Feed back views are a way to run part of the model in a loop adjusting model inputs for each iteration and providing back to the view of the model outputs required. The reasons for using feedback views include handling model feedback links, and trying to get model outputs to track on some know data. There are many other ways of using feed back too.

There are a few built in tool variables that are available to the view writer to track the feedback process and terminate the loop when ready they are:

• $fbViewIteration - the iteration number of the loop
• $fbViewMaxIters - the maximum number of iterations requested by the user
• $fbViewTolerance - the convergence tolerance requested by the user

Note:

• You can declare regular local variables in a feedback view, however be aware that local variables are wiped out at the start of each iteration. If you want to reuse you local variables for each iteration you need to declare it as keep variable, which remain available to the view for every iteration. Usually you create keep variables on the first iteration inside an “if clause” as seen below:

keep A[]

if ($fbViewIteration)
  A[] = create(...)
endif

Here is the code for the .v and .t files for a simple feedback view that modifies a model input until the model output matches some given “golden” value in the last time slice.

viewName.v:

varToAdjust F
modelCalculatedVal	U
goldenValToTrackOn	U

viewname.t:

creview varToAdjust[dims][]
view modelCalculatedVal[dims][]
view goldenValToTrackOn[dims][]

! These are the built in variables available in the .t file
! $fbViewIteration
! $fbViewMaxIters
! $fbViewTolerance

if $background == false
	say ("This view requires a background.")
	quit (-1)
endif

keep percentDecreaseScnLI[dims]
keep goldenToModelRatioOrig[dims]

local goldenToModelRatio[dims] = goldenValToTrackOn[dims][1] / modelCalculatedVal[dims][1]

if $fbViewIteration == 1
	say ("Iteration: 1")

	! Make your adjustment for the first iteration -- in this case we will decrease the value by some percent ...this is just an example
	local percentDecrease_bck[dims] = create (; dimFrom1=varToAdjust[dims][1], dim=SEQ;time:1981:2006:1;year, data=1)
	percentDecrease_scn[dims] = copyshape (percentDecrease_bck[dims])
	percentDecrease_scn[dims] = table (;with=percentDecrease_bck[dims], title="..enter required percent decrease..")

	varToAdjust[dims][2] = percentDecrease_scn[dims] / 100 * -1 + 1 * varToAdjust[dims][1]
	goldenToModelRatioOrig[dims] = goldenToModelRatio[dims]
        percentDecreaseScnLI[dims] = percentDecrease_scn[dims]
	quit (1)
else
	say ("Iteration: ", $fbViewIteration)
	
	local goldenToModelRatioLI[dims] = goldenToModelRatio[dims]

	! Calculate the adjusted variable value 
	local ratioDiff[dims] = percentDecreaseScnLI[dims] - goldenToModelRatioOrig[dims]

	local percentDecrease_scn[dims] = 1 - goldenToModelRatioOrig[dims] * percentDecreaseScnLI[dims] / ratioDiff[dims]

	varToAdjust[dims][2] = percentDecrease_scn[dims] / 100 * -1 + 1 * varToAdjust[dims][1]
	export (percentDecrease_scn[dims]; format=tool, file=$persistence/percentDecreaseScnLI)

	!check tolerances for convergence 
	!**NOTE** this one only checks for convergence in the last year!
	local tol[] = create(; data=$fbViewTolerance)
	local numNotConv[] = sum (boolgt (abs (extract (goldenToModelRatio[dims]; time:-0, shrink=on) - 1), tol[]))
	integer $numNotConv
	getobjinfo(numNotConv[], $numNotConv; info=dataElement)

	if ($numNotConv == 0) or ($fbViewIteration == $fbViewMaxIters)
		mumble ("   numNotConv = ", $numNotConv, " at iteration ",$fbViewIteration, " stopping due to ")
		if ($numNotConv == 0)
			say ("convergence")
		else
			say ("maximum iterations")
		endif		
		quit(0)
	else
		say ("   numNotConv = ", $numNotConv, " at iteration ",$fbViewIteration, " continuing")
		quit (1)
	endif
endif

quit (1) exits the current iteration.

quit (0) exits the entire feedback view.

Declare the feedback view in views.samm:

fbview trackEU compare trackEU.v trackEU.t  "Tracking Energy Use“ 20 0.001

where the line contains the following parameters:

type of view (= fbview)
view name
model scope
.v file name
.t file name
description
default number of iterations
convergence tolerance

Note that feedback views require a background scenario to run (i.e. $background must be true).

Also, the “current” instance identified in the table below refers to the instance of a use-type (U) object in a fbview which - in the first iterations contains the value for the background (or worksheet) scenario - and in subsequent scenarios contains the value corresponding to the most recent iteration. If the background value of such a variable is required in iterations greater than 1, the value should be persisted using a keep object during the first iteration.