How to pass a string with parameters for evaluation in Excel? - excel

Question

In VBA, is there a way to create a function which receives one of it's parameters as a string with a condition to be evaluated by an IF block?
This should give an idea of what I am looking for, but I now it's not that simple:
Function StringAsCondition(a As String) As Boolean
Dim result As Boolean
Dim b As Long
Dim c As Long
b = 4
c = 2
If a Then
result = True
End If
StringAsCondition = result
End Function
Sub Test()
Dim a As String
a = "b >= c"
Dim functionresult As Boolean
functionresult = StringAsCondition(a)
MsgBox functionresult
End Sub

Answer 1

The evaluation of the string is actually a rather easy task in the "fancy" programming languages (all but VBA). There, you would simply use some type of string formatting, replace and evaluate.
In VBA, you may build your own string formatting (credits Is there an equivalent of printf or String.Format in Excel) and use it:
Sub Test()
Dim condition As String
Dim b As Long, c As Long
b = 4
c = 2
condition = "{0} >= {1}"
Debug.Print Application.Evaluate(StringFormat(condition, b, c))
End Sub
Public Function StringFormat(mask As String, ParamArray tokens()) As String
Dim i As Long
For i = LBound(tokens) To UBound(tokens)
mask = Replace$(mask, "{" & i & "}", tokens(i))
Next
StringFormat = mask
End Function

Answer 2

My answer just tries to present the real solution (of #Vitiata) in a way to directly answer the request:
Function StringAsCondition(cond As String) As Boolean
Dim result As Boolean
Dim b As Long
Dim c As Long
b = 4
c = 2
result = CBool(Application.Evaluate(StringFormat(cond, b, c)))
StringAsCondition = result
End Function
Sub TestEV()
Dim a As String
Dim functionresult As Boolean
a = "{0} >= {1}" 'instead of a = "b >= c"
functionresult = StringAsCondition(a)
MsgBox functionresult
End Sub
Private Function StringFormat(mask As String, ParamArray tokens()) As String
Dim i As Long
For i = 0 To UBound(tokens)
mask = Replace$(mask, "{" & i & "}", tokens(i))
Next
StringFormat = mask
End Function

Answer 3

Allow placeholding variables in any order
Just in addition to #Vityata 's clever solution, a similar solution with a mask definition closer to common mathematical formulae with variables in any order.
So an expression (condition) of e.g.
y >= x*c would be transformed easily to a mask string like
expression = "{y} >= {x}*{c}"
Sub TestStringFormat()
' Purp: evaluate mask with place holding variables in any order
Dim expression As String
expression = "{y} >= {x}*{c}" ' "17 >= 4*2" ~~> True
Dim myVars As String: myVars = "c,x,y" ' "c,x,y", c x y
Debug.Print Application.Evaluate(StrFrm(expression, myVars, 2, 4, 17))
End Sub
Public Function StrFrm(mask As String, myVars As String, ParamArray tokens()) As String
' Purpose: mask with place holding variables in any order ' e.g. y >= x*c
' Note: modified by T.M.; credit to https://stackoverflow.com/users/246342/alex-k
' Site: https://stackoverflow.com/questions/17233701/is-there-an-equivalent-of-printf-or-string-format-in-excel
Dim vars: vars = Split(myVars, ",") ' variables c,x,y
Dim i As Long
For i = LBound(vars) To UBound(vars)
mask = Replace$(mask, "{" & vars(i) & "}", tokens(i))
Next
StrFrm = mask
Debug.Print StrFrm
End Function