# Hash into Two Arguments (for REXML)

**URL:** <https://www.ruby-forum.com/t/hash-into-two-arguments-for-rexml/85152>\
**Category:** Ruby\
**Created:** [February 10, 2007, 5:49pm UTC](https://www.ruby-forum.com/t/hash-into-two-arguments-for-rexml/85152 "2007-02-10T17:49:33Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![keith2koool](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/67e7ee/32.png) [@keith2koool](https://www.ruby-forum.com/u/keith2koool)\
**Post date:** [February 10, 2007, 5:49pm UTC](https://www.ruby-forum.com/t/hash-into-two-arguments-for-rexml/85152/1 "2007-02-10T17:49:33Z")

</div>

Hey,

I’m storing my XML namespaces for REXML as Hashes (for XPath.first [1])  
like so:

RDF\_NS = {“rdf” =\> “[http://www.w3.org/1999/02/22-rdf-syntax-ns#](http://www.w3.org/1999/02/22-rdf-syntax-ns#)”}

When I need to do an XPath lookup, these work great (especially as I  
can merge() more than one together), but when I have to create new  
Elements and add their namespaces, the Hash isn’t what’s expected[2]  
(it needs two distinct args).

Is there a cooler way to get from a Hash to two args other than my  
\*Hash.to\_a.flatten ?

require ‘rexml/document’

# =\> true

RDF\_NS = {“rdf” =\> “[http://www.w3.org/1999/02/22-rdf-syntax-ns#](http://www.w3.org/1999/02/22-rdf-syntax-ns#)”}

# =\> {“rdf”=\>“[http://www.w3.org/1999/02/22-rdf-syntax-ns#](http://www.w3.org/1999/02/22-rdf-syntax-ns#)”}

a = REXML::Element.new “rdf:RDF”

# =\> rdf:RDF/

a.add\_namespace(\*RDF\_NS.to\_a.flatten)

# =\> \<rdf:RDF xmlns:rdf=‘[http://www.w3.org/1999/02/22-rdf-syntax-ns#](http://www.w3.org/1999/02/22-rdf-syntax-ns#)’/\>

Thanks,  
Keith

1. namespaces: If supplied, a Hash which defines a namespace mapping:  
XPath.first( node, “a/x:b”, { “x”=\>“[http://doofus](http://doofus)” } )  
[http://www.germane-software.com/software/rexml/doc/classes/REXML/XPath.html#M000452](http://www.germane-software.com/software/rexml/doc/classes/REXML/XPath.html#M000452)

2. add\_namespace( prefix, uri=nil )

Adds a namespace to this element.  
prefix: the prefix string, or the namespace URI if uri is not  
supplied  
uri: the namespace URI. May be nil, in which prefix is used as the  
URI

a.add\_namespace(“foo”, “bar”)  
[http://www.germane-software.com/software/rexml/doc/classes/REXML/Element.html#M000490](http://www.germane-software.com/software/rexml/doc/classes/REXML/Element.html#M000490)

---

<div class="post-metadata">

**Author:** ![keith2koool](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/67e7ee/32.png) [@keith2koool](https://www.ruby-forum.com/u/keith2koool)\
**Post date:** [February 11, 2007, 1:16pm UTC](https://www.ruby-forum.com/t/hash-into-two-arguments-for-rexml/85152/2 "2007-02-11T13:16:10Z")

</div>

On 10.02.2007 17:48, Keith F. wrote:

> (it needs two distinct args).  
> a.add\_namespace(\*RDF\_NS.to\_a.flatten)
> 
> # =\> \<rdf:RDF xmlns:rdf=‘[http://www.w3.org/1999/02/22-rdf-syntax-ns#](http://www.w3.org/1999/02/22-rdf-syntax-ns#)’/\>

Here’s a variant, but this is not very nice either:

a.add\_namespace(\*RDF\_NS…each {|k,v| break k,v}}

> a.add\_namespace(“foo”, “bar”)  
> [http://www.germane-software.com/software/rexml/doc/classes/REXML/Element.html#M000490](http://www.germane-software.com/software/rexml/doc/classes/REXML/Element.html#M000490)

I get the feeling that you’re probably not using Hashes properly,  
especially since you mention you have a lot of them with one key value  
pair and merge them together.

As far as I can see the most reasonable approach would be to have a  
single Hash that records all namespaces. That would be the Hash that  
you pass to XPath.first.

When you want to add a specific namespace to an element, you would then  
rather do something like this:

a.add\_namespace(“foo”, namespaces[“foo”])

If you want to avoid double typing you could do

def namespaces.pair(key)  
(val = self[key]) ? [key, val] : nil  
end

or

def namespaces.pair(key)  
val = self[key] and [key, val]  
end

and then

a.add\_namespace(\*namespaces.pair(“foo”))

Or you reverse the logic and do

def namespaces.add\_ns(elem, ns)  
val = self[ns] and elem.add\_namespace(ns, val)  
end

Kind regards

robert