Calculate average value recursively

Hi,

I'm trying to calculate the average for a set of rainfall values in the 
following node-set:

<month number="1">
	<day number="1">
		<rainfall>1.5</rainfall>
	</day>
	<day number="2">
		<rainfall>0.5</rainfall>
	</day>
	<day number="3">
		<rainfall>TR</rainfall>
	</day>
	...
</month>

Usually, I'd use "sum(month/day/rainfall) div count(month/day)" to get 
the average rainfall, however, as null values are represented as "TR", 
that expression returns a NaN error.

I'm thinking that I would need a recursive method that can catch cases 
such as TR, appends either the rainfall value or 0 to a sequence, which 
it would send back recursively as a parameter until reaching the end and 
then dividing by the number of days? However, I'm not sure about how to 
do this, nor am I sure if it's the right solution!

Thanks in advance.

Regards,


Weiran.

-- 
Weiran Zhang

E-mail: weiran@xxxxxxxxxxx
Website: www.ejecutive.co.uk

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