 to count($values)">
>> >    <xsl:sequence select="$values[.]"/>
>> > </xsl:for-each>
>> >
>> > <xsl:for-each select="1 to count($values)">
>> >    <xsl:sequence select="$values[current()]"/>
>> > </xsl:for-each>
>> >
>> > The first works as expected, the second does not, but the third
>> astonishingly enough gives me the same result as the first. Check it out:
>> >
>> > https://xsltfiddle.liberty-development.net/ei5R4v8/2
>> >
>> > I read/understand that there is a difference between a for expression
>> and a path expression, but since we can use atomized values in
>> xsl:for-each, I would like to see more similarity between for and
for-each.
>> >
>> > Should this be on the wishlist for XSLT 4 or do I have to
>> learn/understand some more concepts?
>> >
>> > Puzzled greeting,
>> >
>> > - Michael
>> >
>> >
>>
>>
>>
>

--
Cheers,
Dimitre Novatchev
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