| By Dimitre Novatchev |
| Language | XPATH |
| Category | designpatterns, xml |
| Posted | 28 Mar 2001 |
| Updated | 01 Apr 2001 |
| Summary | ||||
| This snippet shows how to generate in a single XPath expression a string, which depends on some other boolean XPath expression. It may be easily generalised to generate a string depending on a condition, which has more than 2 outcomes. | ||||
| I love XPath for its power to select a node-set based on complex conditions. | ||||
| At the same time such capabilities for specifying a string as opposed to a nodeset, seem non-existant. How often have you had to use a verbose multi-line xsl:choose construct just to specify that "in case1 use string1, in case2 use string2, ..., in caseN use stringN"? | ||||
| And what if you have decided not to use an XSLT transformation but only the DOM methods -- then you don't have even xsl:choose to use... | ||||
| In all such cases we feel the need of a technique, which would allow us to specify in a single XPath expression a string, which depends on condition(s). | ||||
| Here's how to do it: If you remember another of my snippets for advanced sorting of dynamically constructed strings | ||||
| (http://www.vbxml.com/snippetcentral/main.asp?view=viewsnippet&id=v20010227100924), | ||||
| there I used the concat() function with two operands. The two operands were specified in such a way, that for any different pair of values they'd have, one would be a nonempty string, and the other would be the empty string. | ||||
| We'll use the same approach now. We want an XPath expression, which returns a string when some given condition is true, and returns the empty string if this same condition is false. | ||||
| We can think of "true" as "1" and of "false" as "0". | ||||
| Zero comes handy -- the empty string has a length of zero. | ||||
| But how to fit "1" to any string? | ||||
| And which string - handling function can we use? | ||||
| OK, substring() seems quite convenient. And here's the trick: we can use substring() with only two arguments. | ||||
| substring(str,nOffset) | ||||
| will return the (remainder of the) string str starting at offset nOffset. | ||||
| In particular: | ||||
| substring(str,1) returns the whole string | ||||
| substring(str, nVeryLargeNumber) will return the empty string, if nVeryLargeNumber is guaranteed to be greater than any possible string length. | ||||
| Do we know such a number? Hmm... Infinity? | ||||
| So, the expression we might use would be: | ||||
| concat( substring(str1,exp(Condition)), substring(str2,exp(not(Condition)) ) | ||||
| and we want exp(Condition) to be 1 if Condition is true, and exp(Condition) to be Infinity if Condition is false. | ||||
| A last moment of thought and... Here it is: | ||||
| We express exp(Condition) as: | ||||
| 1 div Condition | ||||
| Because a boolean expression is first converted to a number (true -> 1, false -> 0), we get exactly: | ||||
| exp(true) = 1 exp(false) = Infinity. | ||||
| To summarise: The XPath expression returning Str1 if a condition Cond is true and returning Str2 if this same condition Cond is false -- this is: | ||||
| concat( substring(Str1,1 div Cond), substring(Str2,1 div not(Cond)) ) | ||||
| The technique I described can easily be generalised to specify an XPath expression, which generates one of given N strings depending on the value of another XPath expression, which has exactly N possible different values. | ||||
| I leave this to you as an exercise. | ||||
| To illustrate what we have done, here's a small example. I want to have a template, which generates the text: "My girlfriend" when it is passed a parameter "Mary" and to generate the text "Some other girl" if the value of the parameter is not "Mary". | ||||
| Of course, no xsl:if -s or xsl:when -s are allowed. | ||||
| Here's the code, and when applied to any xml source document, it generates: | ||||
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| XML Source is N/A | ||||
| XSL Stylesheet | ||||
| XSL Stylesheet | ||||
| XML/HTML Result: | ||||
| Text Result: | ||||