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
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