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# evaluating string as numerical expression

Posted on 1999-07-22
Medium Priority
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Is there a built in function which will evaluate a string to  it's numerical/logical equivalent?  Or will I just have to write a parser to do the work myself?  I would like to be able to take a string (i.e. "5 + 1 <= 10 / 2") and have it return true or false appropriately.
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Question by:cwirrgan
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LVL 7

Expert Comment

ID: 1526822
Nopz, you will have to do something on your own.
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LVL 6

Accepted Solution

VBGuru earned 150 total points
ID: 1526823
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LVL 7

Expert Comment

ID: 1526824
Hmm apparently not.
0

LVL 6

Expert Comment

ID: 1526825
ya, these are only pointer not exact solution. You can develop on these
0

LVL 12

Expert Comment

ID: 1526826
In the old DOS days, the INPUT statement used to be able to do this sort of thing. You could put "2+3" in response to an "Input X" statement and the thing would give you "5" as the result. Don't think that will work anymore in the Visual world - so much for progress.

M

0

Author Comment

ID: 1526827
The links provided definitely give me a start.  I'll have to add the logical operators, but these samples will really help out.
0

LVL 18

Expert Comment

ID: 1526828
VERSION 4.00
Begin VB.Form Form1
Caption         =   "Form1"
ClientHeight    =   5880
ClientLeft      =   1140
ClientTop       =   1545
ClientWidth     =   6690
Height          =   6450
Left            =   1080
ScaleHeight     =   5880
ScaleWidth      =   6690
Top             =   1035
Width           =   6810
Begin VB.TextBox Text1
Height          =   375
Left            =   3240
TabIndex        =   1
Top             =   240
Width           =   2655
End
Begin VB.CommandButton Command1
Caption         =   "Calculate"
Height          =   375
Left            =   2520
TabIndex        =   0
Top             =   960
Width           =   1455
End
Begin VB.Label Label3
Height          =   375
Left            =   3240
TabIndex        =   4
Top             =   1680
Width           =   2295
End
Begin VB.Label Label2
Alignment       =   1  'Right Justify
Caption         =   "Value ="
Height          =   375
Left            =   840
TabIndex        =   3
Top             =   1680
Width           =   2295
End
Begin VB.Label Label1
Alignment       =   1  'Right Justify
Caption         =   "Expression : "
Height          =   375
Left            =   840
TabIndex        =   2
Top             =   240
Width           =   2295
End
End
Attribute VB_Name = "Form1"
Attribute VB_Creatable = False
Attribute VB_Exposed = False
Option Explicit
Dim e_input As String     ' Expression input string.
Dim e_tok As String       ' Current token kind.
Dim e_spelling As String  ' Current token spelling.
Dim e_error As Integer    ' Tells if syntax error occurred.

Function e_eval(ByVal s As String, value As Double) As Integer
' Initialize.
e_error = 0
e_input = s
Call e_nxt

' Evaluate.
value = e_prs(1)

' Check for unrecognized input.
If e_tok <> "" And Not e_error Then
MsgBox "syntax error, token = '" + e_spelling + "'"
e_error = -1
End If

e_eval = Not e_error
End Function

' e_prs
'   Parse an expression, allowing operators of a specified
'   precedence or higher. The lowest precedence is 1.
'   This function gets tokens with e_nxt and recursively
'   applies operator precedence rules.
Function e_prs(p As Integer) As Double
Dim n As Double    ' Return value.
Dim fun As String  ' Function name.

' Parse expression that begins with a token (precedence 12).
If e_tok = "num" Then
' number.
n = Val(e_spelling)
Call e_nxt
ElseIf e_tok = "-" Then
' unary minus.
Call e_nxt
n = -e_prs(11)    ' Operand precedence 11.
ElseIf e_tok = "not" Then
' logical NOT.
Call e_nxt
n = Not e_prs(6)  ' Operand precedence 6.
ElseIf e_tok = "(" Then
' parentheses.
Call e_nxt
n = e_prs(1)
Call e_match(")")
ElseIf e_tok = "id" Then
' Function call.
fun = e_spelling
Call e_nxt
Call e_match("(")
n = e_prs(1)
Call e_match(")")
n = e_function(fun, n)
Else
If Not e_error Then
MsgBox "syntax error, token = '" + e_spelling + "'"
e_error = -1
End If
End If

