Anyone who has used XSL will tell you that its nothing short of powerful. For a quick and dirty example of what can be done with XSL, just take a look at what Mark Pilgrim has done to his Atom feed (if you're interested in how he did it, you can read all about it).
A problem I ran into recently when trying to transform an XML document using XSL was that I only wanted my transformation to grab values that were unique. So for instance, consider the following example:
<?xml version="1.0" ?>
<celebrities>
<celebrity>
<name first="Marilyn" last="Monroe" />
<birthdate value="1926-06-01" />
</celebrity>
<celebrity>
<name first="Damon" last="Wayans" />
<birthdate value="1960-09-04" />
</celebrity>
<celebrity>
<name first="Marilyn" last="Manson" />
<birthdate value="1969-01-05" />
</celebrity>
</celebrities>
Now let's suppose I want to extract all the celebrity first names from that document, but I want each item in the list returned to be unique (i.e. I only want 2 names returned instead of 3). A quick search around the Web revealed a consensus as to what approach to take when tackling this problem – the only problem being that I could not, for the life of me, get it to work.
If you attempt to find a solution to this problem, you'll doubtless run across several references to XPath's preceding-sibling selector. The problem is, most people (myself included) can't get this to work in complex situations – mostly since there are so few samples of the selector's usage available online.
This story has a happy ending, though. While sifting through tons of preceding-sibling content, I happened to stumble upon an entirely different solution involving XSL's key element. This approach, which is similar to hashing all the values, is not only clearly documented online, but it's also relatively easy to follow.
The following XSL transformation will return the list I'm after:
...
<xsl:key name="names" match="name" use="@first"/>
<xsl:template match="/">
<xsl:for-each select="//name[generate-id() = generate-id(key('names',@first)[1])]">
<xsl:value-of select="@first"/>
</xsl:for-each>
</xsl:template>
...
As you can see, I first establish a key by the name of "names," telling it to match the name element using its first attribute. Then when it comes time to select each first attribute in the document, I generate an id for the current element and make sure it is equal to the id of the first first attribute in the defined group of keys. Make sense? Well, it doesn't have to, because it works, and that's more than I can say about the preceding-sibling approach.
If you have any questions about the key-based approach or can lend any insight regarding the preceding-sibling approach, please feel free to let me know.
Comments
There's a rather critical typo in the above. The line
generate-id(key('names',@first))[1])]">
should be
generate-id(key('names',@first)[1])]">
with only one close paren following the @first.
Bernie,
this can be done using "following" axis although "preceding" will not work. The example has been taken from Dave Pawson which is a wonderful resource for XSLT solutions.
Warm regards,
Abhinav
<location>
<state>xxxx</state>
</location>
<location>
<state>yyyy</state>
</location>
<location>
<state>xxxx</state>
</location>
The desired output is:
xxxx
yyyy
That is, duplicate values of state should not be printed. This can be done as follows.
<xsl:variable name="unique-list"
select="//state[not(.=following::state)]" />
<xsl:for-each select="$unique-list">
<xsl:value-of select="." />
</xsl:for-each>
Awesome tip...I found the preceding sibling thing a bunch of times and it never worked for me either :(
Anyway, I used your way and it works!
This was extremely helpful and worked perfectly (when the typo was corrected!)
Top tip.
If you're using XSLT 2, then instead you can use:
<xsl:for-each-group select="//state" group-by=".">
<xsl:sequence select="current-group()[1]" />
</xsl:for-each>
(updating Abhinav's example above)
Buddy - You saved my life ;)
Abhinav's tip worked like a charm for me!
I need unique list of combinatin of attributes city and state. I tried the above solution which didn't work. Any help will be welcomed.
<location>
<city>aaaa</city>
<state>xxxx</state>
</location>
<location>
<city>aaaa</city>
<state>yyyy</state>
</location>
<location>
<city>aaaa</city>
<state>yyyy</state>
</location>
<location>
<city>bbbb</city>
<state>xxxx</state>
</location>
<location>
<city>bbbb</city>
<state>xxxx</state>
</location>
<location>
<city>bbbb</city>
<state>yyyy</state>
</location>
Great advice. I have been looking awhile for a way to pick out unique elements and this works. Thanks.
