Re: PREC

[Eric Norum <norume@aps.anl.gov>]

Dirk Zimoch wrote:
> Hi all,
>
> we all know the PREC field of many records. Unforunately, its purpose is 
> not documented very precisely. The Application Developer's Guide says in 
> chapter 3.5.1 (EPICS Overview - Channel Access - Client Services):
>
> "In addition to requesting process variable values, any combination of 
> the following additional information may be requested:
> [...]
>   * Precision: Precision with which to display floating point numbers."
>
> In the Record Reference Manual, one can read e.g. in the ai chapter:
>
> "The PREC field determines the floating point precision with which to 
> display VAL."
>
> But what does this exactly mean? I think, the most intuitive sense is to 
> format the value like "%.<PREC>f". At least this is the default 
> behaviour of medm when using a text monitor. But why not like 
> "%.<PREC>g" or "%.<PREC>e"? And why does neither caget nor camonitor 
> honor PREC? And what about negative PREC values?

I'm not sure that the most intuitive sense is "%.<PREC>f".  I interpret the 
documentation to mean that PREC should result in "%.<PREC>g".

> I suggest the following:
> PREC >= 0 means to format the value like printf("%.*f",PREC,VAL);
> PREC < 0  means to format the value like printf("%.*e",-PREC,VAL);

I worry that there are a lot of tools out there that might not take well to 
negative PREC values.

> All tools including caget and camonitor should honor PREC.
>
> Any comments?

I'd argue that PREC should specify the number of significant digits and that 
the style (f/e) should be chosen according to the printf rules:

           If the precision is missing, 6 digits are given; if the
           precision is zero, it is treated as 1.  Style e is used if the
           exponent from its conversion is less than -4 or greater than or
           equal to the precision.  Trailing zeros are removed from the
           fractional part of the result; a decimal point appears only if it
           is followed by at least one digit.

With PREC=2, pi would print as 3.1, pi/100 as 0.031, pi/10000 as 0.00031, 
pi/100000 as 3.1e-5, pi*10 as 31 and pi*100 as 3.1e2.


--
Eric Norum                                 norume@aps.anl.gov
Advanced Photon Source                     Phone: (630) 252-4793
Argonne National Laboratory