Return a string value based on XPATH condition

Question

If I have the below XML, how to specify a xpath to return a string based on a condition. For example here if //b[@id=23] then "Profit" else "Loss"

<a>
  <b id="23"/>
  <c></c>
  <d></d>
  <e>
    <f id="23">
       <i>123</i>
       <j>234</j>
    <f>
    <f id="24">
       <i>345</i>
       <j>456</j>
    <f>
    <f id="25">
       <i>678</i>
       <j>567</j>
    <f>
  </e>
</a>

Answer 1

I. XPath 2.0 solution (recommended if you have access to an XPath 2.0 engine)

   (: XPath 2.0 has if ... then ... else ... :) 

   if(//b[@id=23]) 
     then 'Profit' 
     else 'Loss'

II. XPath 1.0 solution:

Use:

concat(substring('Profit', 1 div boolean(//b[@id=23])),
       substring('Loss', 1 div not(//b[@id=23]))
      )

Verification using XSLT 1.0:

This transformation:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>

 <xsl:template match="/">
  <xsl:value-of select=
   "concat(substring('Profit', 1 div boolean(//b[@id=23])),
           substring('Loss', 1 div not(//b[@id=23]))
          )"/>
 </xsl:template>
</xsl:stylesheet>

when applied on the provided XML document (corrected to make it well-formed):

<a>
    <b id="23"/>
    <c></c>
    <d></d>
    <e>
        <f id="23">
            <i>123</i>
            <j>234</j>
        </f>
        <f id="24">
            <i>345</i>
            <j>456</j>
        </f>
        <f id="25">
            <i>678</i>
            <j>567</j>
        </f>
    </e>
</a>

produces the wanted, correct result:

Profit

When we replace in the XML document:

<b id="23"/>

with:

<b id="24"/>

again the correct result is produced:

Loss

Explanation:

We use the fact that:

substring($someString, $N)

is the empty string for all $N > string-length($someString).

Also, the number Infinity is the only number greater than the string-length of any string.

Finally:

number(true()) is 1 by definition,

number(false()) is 0 by definition.

Therefore:

1 div $someCondition

is 1 exactly when the $someCondition is true()

and is Infinity exactly when $someCondition is false()

Thus it follows from this that if we want to produce $stringX when $Cond is true() and to produce $stringY when $Cond is false(), one way to express this is by:

concat(substring($stringX, 1 div $cond),
       substring($stringY, 1 div not($cond)),
      )

In the above expression exactly one of the two arguments of the concat() function is non-empty.