' Parse binary operators.
Do While Not e_error
If 0 Then  ' To allow ElseIf .
ElseIf p <= 11 And e_tok = "^" Then: Call e_nxt: n = n ^ e_prs(12)
ElseIf p <= 10 And e_tok = "*" Then: Call e_nxt: n = n * e_prs(11)
ElseIf p <= 10 And e_tok = "/" Then: Call e_nxt: n = n / e_prs(11)
ElseIf p <= 9 And e_tok = "\" Then: Call e_nxt: n = n \ e_prs(10)
ElseIf p <= 8 And e_tok = "mod" Then: Call e_nxt: n = n Mod e_prs(9)
ElseIf p <= 7 And e_tok = "+" Then: Call e_nxt: n = n + e_prs(8)
ElseIf p <= 7 And e_tok = "-" Then: Call e_nxt: n = n - e_prs(8)
ElseIf p <= 6 And e_tok = "=" Then: Call e_nxt: n = n = e_prs(7)
ElseIf p <= 6 And e_tok = "<" Then: Call e_nxt: n = n < e_prs(7)
ElseIf p <= 6 And e_tok = ">" Then: Call e_nxt: n = n > e_prs(7)
ElseIf p <= 6 And e_tok = "<>" Then: Call e_nxt: n = n <> e_prs(7)
ElseIf p <= 6 And e_tok = "<=" Then: Call e_nxt: n = n <= e_prs(7)
ElseIf p <= 6 And e_tok = ">=" Then: Call e_nxt: n = n >= e_prs(7)
ElseIf p <= 5 And e_tok = "and" Then: Call e_nxt: n = n And e_prs(6)
ElseIf p <= 4 And e_tok = "or" Then: Call e_nxt: n = n Or e_prs(5)
ElseIf p <= 3 And e_tok = "xor" Then: Call e_nxt: n = n Xor e_prs(4)
ElseIf p <= 2 And e_tok = "eqv" Then: Call e_nxt: n = n Eqv e_prs(3)
ElseIf p <= 1 And e_tok = "imp" Then: Call e_nxt: n = n Imp e_prs(2)
Else
Exit Do
End If
Loop

e_prs = n
End Function

' e_function.
'   Evaluate a function. This is a helper function to simplify
'   e_prs.
Function e_function(fun As String, arg As Double) As Double
Dim n As Double

Select Case LCase\$(fun)
Case "abs": n = Abs(arg)
Case "atn": n = Atn(arg)
Case "cos": n = Cos(arg)
Case "exp": n = Exp(arg)
Case "fix": n = Fix(arg)
Case "int": n = Int(arg)
Case "log": n = Log(arg)
Case "rnd": n = Rnd(arg)
Case "sgn": n = Sgn(arg)
Case "sin": n = Sin(arg)
Case "sqr": n = Sqr(arg)
Case "tan": n = Tan(arg)

'New functions
Case "arccos": n = arccos(arg)
Case "arcsin": n = arcsin(arg)

Case "arcsec": n = arcsec(arg)
Case "arccosec": n = arccosec(arg)
Case "arccot": n = arccot(arg)

Case "cot": n = cot(arg)
Case "cosec": n = cosec(arg)
Case "sec": n = sec(arg)

Case "sinh": n = sinh(arg)
Case "cosh": n = cosh(arg)
Case "tanh": n = tanh(arg)

Case "arcsinh": n = arcsinh(arg)

Case "coth": n = coth(arg)
Case "cosech": n = cosech(arg)
Case "sech": n = sech(arg)

Case Else
If Not e_error Then
MsgBox "undefined function '" + fun + "'"
e_error = -1
End If
End Select

e_function = n
End Function

Private Function arccos(ByVal x As Double) As Double

If Abs(x) = 1 Then

If x = 1 Then arccos = 0 Else arccos = Atn(1) * 4

Else

arccos = Atn(-x / Sqr(-x * x + 1)) + 2 * Atn(1)

End If

End Function

Private Function arcsec(ByVal x As Double) As Double

arcsec = arccos(1 / x)

