Eliminate Circular Reference Tableau/Order of Calculation

Question

I am new here but hoping that I can get some better answers than on the Tableau community.

I am working to create a calculator tool within Tableau. It is a bit of a strange thing to do within Tableau but so far it is working well until I came across this problem.

I want to calculate a tax asset value available for each year and applied (or used) for each year then calculate the remaining value and carry it forward to the next year.

The calculation is: (best seen in the Tableau image)

Added tax deduction (D) =

IF EBITDA**(A)**<0 then EBITDA (A) * - 1 + Depreciation (B)

ELSE Depreciation (B)

Opening Balance (C) = lookup CF Tax Deduction (G) ,-1

Available Tax deduction (E) =

Opening Balance (OB tax) (C) + Added Tax Deduction (D)

Used Tax Deduction (F) =

IF EBITDA (A) >0 then min(Available Tax Deduction (E) , EBITDA (A) ) //-this is because you cannot deduct more tax than your income

Carryforward (CF Tax Deduction) (G) =

Available Tax Deduction (E) - Used Tax Deduction (F)

The calculation works in excel but creates a circular reference in Tableau.

Anyone have suggestions as to how to avoid the circular reference? As soon as I enter in the formula for Used Tax Deduction I get a circular reference. I am now trying to solve using tabpy/python as well.

How it looks in Excel

How it looks in Tableau

Answer 1

Lookup cannot handle recursiveness that you implicitly entered through the circular reference of G(t) to G(t-1). (Where G(t) means the carryforward of the current time period, and G(t-1) is the same of the previous period.) If I simplify a bit the above calculations, this is what I get: G(t) = E(t) - F(t) Then replacing E(t) gives: G(t) = C(t) + D(t) - F(t) Where C(t) is effectively G(t-1), therefore: G(t) = G(t-1) + D(t) - F(t)

Tableau's recommended solution for such recursive formulae is the PREVIOUS_VALUE(). Try to implement this G(t) = PREVIOUS_VALUE(0) + D(t) - F(t) or simply put: G = PREVIOUS_VALUE(0) + D - F (PREVIOUS_VALUE(0) starts with zero value in the first round of the calculations.)