Abhinav's tip is excellent! One quibble: the code as written will pick up all state elements, whether or not they are contained in location elements. It might be better to write:
Dude this article and those comments saved my life. Thank you!
Thanks, this is realy great helpfull refrence.
Whoops, guess I'll have to encode this comment myself. Here goes a second try :
A followup to Abhinav Maheshwari's comment above. If you'd like to do this with an attribute instead here is an example :
XML :
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" rel="nofollow" rel="nofollow" href="b.xsl"?>
<xml>
<location state="xxxx">a</location>
<location state="yyyy">b</location>
<location state="xxxx">c</location>
</xml>
XSL :
<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
<xsl:template match="/">
<xsl:variable name="unique-list" select="/xml/location[not(@state=following::location/@state)]" />
<xsl:for-each select="$unique-list">
<xsl:value-of select="@state" />
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
Thanks for the example using the attribute, it worked fine. But i need to set a parameter to get the attribute name so that i can use $attributeName to fetch the unique records based on the attribute. Any idea about this ?
if you add one more attribute to it than @state will contain values of both of the attribute
i.e
Works! Thank you very much.
However, all the future readers of above article would probably appreciate a correction regarding the typo mentioned by Jesse Weinstein in the second comment.
Btw: The links to the original documentation of the trick are broken.
Apart from that, it helped me very much and saved my day. Thanks again.
it is really excellent i used the xslt tranformation very well
Thanks to Jesse for pointing out the typo and to Tom for reminding me I need to update the original article. I've now updated it, so hopefully fewer people will be confused by the example given going forward.
Yeeesss! Thank you very much.
I believe that this is actually called the muenchian method.
http://www.jenitennison.com/xslt/grouping/muenchian.html
Just nice to know, and give credit where it's due.
I don't know if this will help anyone out in the future, but I have been struggling for a few hours with the following XML:
I wanted to select unique years and started off with the obvious xpath:
years/year[@value != preceding-sibling::year/@value]
However it only returned the 2009 node and no 2010 nodes. After much searching and messing around with XPath I finally made it work with only a very slight and curious tweak. I replaced the '!=' with a 'not', ie.:
years/year[not(@value = preceding-sibling::year/@value)]
This works a treat, but I con't really see what the difference between them is apart from a few hours of my life.
Hey ho! Maybe this will help someone else.
Cheers
Steve
Okay, I'll encode the XML myself for you to see:
<years>
<year value="2010"></year>
<year value="2010"></year>
<year value="2010"></year>
<year value="2009"></year>
</years>
Cheers
Steve
Hi,
Thankyou so much for the post. U saved me.
hi thanks.
It's working perfectly. I need to get distinct values from the various nodes and i finally got by using not(.following) sentax.
Thanks Again.
Still a great tip more than 6 years on :-)
Awesome, it worked! Thanks!
This was a lifesaver since I don't appear to have an XSLT 2.0 capable processor (still using classic ASP). Need to process the following nodes too now, so will have to figure that one.
XSLT is very elegant in my opinion.
Still good value. Its worked from me too
Thanks
Thank youv very much. Still valuable and helpful.
You could mention that this mechanism works also in selecting distinct child nodes instead of attributes. All you have to do is omit the @.
Have a good day,
Marco
Abhinav's sample saved my weekend. Thanks to all the contributors, this is such a useful article.
Thanks Abhinav.. Ur code saved me..
Also, a big thanks to Berni for creating this forum
U folks rock :D
Thanks heaps, this code was very useful.
Great Tutorial, honesty and clarity.
You have also saved my life.....either that or the Computer from having a swim in the ocean.
Need only loop first and second values
If there multiple unique value ..please help
Thanks
Post Comments
If you feel like commenting on the above item, use the form below. Your email address will be used for personal contact reasons only, and will not be shown on this website.
The following thread and site were helpful in understanding why my preceding-sibling wasn't working. I was having a similar problem, but wanted unique elements within a specific nodes, not the entire document. It turns that preceding-sibling itself returns a node-set, which is in the order of the document...which means logical usage always gets you the first member in the document. ...it's expained better in the links:
http://www.biglist.com/lists/xsl-list/archives/200302/msg00368.html
http://www.dpawson.co.uk/xsl/sect2/N1641.html#d2340e325
Permalink