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Date: Thu, 30 Jan 2014 03:58:10 +0000 (UTC)
From: "Michele Vivoda (JIRA)" <jira@apache.org>
To: issues@commons.apache.org
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Subject: [jira] [Commented] (JXPATH-163) String variables cannot be used as
 indices
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    [ https://issues.apache.org/jira/browse/JXPATH-163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13886230#comment-13886230 ] 

Michele Vivoda commented on JXPATH-163:
---------------------------------------

Hi,

>From XPath specification:

http://www.w3.org/TR/xpath/#NT-PredicateExpr

[8]   	Predicate	   ::=   	'[' PredicateExpr ']'	
[9]   	PredicateExpr	   ::=   	Expr

A predicate filters a node-set with respect to an axis to produce a new node-set. For each node in the node-set to be filtered, the PredicateExpr is evaluated with that node as the context node, with the number of nodes in the node-set as the context size, and with the proximity position of the node in the node-set with respect to the axis as the context position; if PredicateExpr evaluates to true for that node, the node is included in the new node-set; otherwise, it is not included.

A PredicateExpr is evaluated by evaluating the Expr and converting the result to a boolean. If the result is a number, the result will be converted to true if the number is equal to the context position and will be converted to false otherwise; if the result is not a number, then the result will be converted as if by a call to the boolean function. Thus a location path para[3] is equivalent to para[position()=3].

--------------

I read that if the Expr is a string, it is converted to true if the string is not empty, meaning 'any node' and so the first node is selected, if it were a "node set" selection instead of a "single node" then all nodes would have been selected. JXPath seems to comply with the spec.

You may probably consider to use  number() to force conversion to a number that then is checked against the position.

/se:someArray[number($var)]

 
> String variables cannot be used as indices
> ------------------------------------------
>
>                 Key: JXPATH-163
>                 URL: https://issues.apache.org/jira/browse/JXPATH-163
>             Project: Commons JXPath
>          Issue Type: Bug
>    Affects Versions: 1.3
>            Reporter: Alexander Koledzhikov
>            Priority: Minor
>         Attachments: commons-jxpath.patch
>
>
> If I have a string variable, I cannot use it as an index in an xpath query.
> For example, if var = "5", this query will return the 1st element, not the fifth:
> /se:someArray[$var]


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