Excel VBA - Assigning Variable to a User-Defined Formula Located in Some Cell Range

Question

In Cell(1,1) of one sheet, I have this formula: 5 * a ^ 2 + 3 * a ^ 1 + 1 * a + 5

In Cell(2,1) of the same sheet, I have another formula: 3 * b ^ 2 + 5 * b ^ 1 + 1 * b + 3

Screenshot of the formulas

I want to calculate values for these formulas and assign it to variables within VBA. For example, I've been trying something like this

Function calculate_y(a,b)

answer1 = [A1]
answer2 = [A2]

calculate_y = answer1 + answer2 

End Function  

But I keep getting errors. I have tried playing around with Evaluate() and characters like & but to no avail. The reason I'm not directly entering the formulas into vba is because these formulas will be changing. I don't want to have to update the formulas manually.

Answer 1

Use evaluate while replacing the variables in the string with the values provided:

Function calculate_y(a, b)

With Application.Caller.Parent
    answer1 = .Evaluate(Replace(.Range("A1"), "a", a))
    answer2 = .Evaluate(Replace(.Range("A2"), "b", b))
End With

calculate_y = answer1 + answer2

End Function

---

If you are not planning on using it as a UDF but calling it from vba, you should also pass the worksheet on which the formulas reside:

Function calculate_y(ws As Worksheet, a As Double, b As Double)

With ws
    answer1 = .Evaluate(Replace(.Range("A1"), "a", a))
    answer2 = .Evaluate(Replace(.Range("A2"), "b", b))
End With

calculate_y = answer1 + answer2

End Function