[rrd-users] Simple RPN with EXP goes wrong

[Steve Shipway]

> Example: 3*x^2
> I thought this would be in rpn: 3,x,2,EXP,*
> However, rrdtool is complaining "ERROR: RPN final stack size != 1"

EXP is the reverse of the natural log, sometimes called 'antilog' ; IE, e^X, NOT X squared.  It also does not take a second argument as it always uses e (2.71828etc). Thus, your RPN above ens up with two items in the stack (3, and x(e^2))

If you want to raise X to the power of Y, then there is no single RPN function to do it.  Instead, you need to work through logarithms.

e^( log(x) + log(y) ) == x * y ( a long way to go about it )

e^( log(x) * y ) == x ^ y

Or, in RPN, 

x,LOG,y,*,EXP

I agree it would be helpful to have a new command (eg. 'x,y,POW') to do this in a single operation, but nobody has submitted it yet...

Steve

Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.shipway at auckland.ac.nz
Ph: +64 9 373 7599 ext 86487