Guide for Module Developers — EnergyPlus 9.6

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Code Readability vs. Speed of Execution[LINK]

Programmers throughout time have had to deal with speed of code execution and it’s an ongoing concern. However, compilers are pretty smart these days and, often, can produce speedier code for the hardware platform than the programmer can when he or she uses “speed up” tips. The EnergyPlus development team would rather the code be more “readable” to all than to try to outwit the compilers for every platform. First and foremost, the code is the true document of what EnergyPlus does – other documents will try to explain algorithms and such but must really take a back seat to the code itself.

However, many people may read the code – as developers, we should try to make it as readable at first glance as possible. For a true example from the code and a general indication of preferred style, take the case of the zone temperature update equation. In the Engineering Reference document, the form is recognizable and usual:

And, this equation appears in the code (ZoneTempPredictorCorrector Module), as:

ZT(ZoneNum) = (CoefSumhat + CoefAirrat*(3.0*ZTM1(ZoneNum) - (3.0/2.0)*ZTM2(ZoneNum) &

+ (1./3.)* ZTM3(ZoneNum))) &

/ ((11.0/6.0)*CoefAirrat+CoefSumha)

Somewhat abbreviated here due to lack of page width but still recognizable from the original. A better version would actually be:

ZT(ZoneNum) = (CoefSumhat - CoefAirrat*(-3.0*ZTM1(ZoneNum) + (3.0/2.0)*ZTM2(ZoneNum) &

- (1./3.)* ZTM3(ZoneNum))) &

/ ((11.0/6.0)*CoefAirrat+CoefSumha)

Whereas the natural tendency of programming would lead to the less readable:

ZT(ZoneNum) = (CoefSumhat + CoefAirrat*(3.0*ZTM1(ZoneNum) – 1.5*ZTM2(ZoneNum) + .333333* ZTM3(ZoneNum))) &

/ (1.83333*CoefAirrat+CoefSumha)

The final version is a correct translation (more or less) from the Engineering/usual representation but much harder to look at in code and realize what is being represented.

Speed of Execution[LINK]

A critical consideration in speed of execution is character string comparisons. These are typically quite slow and should not be used in the core routines (i.e. those that are executed every zone or hvac time step). An alternative to string comparisons is to define module-level integer parameters, equate a string to a parameter during the initial subroutine call (e.g. GetInput), and then do integer comparisons through the remainder of the calls to the module. Doing this does not deter readability, yet assists in reducing execution time.

For example, in the module shown previously (Module Fans), the parameters for fan types are set as Integers:


INTEGER, PARAMETER :: FanType_SimpleConstVolume = 1


INTEGER, PARAMETER :: FanType_SimpleOnOff = 3

INTEGER, PARAMETER :: FanType_ZoneExhaust = 4

During the GetInput, string types are shown (this is getting these objects):


SimpFanNum,AlphArray, &


. . .

Fan(FanNum)%FanName = AlphArray(1)

Fan(FanNum)%FanType = ‘SIMPLE’

. . .

Fan(FanNum)%Control = ‘CONSTVOLUME’

Fan(FanNum)%FanType_Num = FanType_SimpleConstVolume

Then, during the simulation the integer parameters are used:

! Calculate the Correct Fan Model with the current FanNum

IF (Fan(FanNum)%FanType_Num = = FanType_SimpleConstVolume) THEN

Call SimSimpleFan(FanNum)

Else IF (Fan(FanNum)%FanType_Num = = FanType_SimpleVAV) THEN

Call SimVariableVolumeFan(FanNum)

Else If (Fan(FanNum)%FanType_Num = = FanType_SimpleOnOff) THEN

Call SimOnOffFan(FanNum)

Else If (Fan(FanNum)%FanType_Num = = FanType_ZoneExhaust) THEN

Call SimZoneExhaustFan(FanNum)

End If

This does not detract from code readability at all but execution is much speedier with this versus the string comparisons.