Is there an "if -then - else " statement in XPath?

yossale_4038 0
Viewed 128229

It seems with all the rich amount of function in xpath that you could do an "if" . However , my engine keeps insisting "there is no such function" , and I hardly find any documentation on the web (I found some dubious sources , but the syntax they had didn't work)

I need to remove ':' from the end of a string (if exist), so I wanted to do this:

if (fn:ends-with(//div [@id='head']/text(),': '))
            then (fn:substring-before(//div [@id='head']/text(),': ') )
            else (//div [@id='head']/text())

Any advice?

if-statement xpath
7 Answers

Yes, there is a way to do it in XPath 1.0:

concat(
  substring($s1, 1, number($condition)      * string-length($s1)),
  substring($s2, 1, number(not($condition)) * string-length($s2))
)

This relies on the concatenation of two mutually exclusive strings, the first one being empty if the condition is false (0 * string-length(...)), the second one being empty if the condition is true. This is called "Becker's method", attributed to Oliver Becker (original link is now dead, the web archive has a copy).

In your case:

concat(
  substring(
    substring-before(//div[@id='head']/text(), ': '),
    1, 
    number(
      ends-with(//div[@id='head']/text(), ': ')
    )
    * string-length(substring-before(//div [@id='head']/text(), ': '))
  ),
  substring(
    //div[@id='head']/text(), 
    1, 
    number(not(
      ends-with(//div[@id='head']/text(), ': ')
    ))
    * string-length(//div[@id='head']/text())
  )
)

Though I would try to get rid of all the "//" before.

Also, there is the possibility that //div[@id='head'] returns more than one node.
Just be aware of that — using //div[@id='head'][1] is more defensive.

Tomalak_187710

The official language specification for XPath 2.0 on W3.org details that the language does indeed support if statements. See Section 3.8 Conditional Expressions, in particular. Along with the syntax format and explanation, it gives the following example:

if ($widget1/unit-cost < $widget2/unit-cost) 
  then $widget1
  else $widget2

This would suggest that you shouldn't have brackets surrounding your expressions (otherwise the syntax looks correct). I'm not wholly confident, but it's surely worth a try. So you'll want to change your query to look like this:

if (fn:ends-with(//div [@id='head']/text(),': '))
  then fn:substring-before(//div [@id='head']/text(),': ')
  else //div [@id='head']/text()

I do strongly suspect this may fix it however, as the fact that your XPath engine seems to be trying to interpret if as a function, where it is in fact a special construct of the language.

Finally, to point out the obvious, insure that your XPath engine does in fact support XPath 2.0 (as opposed to an earlier version)! I don't believe conditional expressions are part of previous versions of XPath.

user_443890

How about using fn:replace(string,pattern,replace) instead?

XPATH is very often used in XSLTs and if you are in that situation and does not have XPATH 2.0 you could use:

  <xsl:choose>
    <xsl:when test="condition1">
      condition1-statements
    </xsl:when>
    <xsl:when test="condition2">
      condition2-statements
    </xsl:when>
    <xsl:otherwise>
      otherwise-statements
    </xsl:otherwise>
  </xsl:choose>
jonelf_446200

Somewhat simpler XPath 1.0 solution, adapted from Tomalek's (posted here) and Dimitre's (here):

concat(substring($s1, 1 div number($cond)), substring($s2, 1 div number(not($cond))))

Note: I found an explicit number() was required to convert the bool to an int otherwise some XPath evaluators threw a type mismatch error. Depending on how strict your XPath processor is type-matching you may not need it.

user_39360650
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