Numercial Reduction question

[Fernando Pérez]

> For example,  I might have a 31 by 12 array (days by months) of
> data.  Some cells have a special value in them to denote that there is
> no data for that day (either it's an invalid date, or there was no data
> for that day).  I want to be able to collapse along the day axis (31) to
> create monthly data, say summing the values, or averaging the
> values.

I just mentioned masked arrays. But if you don't have fancy requirements, 
just using where might do:

In[1]:= x=array([1,2,3,-99,4])
 
In[2]:= sum(x)
 
Out[2]= -89
 
In[3]:= sum(where(x==-99,0,x))
 
Out[3]= 10

cheers,

f