Re: A bug of calc record?

[Dirk Zimoch <dirk.zimoch@psi.ch>]

Hi Geyang,

One should generally avoid to accumulate floating point values which have an
infinite number digits in dual representation, such as 0.1 or 0.2. It is like
adding 0.3333 instead of 1/3 in decimal. Integer values are much better for
counting than floats.

Floating point values are very bad in:
* accumulating small numbers to a large sum
* subtracting two numbers with a small difference
* deciding if two numbers are equal

Adding and subtracting is much more sensitive to rounding errors than
multiplying and dividing.

Dirk

Tim Mooney wrote:
> This is not actually a problem. 1.38777878078145e-16 is an excellent
> approximation to the desired value of zero, and an excellent approximation
> is the best you can ever hope for, with finite-precision arithmetic.
> You're accumulating round-off error each time you add or subtract 0.1.
> You could set PREC to, say, 6, and clients would only show 6 digits
> of the result.  (I don't know right off if dbpr honors PREC, but it's
> just a diagnostic routine, so it won't usually matter.)
> 
> Silver wrote:
>> hi, all:
>>
>> I am trying to use the record suggested by Ned Arnold. It's good work when
>> increment is 1. but when I change to 0.1, it has a strange problem. the db
>> file is like following:
>>
>> record(ai, "$(user):aiExample") { field(DESC, "Analog input") field(INP,
>> "$(user):calcExample.VAL  NPP NMS") } record(calc, "$(user):calcExample") { 
>> field(DESC, "Counter") field(SCAN,"1 second") field(FLNK,
>> "$(user):aiExample") field(CALC, "A-B>0?A>=0.9?0.8:A+0.1:A<0.1?0.1:A-0.1") 
>> field(INPA, "$(user):calcExample.VAL  NPP NMS") field(INPB,
>> "$(user):calcExample.LA NPP NMS") field(EGU, "Counts") }
>>
>> Following is dbtr message, we can see when the value change to 0.1, after
>> caculation, it's value change to VAL: 1.38777878078145e-16, can anyone give
>> some suggestion? thanks a lot in advance.
>>
>> epics> dbtr tcl:calcExample A: 0.2              ACKS: NO_ALARM      ACKT: YES
>> ADEL: 0 ALST: 0.1           ASG:                B: 0.3              BKPT:
>> 0x00 C: 0                CALC: A-B>0?A>=0.9?0.8:A+0.1:A<0.1?0.1:A-0.1
>>  D: 0                DESC: Counter       DISA: 0             DISP: 0
>>  DISS: NO_ALARM      DISV: 1             DTYP: <nil>         E: 0
>>  EGU: Counts         EVNT: 0             F: 0 FLNK:DB_LINK tcl:aiExample
>> G: 0                H: 0 HHSV: NO_ALARM      HIGH: 0             HIHI: 0
>> HOPR: 0 HSV: NO_ALARM       HYST: 0             I: 0 INPA:DB_LINK
>> tcl:calcExample.VAL NPP NMS INPB:DB_LINK tcl:calcExample.LA NPP NMS
>> INPC:CONSTANT       INPD:CONSTANT INPE:CONSTANT       INPF:CONSTANT
>> INPG:CONSTANT       INPH:CONSTANT INPI:CONSTANT       INPJ:CONSTANT
>> INPK:CONSTANT       INPL:CONSTANT J: 0                K: 0                L:
>> 0                LA: 0.2 LALM: 0.1           LB: 0.3             LC: 0
>> LCNT: 0 LD: 0               LE: 0               LF: 0               LG: 0
>>  LH: 0               LI: 0               LJ: 0               LK: 0
>>  LL: 0               LLSV: NO_ALARM      LOLO: 0             LOPR: 0
>>  LOW: 0              LSV: NO_ALARM       MDEL: 0             MLST: 0.1
>>  NAME: tcl:calcExample                   NSEV: NO_ALARM      NSTA: NO_ALARM
>>  PACT: 0             PHAS: 0             PINI: NO            PREC: 0
>>  PRIO: LOW           PROC: 0             PUTF: 0             RPRO: 0
>>  SCAN: 1 second      SDIS:CONSTANT       SEVR: NO_ALARM      STAT: NO_ALARM
>>  TPRO: 0             TSE: 0              TSEL:CONSTANT       UDF: 0
