Re: for vs. for-each

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