Question

I have a sequence of length $n initialized to zeroes:

let $seq := (for $i in (1 to $n) return 0)

I can access a position easily…

return $seq[5]

…but how do I update it? (the following doesn’t work)

let $seq[5] := $seq[5] + 1

Answer 1

If you’re using an XQuery implementation which supports XQuery 3 maps (eg. Saxon, BaseX), you could use these:

declare namespace map="http://www.w3.org/2005/xpath-functions/map";

(: Fill map with square numbers :)
let $map := map:new(
  for $i in (1 to 10)
  return map:entry($i, $i*$i)
)
(: Overwrite a single value :)
let $map := map:new(($map, map:entry(2, 5)))
(: Fetch this value :)
return map:get($map, 2)

But generally it is possible to solve a problem without maps and in most cases this code will probably run faster as it will get better optimized.