>>  VAL: 0.1 epics> dbtr tcl:calcExample A: 0.1              ACKS: NO_ALARM
>> ACKT: YES           ADEL: 0 ALST: 1.38777878078145e-16              ASG:
>> B: 0.2 BKPT: 0x00          C: 0 CALC: A-B>0?A>=0.9?0.8:A+0.1:A<0.1?0.1:A-0.1
>> D: 0 DESC: Counter       DISA: 0             DISP: 0             DISS:
>> NO_ALARM DISV: 1             DTYP: <nil>         E: 0                EGU:
>> Counts EVNT: 0             F: 0                FLNK:DB_LINK tcl:aiExample
>>  G: 0                H: 0                HHSV: NO_ALARM      HIGH: 0
>>  HIHI: 0             HOPR: 0             HSV: NO_ALARM       HYST: 0
>>  I: 0                INPA:DB_LINK tcl:calcExample.VAL NPP NMS
>>  INPB:DB_LINK tcl:calcExample.LA NPP NMS INPC:CONSTANT       INPD:CONSTANT
>>  INPE:CONSTANT       INPF:CONSTANT       INPG:CONSTANT       INPH:CONSTANT
>>  INPI:CONSTANT       INPJ:CONSTANT       INPK:CONSTANT       INPL:CONSTANT
>>  J: 0                K: 0                L: 0                LA: 0.1
>>  LALM: 1.38777878078145e-16              LB: 0.2             LC: 0
>>  LCNT: 0             LD: 0               LE: 0               LF: 0
>>  LG: 0               LH: 0               LI: 0               LJ: 0
>>  LK: 0               LL: 0               LLSV: NO_ALARM      LOLO: 0
>>  LOPR: 0             LOW: 0              LSV: NO_ALARM       MDEL: 0
>>  MLST: 1.38777878078145e-16              NAME: tcl:calcExample
>>  NSEV: NO_ALARM      NSTA: NO_ALARM      PACT: 0             PHAS: 0
>>  PINI: NO            PREC: 0             PRIO: LOW           PROC: 0
>>  PUTF: 0             RPRO: 0             SCAN: 1 second      SDIS:CONSTANT
>>  SEVR: NO_ALARM      STAT: NO_ALARM      TPRO: 0             TSE: 0
>>  TSEL:CONSTANT       UDF: 0              VAL: 1.38777878078145e-16
>>  epics> dbtr tcl:calcExample A: 1.38777878078145e-16                 ACKS:
>> NO_ALARM      ACKT: YES ADEL: 0             ALST: 0.1           ASG:
>> B: 0.1 BKPT: 0x00          C: 0 CALC: A-B>0?A>=0.9?0.8:A+0.1:A<0.1?0.1:A-0.1
>> D: 0 DESC: Counter       DISA: 0             DISP: 0             DISS:
>> NO_ALARM DISV: 1             DTYP: <nil>         E: 0                EGU:
>> Counts EVNT: 0             F: 0                FLNK:DB_LINK tcl:aiExample
>>  G: 0                H: 0                HHSV: NO_ALARM      HIGH: 0
>>  HIHI: 0             HOPR: 0             HSV: NO_ALARM       HYST: 0
>>  I: 0                INPA:DB_LINK tcl:calcExample.VAL NPP NMS
>>  INPB:DB_LINK tcl:calcExample.LA NPP NMS INPC:CONSTANT       INPD:CONSTANT
>>  INPE:CONSTANT       INPF:CONSTANT       INPG:CONSTANT       INPH:CONSTANT
>>  INPI:CONSTANT       INPJ:CONSTANT       INPK:CONSTANT       INPL:CONSTANT
>>  J: 0                K: 0                L: 0 LA: 1.38777878078145e-16
>> LALM: 0.1           LB: 0.1 LC: 0               LCNT: 0             LD: 0
>> LE: 0 LF: 0               LG: 0               LH: 0               LI: 0
>>  LJ: 0               LK: 0               LL: 0               LLSV: NO_ALARM
>>  LOLO: 0             LOPR: 0             LOW: 0              LSV: NO_ALARM
>>  MDEL: 0             MLST: 0.1           NAME: tcl:calcExample
>>  NSEV: NO_ALARM      NSTA: NO_ALARM      PACT: 0             PHAS: 0
>>  PINI: NO            PREC: 0             PRIO: LOW           PROC: 0
>>  PUTF: 0             RPRO: 0             SCAN: 1 second      SDIS:CONSTANT
>>  SEVR: NO_ALARM      STAT: NO_ALARM      TPRO: 0             TSE: 0
>>  TSEL:CONSTANT       UDF: 0              VAL: 0.1 epics>
>>
>> Best regards Geyang  2008-11-23
>>
>>
> 

-- 
Dr. Dirk Zimoch
Paul Scherrer Institut, WBGB/006
5232 Villigen PSI, Switzerland
Phone +41 56 310 5182