xsl:iterate
Used to iterate over a sequence, with the option to set parameters for use in the next iteration.
Category: instruction
Content: (
xsl:param*
, xsl:on-completion?
, sequence-constructor
)
Permitted parent elements:
any XSLT element whose content model is
sequence-constructor; any literal result element
Attributes
|
| Input sequence over which the iteration is processed. |
Saxon availability
Available in XSLT 3.0. From Saxon 9.8, available in all editions. Implemented in Saxon-PE and Saxon-EE since Saxon 9.6. Available for all platforms.
Notes on the Saxon implementation
The ordering of elements within xsl:iterate has changed since the
original W3C working draft (xsl:on-completion now comes immediately
after xsl:param). Since Saxon 9.7, only the new ordering is
allowed. Saxon 9.6 allowed both the new and the old ordering, but gives a warning
if the old ordering is used.
Earlier Saxon releases implemented a prototype of xsl:iterate as an
extension in the Saxon namespace (saxon:iterate). This was dropped
in Saxon 9.5.
For xsl:iterate to be streamable, the W3C rules require that the
select expression must be "striding", which essentially means
that it may use the child axis but not the descendant axis (to ensure that
selected nodes do not overlap each other). Prior to Saxon 9.5, Saxon attempted
to be more liberal than this, and allow limited streaming also when the
descendant axis was used. Since Saxon 9.6, Saxon has been brought into line with
the W3C specification. In many cases the restriction can be circumvented by
using the outermost function, for example the expression
outermost(//title) is striding even though it uses the
descendant axis.
Details
The xsl:iterate instruction is new in XSLT 3.0. It is similar to xsl:for-each, except that the
items in the input sequence are processed sequentially, and after processing
each item in the input sequence it is possible to set parameters for use in the
next iteration. It can therefore be used to solve problems that in XSLT 2.0
require recursive functions or templates.
The xsl:iterate instruction is motivated by use-cases for streaming,
but it can also be used profitably in non-streaming situations. For more
information see Streaming with
xsl:iterate.
The instruction allows a child element xsl:on-completion which defines processing to be
carried out when the input sequence is exhausted. The instructions within
xsl:on-completion have access to the final values of the
parameters declared in the xsl:next-iteration instruction set while processing the last item in
the sequence. An xsl:break element
may be used within the enclosed sequence constructor of an
xsl:iterate instruction, which causes premature completion
before the entire input sequence has been processed.
Examples
Example 1
Computes the running balance of a sequence of financial transactions:
<xsl:iterate select="transactions/transaction"> <xsl:param name="balance" select="0.00" as="xs:decimal"/> <xsl:variable name="newBalance" select="$balance + xs:decimal(@value)"/> <balance date="{@date}" value="{$newBalance}"/> <xsl:next-iteration> <xsl:with-param name="balance" select="$newBalance"/> </xsl:next-iteration> </xsl:iterate>Example 2
Copies the input sequence up to the first <br>
element: