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I initially created this to improve my C# understanding and what a learning curve it was. So I decided to directly translate the C# source code I had into AutoIt as a way of showing off the >Stack UDF (which you will need). Of course it can be changed not to use the Stack UDF, but why reinvent the wheel and make life even more difficult. If you don't understand RPN or the Shunting-Yard algorithm, then please look at the links below. Sources: https://en.wikipedia.org/wiki/Shunting_yard_algorithm, https://en.wikipedia.org/wiki/Reverse_Polish_notation and http://www.slideshare.net/grahamwell/shunting-yard #include <StringConstants.au3> #include "Stack.au3" Example() Func Example() ; Create postfix notation (RPN) from infix notation. ConsoleWrite(StringFormat('%s: == %s', '3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3', ShuntingYard_Parse('3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3')) & @CRLF) ConsoleWrite('Reference: 3 4 2 * 1 5 - 2 3 ^ ^ / +' & @CRLF) ConsoleWrite(@CRLF); A new line. ConsoleWrite(StringFormat('%s: == %s', '3 + 4', ShuntingYard_Parse('3 + 4')) & @CRLF) ConsoleWrite('Reference: 3 4 +' & @CRLF) ConsoleWrite(@CRLF); A new line. ; Expression from: http://www.slideshare.net/grahamwell/shunting-yard. ConsoleWrite(StringFormat('%s: == %s', '2 + (3 * (8 - 4))', ShuntingYard_Parse('2 + (3 * (8 - 4))')) & @CRLF) ConsoleWrite('Reference: 2 3 8 4 - * +' & @CRLF) ConsoleWrite(@CRLF); A new line. ; Calculate postfix notation (RPN) examples. ConsoleWrite('3 4 + == ' & ShuntingYard_Calculate('3 4 +') & @CRLF) ConsoleWrite('3 4 2 * 1 5 - 2 3 ^ ^ / + == ' & ShuntingYard_Calculate('3 4 2 * 1 5 - 2 3 ^ ^ / +') & @CRLF) ConsoleWrite('2 3 8 4 - * + == ' & ShuntingYard_Calculate('2 3 8 4 - * +') & @CRLF) EndFunc ;==>Example Func ShuntingYard_Calculate($sExpression) Local $aChar = StringSplit($sExpression, '', $STR_NOCOUNT), _ $hStack = Stack(), _ ; Create a stack to push/pop to. $iNumber1 = 0, $iNumber2 = 2, _ $sDigits = '', _ ; Hold string digits e.g. in case there are numbers greater than 1 digit long. $sToken = '' ; Token. For $i = 0 To UBound($aChar) - 1 $sToken = $aChar[$i] If StringIsDigit($sToken) Then $sDigits &= $sToken ElseIf __ShuntingYard_IsOperator($sToken) Then $iNumber2 = Stack_Pop($hStack) $iNumber1 = Stack_Pop($hStack) Stack_Push($hStack, __ShuntingYard_Eval($iNumber1, $iNumber2, $sToken)) ElseIf StringIsSpace($sToken) And Not ($sDigits == '') Then If StringIsInt($sDigits) Or StringIsFloat($sDigits) Then $iNumber1 = Number($sDigits) Stack_Push($hStack, $iNumber1) EndIf $sDigits = '' $iNumber1 = 0 EndIf Next Return Stack_Pop($hStack) EndFunc ;==>ShuntingYard_Calculate Func ShuntingYard_Parse($sExpression) Local $aChar = StringSplit($sExpression, '', $STR_NOCOUNT), _ $bIsBracket = False, _ ; Is parenthesis. $hStack = Stack(), _ ; Create a stack to push/pop to. $sOutput, _ ; Output string. $sToken = '' ; Token. For $i = 0 To UBound($aChar) - 1 $sToken = $aChar[$i] If StringIsDigit($sToken) Then ; If a digit add to the output string. $sOutput &= $sToken ElseIf __ShuntingYard_IsOperator($sToken) Then ; Check if a valid token. If __ShuntingYard_IsBracket($sToken) Then $bIsBracket = $sToken == '(' ; True if open bracket else False if closing. If $bIsBracket Then ; Open bracket. Stack_Push($hStack, $sToken) Else ; Closing bracket. $sOutput &= ' ' Do ; Repeat until '(' is found. $sOutput &= Stack_Pop($hStack) Until (Stack_Peek($hStack) == '(') Stack_Pop($hStack) EndIf Else $sOutput &= " " ; Workaround for spacing and certain expressions. If Stack_Count($hStack) > 0 And __ShuntingYard_HasHigherPrecedence($sToken, Stack_Peek($hStack)) Then $sOutput &= Stack_Pop($hStack) Stack_Push($hStack, $sToken) $sOutput &= " " ; Workaround for spacing and certain expressions. Else Stack_Push($hStack, $sToken) EndIf EndIf EndIf Next If Stack_Count($hStack) > 0 Then ; Append what is left on the stack in reverse order. $sOutput &= ' ' While (Stack_Count($hStack) > 0) $sOutput &= Stack_Pop($hStack) & ' ' WEnd EndIf Return $sOutput EndFunc ;==>ShuntingYard_Parse Func __ShuntingYard_Eval($iNumber1, $iNumber2, $sChar) Switch $sChar Case '+' Return $iNumber1 + $iNumber2 Case '-', ChrW(8722) ; Char 8722 is - but the unicode char code Return $iNumber1 - $iNumber2 Case '*' Return $iNumber1 * $iNumber2 Case '/' Return $iNumber1 / $iNumber2 Case '^' Return $iNumber1 ^ $iNumber2 Case Else Return $iNumber1 EndSwitch EndFunc ;==>__ShuntingYard_Eval Func __ShuntingYard_HasHigherPrecedence($sChar1, $sChar2) Local $iChar1 = __ShuntingYard_PrecendenceValue($sChar1), _ $iChar2 = __ShuntingYard_PrecendenceValue($sChar2) If $iChar1 = 0 And $iChar2 = 0 Then Return False EndIf Return $iChar1 <= $iChar2 And Not ($sChar1 == '^') EndFunc ;==>__ShuntingYard_HasHigherPrecedence Func __ShuntingYard_IsBracket($sChar) Return $sChar == '(' Or $sChar == ')' EndFunc ;==>__ShuntingYard_IsBracket Func __ShuntingYard_IsOperator($sChar) Return $sChar == '+' Or $sChar == '-' Or $sChar == ChrW(8722) Or $sChar == '*' Or $sChar == '/' Or $sChar == '^' Or $sChar == '(' Or $sChar == ')'; Char 8722 is - but the unicode char code. EndFunc ;==>__ShuntingYard_IsOperator Func __ShuntingYard_PrecendenceValue($sChar) Switch $sChar Case '+', '-' Return 2 Case '*', '/' Return 3 Case '^' Return 4 Case Else Return 0 EndSwitch EndFunc ;==>__ShuntingYard_PrecendenceValue