Subject: [xsl] Subtle (or not?) Map Processing Bug in XQuery From: "Eliot Kimber ekimber@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> Date: Fri, 7 Jan 2022 16:18:57 -0000 |
Ibm starting to use XQuery again after being away from it for a few years and also trying to re-learn how to operate on maps efficiently as my current XQuery use is largely data analysis and reporting over large bodies of DITA content, so therebs lots of reference management and resolution to be done, lots of fiddly bits to keep track of. I ran into a coding bug that threw me for a minute and thought Ibd mention it here just in case it throws anyone else in the future (including future me). In my XQuery script Ibm building a map from all the image files in a directory tree (using BaseXbs file extensions): let $files := file:list($rootDir, true(), b*.pngb) let $imageMap := map:merge( for $file in $files map:entry(local:getFilename($file), map{bpathb : $file, bkeynameb, local:getKeyName($file)) ) At this point $imageMap is a map where each entrybs value is also a map. I then iterate over the map to find images not referenced from any topics in my doc set: let $orphanMap := map:merge( for $key in map:keys($imageMap) order by $key return if (local:notReferenced($orphanMap($key)(bkeynameb))) then $orphanMap($key) else () ) I then report the items in $orphanMap. When I ran the code as shown I was surprised to only get one item in the map even though there are 199 orphaned images in my test set. My bug of course is that I forgot that I have to construct a new map entrybwhat the code above does is add the map that is the value of the entry to the result map. These maps all have the same keys so of course the default combine behavior results in a single entry in the result map. Doh! I clearly had it in my head that b$orphanMap($key)b would return the *entry* with that key, not the *value* of the entry with that key. What threw me at first was that the map:merge() worked because it was being given map entries to merge, just not the right ones. If the values in the input map had been something else then the map:merge() would have failed and Ibd have immediately realized my mistake. The correct code is: let $orphanMap := map:merge( for $key in map:keys($imageMap) order by $key return if (local:notReferenced($orphanMap($key)(bkeynameb))) then map:entry($key, $orphanMap($key)) else () ) Ibm also wondering if therebs any general source of XQuery coding patterns for working with maps in non-trivial ways? I havenbt run across one but I havenbt looked too hard yet. Cheers, Eliot -- Eliot Kimber http://contrext.com
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