I’d like to define a function in Visual Basic which computes income tax in a given bracket. The inputs should be the income, the marginal tax rate, the lower bracket boundary, and – optionally – the upper bracket boundary. (For the top bracket there is no upper boundary).

Here’s how I went about it. First, I define a “ramp” function as follows:

```
Public Function ramp(x)
ramp = (x + Abs(x)) / 2
End Function
```

which is basically the same as IF(x<0,0,x). Then I define the function (in Dutch) for the tax as

```
Public Function schijfbelasting(inkomen, ondergrens, bovengrens, tarief)
schijfbelasting = ramp(tarief * (inkomen - ondergrens)) - ramp(tarief * (inkomen - bovengrens))
End Function
```

Here “inkomen”=income, “ondergrens”=lower bracket boundary, “bovengrens”=upper bracket boundary, “tarief”=marginal tax rate, and the output “schijfbelasting”=tax in the specified bracket.

This all works fine, except that I’d like to make the “bovengrens” (upper bracket boundary) optional using

```
Optional bovengrens
```

In Matlab, I would use the “nargin” (number of arguments) function to do something like the following:

```
Public Function schijfbelasting(inkomen, ondergrens, Optional bovengrens, tarief)
If nargin==4
schijfbelasting = ramp(tarief * (inkomen - ondergrens)) - ramp(tarief * (inkomen - bovengrens))
Elseif nargin==3
schijfbelasting = ramp(tarief*(inkomen-ondergrens))
End If
End Function
```

However, I’m not aware of a function similar to “nargin” in Visual Basic. It could also be something like “if the argument “bovengrens” is defined”. Does anybody know how to approach this problem? Thanks in advance.

P.S. I am aware that I can make the code ‘work’ by filling in a very large number for the bracket ‘boundary’ in the top bracket, but I do not consider this elegant coding.

You can use VBA’s `IsMissing`

function to test for optional parameters. Here’s an example:

```
Public Sub OptionalArg(arg1, Optional arg2)
Debug.Print arg1; IIf(IsMissing(arg2), "missing", arg2)
End Sub
```

Test it like this:

```
Sub Test()
OptionalArg 1
OptionalArg 1, 2
End Sub
```

### Answer：

Just test the arguments using the `IsMissing`

function:

```
If IsMissing(bovengrens) Then ...
```