Cool and Bizzare error in IDL 5.2.1

["R.G. Stockwell" <stockwell(at)co-ra.comremove>]

Well, I've spent the last hour or so tracking down an error,
and finally found the cuplrit. Check out the result of the following
commands

IDL> help,current_mean
CURRENT_MEAN    DOUBLE    =       -1.0853418
IDL> help,newmean
CURRENT_MEAN    DOUBLE    =       -1.0853418


I have two variables, current_mean and newmean. There are no pointers
used, and the code is very straighforward (just iteratively calculating the
mean).
Note how  IDL>help,newmean give the result of current_mean, is that weird or
what??

The error is that when newmean was calculated, current mean was being
modified.
The code fragment that demonstrates this is:

 print,'current mean: ',current_mean
 newmean   =  (current_mean*npoints+newpoint)/(npoints+1)
 print,'current mean: ',current_mean

And the resulting output is:
current mean:       -1.0853418
current mean:      -0.13970473


I'm not sure how I was able to do this, but if it's repeatable, I think I
smell
some obsfucated IDL code in my future!

better get back to work, I just thought it was cool.  If any one can explain
this,
I'd be interested to hear it. But I'm guessing it will be difficult to
reproduce.

Cheers,
bob