VB Forum / General 2 / February 2004

You'll have to write a parser using a loop.  Use Mid$() to step through the text
and pare each number, letter, and operator into separate variables.  The parser
would read from the string until the type of character changed (alpha, numeric,
whitespace, operator), then place that group of characters into a variable.

Unless you specifically need the parts of the expression, you could use an
expression evaluator plug-in to perform the operation.

Even small parsers are a lot of work because of the number of things that must
be tracked. The type of character can be alpha, numeric including formatting
(comma, decimal, exponent, scientific notation), operator (including
parentheses), whitespace, and the set of characters that are invalid in the
format you are using.

I've written one parser (in C++) to use with an existing calculator engine
(textbook code that had its own set of bugs).  If you'd like the C++ code as a
reference, send email (fix the address in this message or go to the page in my
signature and use the webmaster link).

More about me: http://www.jecarter.com/
VB3/VB6/C/PowerBasic source code: http://www.jecarter.com/programs.html
Freeware for the Palm with NS Basic source code: http://nsb.jecarter.com
Drivers for Pablo graphics tablet and JamCam cameras: http://home.earthlink.net/~mwbt/
johnecarter at@at mindspring dot.dot com. Fix the obvious to reply by email.

The following subroutine assumes the equations will always have constant
coefficients; the signs separating the variable and its coefficient will
only be plus or minus signs; the order of the terms is the same as the order
of your variable letter assignments to var1, var2, etc. (you might want to
consider using an array instead of hard-coded, sequential variable names);
the equal sign along with the term following it is always at the end of the
equation; and, finally, if a term is "missing", it will still be shown in
the equation with a coefficient of zero. I'm thinking a routine like this
would work (created off the top of my head):

Sub AssignCoefficients(ByVal Equation As String, _
                    ParamArray Values())
 Dim Index As Long
 Dim Position As Long
 Dim EqualSign As Long
 Dim BeginningOfNumber As Long
 Dim EndOfNumber As Long
 Equation = Replace$(Equation, " ", "")
 EqualSign = InStr(Equation, "=")
 Values(UBound(Values)) = Mid$(Equation, EqualSign + 1)
 Position = 1
 BeginningOfNumber = Position
 Do While Index < UBound(Values)
   Do While Position <= EqualSign
     If Mid$(Equation, Position, 1) Like "[!0-9+-]" Then
       EndOfNumber = Position - 1
       Exit Do
     End If
     Position = Position + 1
   Loop
   Values(Index) = Mid$(Equation, BeginningOfNumber, _
                        EndOfNumber - BeginningOfNumber + 1)
   Index = Index + 1
   Position = Position + 1
   BeginningOfNumber = Position
 Loop
End Sub

And an example of calling it, using the names for the coefficients that you
post showed, would be

Private Sub Command1_Click()
 Dim a1 As Double
 Dim b1 As Double
 Dim c1 As Double
 Dim d1 As Double
 Dim e1 As Double
 ' Assume TextBox with entry of "10w + 7x - 9y + 8z = 10
 AssignCoefficients Text1.Text, a1, b1, c1, d1, e1
 ' Show that the assignments took place
 Debug.Print a1
 Debug.Print b1
 Debug.Print c1
 Debug.Print d1
 Debug.Print e1
End Sub

You might want to consider using an array for those coefficient variables.
If you do, simply removed the Parameter keyword from the Sub declaration,
change the Values type to Double and pass that array into the Sub instead.

Rick - MVP