Performing mathematical calculations when operator is a custom character

Question

What would be the proper way to perform a mathematical calculation when the operator is a variable character. In other words, the operation is not known at compile-time. The operation needs to be dynamically chosen at run-time.

For instance, let's say I need to calculate the sum of 20 + 40, but the + operator is stored in a Char variable, like this:

Dim operator As Char = "+"c    
Dim result As Long = 10 operator 20

what is a "custom char"? You generally dont do math on characters at all — Ňɏssa Pøngjǣrdenlarp, Nov 10 '15 at 21:17
Why is this tagged `C#`? Where is `c` defined? `10 iPlus 20`? What is that even supposed to do? — sab669, Nov 10 '15 at 21:22
@sab669 the `c` is the way to designate char since otherwise `"+"` would be a string — Ňɏssa Pøngjǣrdenlarp, Nov 10 '15 at 21:30
Are you asking how to define the "Operator" at run-time? IE, Maybe it is plus(+), maybe it is negate(-), maybe it is divide(/), etc??? — Steve, Nov 10 '15 at 21:32
I suspect this is almost a duplicate of this post, but not quite. Have a look ... http://stackoverflow.com/questions/13178637/evaluate-mathematical-expression-from-a-string-using-vb ..... — David Wilson, Nov 11 '15 at 21:41

Steven Doggart · Accepted Answer · 2015-11-11T15:15:15.903

The simple way would be to use a Select Case statement to perform the calculation using the correct operation based on the input character. For instance:

' Binary, as in not unary nor ternary
Public Function PerformBinaryOperation(operand1 As Long, operator As Char, operand2 As Long)
    Select Case operator
        Case "+"c
            Return operand2 + operand2
        Case "-"c
            Return operand2 - operand2
        ' ...
    End Select
End Function

The more advanced solution is to write an expression parser to analyze an expression string, such as "10 + 20", and turns it into an expression tree. If you don't want to go through the trouble of writing your own parser, you could use one of the available libraries to do it, such as NCalc.

Another alternative to expression tree parsing is to generate a snippet of .NET code, and then dynamically build and execute that code at run-time. For instance, if you did something like this:

Dim operator As Char = "+"c
Dim codeBuilder As New StringBuilder()
codeBuilder.AppendLine("Public Class MyOperation")
codeBuilder.AppendLine("    Public Function Calculate() As Long")
codeBuilder.AppendLine("         Return 10 " & operator & " 20")
codeBuilder.AppendLine("    End Function")
codeBuilder.AppendLine("End Class")
Dim myOperation As Object = ' ... compile code and instantiate object dynamically
Dim result As Long = myOperation.Calculate()

Traditionally, you would do that using the CodeDom classes to perform the dynamic compilation, but now with the new Roslyn compiler in .NET 4.6, you can use the new Emit functionality. Here is an article that introduces the various dynamic compilation options.

@ChrisDunaway Thanks, though you could have edited it yourself just as easily :) — Steven Doggart, Nov 11 '15 at 15:16

Performing mathematical calculations when operator is a custom character

1 Answers1