Changes in sequences
We have already introduced the concatenation of sequences with the comma operator. With this operator, also single items can be added to a sequence:
let $seq := (1, 2, 3)
return ($seq, 4)
returns:
(1, 2, 3, 4)
With the help of the fn:insert-before() function, it is also possible to insert items into other positions of the sequence. The deletion of items can be achived with fn:remove() by indicating the relevant position. In the following example the item at position 3 is replaced by the item "New".
let $seq := ("I1", "I2", "I3", "I4", "I5")
return fn:insert-before(fn:remove($seq, 3), 3, "New")
Result:
("I1", "I2", "New", "I4", "I5")
The order of the items in a sequence can be reversed with fn:reverse(). With fn:subsequence() it is possible to build an excerpt from a sequence by indicating the start position and the number of elements. Here, these indications are values of xs:double type. They are initially rounded in order to get values of xs:integer type. In case the indication of a number is missing, the input sequence is outputted from the start position to the end. For example, the following expression:
let $seq := (6,5,4,3,2,1,0)
return subsequence(($seq, fn:reverse($seq)),
fn:count($seq) div 2,
fn:count($seq))
results in the sequence:
(3,2,1,0,0,1,2)
A special role plays the fn:unordered() function. It only represents the waiver of a specific order. A XQuery implementation may use this for optimisation purposes. In this way, for example, it is not required to restore the document order when uniting sequences with the union operator:
fn:unordered($seq1 union $seq2)
The following table provides an overview of the functions discussed.
| Signature | Description |
|---|---|
| fn:insert-before( $seq as item()*, $position as xs:integer, $seqnew as item()*) as item()* | returns a new sequence in which the items of the $seqnew sequence have been inserted into the $position position in the $seq sequence; if $position is greater than the number of the items in $seq, $seqnew is added to $seq; if $position has the value 1 or less, $seqnew is put in front of the items of $seq. |
| fn:remove( $seq as item()*, $position as xs:integer) as item()* | returns a new sequence which is made up by removing in $seq the item at position $position; in case of invalid position indications, $seq is returned. |
| fn:reverse( $seq as item()*) as item()* | returns a new sequence containing all items of $seq in reversed order. |
| fn:subsequence( $seq as item()*, $start as xs:double[, $length as xs:double]) as item()* | returns a new sequence beginning with the item at position round($start) in $seq; the rest of $seq is outputted in case $length is missing, otherwise only round($length) items are returned. |
| fn:unordered( $seq as item()*) as item()* | returns a sequence containing all items of $seq, whereby their order is undetermined. |
Source: "XQuery – Grundlagen und fortgeschrittene Methoden", dpunkt-Verlag, Heidelberg (2004)
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