xsl-list
[Top] [All Lists]

[xsl] Subtle (or not?) Map Processing Bug in XQuery

2022-01-07 10:19:04
I’m 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 there’s 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 I’d mention it 
here just in case it throws anyone else in the future (including future me).

 

In my XQuery script I’m building a map from all the image files in a directory 
tree (using BaseX’s file extensions):

 

let $files := file:list($rootDir, true(), ‘*.png’)

let $imageMap := map:merge(
for $file in $files

map:entry(local:getFilename($file), map{‘path’ : $file, ‘keyname’, 
local:getKeyName($file))

)

 

At this point $imageMap is a map where each entry’s 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)(‘keyname’)))

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 entry—what 
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 “$orphanMap($key)” 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 I’d 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)(‘keyname’)))

then map:entry($key, $orphanMap($key))

else ()

)

 

I’m also wondering if there’s any general source of XQuery coding patterns for 
working with maps in non-trivial ways? I haven’t run across one but I haven’t 
looked too hard yet.

 

Cheers,

 

Eliot

--

Eliot Kimber

http://contrext.com

 

 
--~----------------------------------------------------------------
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
EasyUnsubscribe: http://lists.mulberrytech.com/unsub/xsl-list/1167547
or by email: xsl-list-unsub(_at_)lists(_dot_)mulberrytech(_dot_)com
--~--
<Prev in Thread] Current Thread [Next in Thread>