[rrd-users] Simple RPN with EXP goes wrong
Steve Shipway
s.shipway at auckland.ac.nz
Sun Feb 14 00:10:03 CET 2016
> 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
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