To add values in cumulative format - xpath

Question

I have a xml structure as follows:
<bookstore>
<book>
<name>story</name>
<price>50.00</price>
<author>smith</smith>
</book>
<book>
<name>history</name>
<price>150.00</price>
<author>kelly</smith>
</book>
<book>
<name>epic</name>
<price>300.00</price>
<author>jones</smith>
</book>
</bookstore>
In the above example i want to add the price as follows:
first book price should be returned as it is.
second book price should be added with first book price 50.00+150=200.00
Third book price should be added with first & second book price 50.00+150.00+300.00=500.00
& return the values of price as below
<pricelist>
<price>50.00</price>
<price>200.00</price>
<price>500.00</price>
</pricelist>
can anyone help me on this??
Thanks.

Answer 1

There are two ways to solve your problem. One would be to sum up all preceding sibling books, which is easy to read and code, but has O(n^2) complexity and thus does not scale well for large input, but will be fine for rather small sets (complexity might even be worse, depending on how your XQuery processor resolves the preceding siblings).
for $book in /bookstore/book
return
<price>{ sum(($book/price, $book/preceding-sibling::book/price)) }</price>
As a declarative programming language, XQuery lacks variables (that can be modified, eg. in a loop). As an alternative, write a recursive function, which calculates the sum in O(n).
declare function local:sum($books, $sum) {
if ($books) then
let $price := $sum + $books[1]/price
return
(
<price>{ $price }</price>,
local:sum($books[position() > 1], $price)
)
else
()
};
local:sum(/bookstore/book, 0)