End Function

Private Function arcsinh(ByVal x As Double) As Double

arcsinh = Log(x + Sqr(x * x + 1))

End Function

Private Function arccosec(ByVal x As Double) As Double

arccosec = arcsin(1 / x)

End Function

Private Function arccot(ByVal x As Double) As Double

arccot = Atn(1 / x)

End Function

Private Function sec(ByVal x As Double) As Double

sec = 1 / Cos(x)

End Function

Private Function cot(ByVal x As Double) As Double

cot = 1 / Tan(x)

End Function

Private Function cosec(ByVal x As Double) As Double

cosec = 1 / Sin(x)

End Function

Private Function sinh(ByVal x As Double) As Double

sinh = (Exp(x) - Exp(-x)) / 2

End Function

Private Function cosh(ByVal x As Double) As Double

cosh = (Exp(x) + Exp(-x)) / 2

End Function

Private Function tanh(ByVal x As Double) As Double

tanh = sinh(x) / cosh(x)

End Function

Private Function coth(ByVal x As Double) As Double

coth = 1 / tanh(x)

End Function

Private Function sech(ByVal x As Double) As Double

sech = 1 / cosh(x)

End Function

Private Function cosech(ByVal x As Double) As Double

cosech = 1 / sinh(x)

End Function

Private Function arcsin(ByVal x As Double) As Double

If Abs(x) = 1 Then

arcsin = Atn(1) * 2 * Sgn(x)

Else

arcsin = Atn(x / Sqr(-x * x + 1))

End If

End Function

' e_nxt
'   Get the next token into e_tok and e_spelling and remove the
'   token from e_input.
'   This function groups the input into "words" like numbers,
'   operators and function names.
Sub e_nxt()
Dim is_keyword As Integer
Dim c As String  ' Current input character.
Dim is_id As Integer

e_tok = ""
e_spelling = ""

' Skip whitespace.
Do
c = Left\$(e_input, 1)
e_input = Mid\$(e_input, 2)
Loop While c = " " Or c = Chr\$(9) Or c = Chr\$(13) Or c = Chr\$(10)

Select Case LCase\$(c)

' Number constant. Modify this to support hexadecimal, etc.
Case "0" To "9", "."
e_tok = "num"
Do
e_spelling = e_spelling + c
c = Left\$(e_input, 1)
e_input = Mid\$(e_input, 2)
Loop While (c >= "0" And c <= "9") Or c = "."
e_input = c + e_input

' Identifier or keyword.
Case "a" To "z", "_"
e_tok = "id"
Do
e_spelling = e_spelling + c
c = LCase\$(Left\$(e_input, 1))
e_input = Mid\$(e_input, 2)
is_id = (c >= "a" And c <= "z")
is_id = is_id Or c = "_" Or (c >= "0" And c <= "9")
Loop While is_id
e_input = c + e_input

' Check for keyword.
is_keyword = -1
Select Case LCase\$(e_spelling)
Case "and"
Case "eqv"
Case "imp"
Case "mod"
Case "not"
Case "or"
Case "xor"
Case Else: is_keyword = 0
End Select
If is_keyword Then
e_tok = LCase\$(e_spelling)
End If

' Check for <=, >=, <>.
Case "<", ">"
e_tok = c
c = Left\$(e_input, 1)
If c = "=" Or c = ">" Then
e_tok = e_tok + c
e_input = Mid\$(e_input, 2)
End If

' Single character token.
Case Else
e_tok = c
End Select

If e_spelling = "" Then
e_spelling = e_tok
End If
End Sub

' e_match
'   Check the current token and skip past it.
'   This function helps with syntax checking.
Sub e_match(token As String)
If Not e_error And e_tok <> token Then
MsgBox "expected " + token + ", got '" + e_spelling + "'"
e_error = -1
End If
Call e_nxt
End Sub
Private Sub Command1_Click()

Dim n As Double

If e_eval(Text1.Text, n) Then
Label3.Caption = Format\$(n)
End If

End Sub

0

Author Comment

ID: 1526829
thanks deighton.  It looks like a lot of work was put into that.  I'll have to give this a try.
0

LVL 18

Expert Comment

ID: 1526830
Yes but I didn't do much of the work!  I got it from a VB site.
0

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