User defined warning in custom VBA Function for EXCEL - excel

Question

I have defined, inside an Excel workbook, a function that calculates a physical property taking as an input a Temperature and a Pressure.
It operates interpolating discrete data from a T/P matrix on a dedicated spreadsheet. The matrix can be modified as needed.
In case one of the given input data falls outside the data matrix range, the function is forced to calculate using the boundary. For example: If discrete Temperatures data in the matrix are from 40°C to 90°C, in the case that a temperature below 40°C is given to the function as input it will proceed to calculate using 40°C.
What I would like to do is to give a warning inside the cell containing the function call. In order to warn the user that the result is extrapolated and may not be accurate. Something similar to the warnings that excel gives to you, inside the cells, when a number is formatted as text or a formula inside a cell is different from the formulas in cells of the same column or row.
Is it possible to do it?
For sake of clarity, I add the code I wrote:
Function Z(T As Double, P As Double)
Dim i As Integer, j As Integer, r As Integer, c As Integer
Dim T1 As Double, T2 As Double, P1 As Double, P2 As Double, Z1 As Double, Z2 As Double
Dim ckp As Boolean, ckt As Boolean
Dim wks As Worksheet
Set wks = ThisWorkbook.Sheets("Z")
i = 2
j = 2
ckp = False
cht = False
'check if T < T min; if yes, set T = T min
If T < wks.Cells(1, 2) Then
i = 3
T = wks.Cells(1, 2).Value
ckt = True
End If
'check if P < P min; if yes, set P = P min
If P < wks.Cells(2, 1) Then
j = 3
P = wks.Cells(2, 1).Value
ckp = True
End If
'if temperature is not below the minimum, find the column containing the T_
immediately above the given one.
If ckt = False Then
Do
i = i + 1
'Check if T is > T max; if a blank cell is found during seeking, T_
is set at maximum.
If wks.Cells(1, i) = "" Then
T = wks.Cells(1, i - 1)
i = i - 1
ckt = True
End If
Loop While wks.Cells(1, i) < T And ckt = False
End If
'if pressure is not below the minimum, find the row containing the P_
immediately above the given one.
If ckp = False Then
Do
j = j + 1
'Check if P is > P max; if a blank cell is found during seeking, P_
is set at maximum.
If wks.Cells(j, 1) = "" Then
P = wks.Cells(j - 1, 1)
j = j - 1
ckp = True
End If
Loop While wks.Cells(j, 1) < P And ckp = False
End If
'Calculate the function by sequentially using Line Passing Through Two_
Points (RDP)user defined function.
'T1 is the temperature immediately below the given one and T2 is the_
temperature immediately above the given one
'T will be between T1 and T2
'P1 is the pressure immediately below the given one and P2 is the pressure_
immediately above the given one
'P will be between P1 and P2
T1 = wks.Cells(1, i - 1)
T2 = wks.Cells(1, i)
P1 = wks.Cells(j - 1, 1)
P2 = wks.Cells(j, 1)
'Calculate function at T1 and P
Z1 = RDP(P1, P2, wks.Cells(j - 1, i - 1), wks.Cells(j, i - 1), P)
'Calculate function at T2 and P
Z2 = RDP(P1, P2, wks.Cells(j - 1, i), wks.Cells(j, i), P)
'Calculate function at T and P
Z = RDP(T1, T2, Z1, Z2, T)
End Function
I also add a snapshot of the "Z" spreadsheet. Data are calculated by means of a process simulation software. In this way I can use the same spreadsheet in calculations where T and P vary between different ranges and steps, being the function still able to operate. All I need is the function to tell me: "I gave you a result but consider that you gave me an out of range parameter, so pay attention", without stopping processing data (it is used in hundreds of cells, together with other similar functions calculating other parameters)
In the "Z" spreadsheet snapshot, T are in the first row and P in the first column. In "J2" cell you can see the result inputing a Temperature of 100°C and a pressure of 42.5 bar (average between 40 and 45). The result is an average between the Z "90°C and 40 bar" and the Z "90°C and 45 bar".
I hope the issue to be more clear, now.

Answer 1

I found a solution to the posted problem and I share it, hoping that it could be useful. Even if it does not involve warning messages, the user is warned that something is not optimal and the given result could be not accurate. I proceeded in this way:
1) I modified the function to give a variant (1,2) array as a result. The first data is the actual function output, while the second one is a boolean value which can be "TRUE" if the calculation has been forced (not accurate) or "FALSE" otherwise.
2) I asked excel to evaluate "INDEX(Z(cellA,cellB),2)" to conditionally format the cell where the function is used, using the same arguments of the function inside the cell.
In this way, every time the function has been forced to give an output the cell results to be formatted in a different way (for example, with bold and red character).
Here below the modified Function:
Function Z(T As Double, P As Double)
Dim i As Integer, j As Integer, r As Integer, c As Integer
Dim T1 As Double, T2 As Double, P1 As Double, P2 As Double, Z1 As Double, _
Z2 As Double
Dim ckp As Boolean, ckt As Boolean
Dim wks As Worksheet
Dim returnval(2)
Set wks = ThisWorkbook.Sheets("Z")
i = 2
j = 2
ckp = False
ckt = False
'check if T < T min; if yes, set T = T min
If T < wks.Cells(1, 2) Then
i = 3
T = wks.Cells(1, 2).Value
ckt = True
End If
'check if P < P min; if yes, set P = P min
If P < wks.Cells(2, 1) Then
j = 3
P = wks.Cells(2, 1).Value
ckp = True
End If
'if temperature is not below the minimum, find the column containing the T_
immediately above the given one.
If ckt = False Then
Do
i = i + 1
'Check if T is > T max; if a blank cell is found during seeking, T _
is set at maximum.
If wks.Cells(1, i) = "" Then
T = wks.Cells(1, i - 1)
i = i - 1
ckt = True
End If
Loop While wks.Cells(1, i) < T And ckt = False
End If
'if pressure is not below the minimum, find the row containing the P_
immediately above the given one.
If ckp = False Then
Do
j = j + 1
'Check if P is > P max; if a blank cell is found during seeking, P_
is set at maximum.
If wks.Cells(j, 1) = "" Then
P = wks.Cells(j - 1, 1)
j = j - 1
ckp = True
End If
Loop While wks.Cells(j, 1) < P And ckp = False
End If
'Calculate the function by sequentially using Line Passing Through Two_
Points (RDP)user defined function.
'T1 is the temperature immediately below the given one and T2 is the_
temperature immediately above the given one
'T will be between T1 and T2
'P1 is the pressure immediately below the given one and P2 is the_
pressure immediately above the given one
'P will be between P1 and P2
T1 = wks.Cells(1, i - 1)
T2 = wks.Cells(1, i)
P1 = wks.Cells(j - 1, 1)
P2 = wks.Cells(j, 1)
'Calculate function at T1 and P
Z1 = RDP(P1, P2, wks.Cells(j - 1, i - 1), wks.Cells(j, i - 1), P)
'Calculate function at T2 and P
Z2 = RDP(P1, P2, wks.Cells(j - 1, i), wks.Cells(j, i), P)
'Calculate function at T and P
returnval(0) = RDP(T1, T2, Z1, Z2, T)
If ckt = True Or ckp = True Then
returnval(1) = True
Else
returnval(1) = False
End If
Z = returnval
End Function