RPN ( expression )

Evaluate an expression with Reverse Polish Notation

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Stephen Dolenski - Show more from this author
Ocean West Inc.
http://fmforums.com

Sample input:

10 25 2 * + 40 +

Sample output:

100

Function definition: (Copy & paste into FileMaker's Edit Custom Function window)

// RPN( expression )  or Reverse Polish Notation 
// by Stephen Dolenski 

Let ([
exp = Substitute( expression ; " " ; ¶) ; // pivot expression into a return sep list.
exp_len = ValueCount ( exp ) // count entries.
];


//============================

If(  $i   >   exp_len ;   

Let( $i = 1 ; $stack)
;


If(  Filter( GetValue(exp ; $i) ; "*+-/")  ≠ $null ;   // value is operator

     Let ([
          $result =  Evaluate(GetValue ($stack ; 1 )  &  GetValue(exp ; $i)  & GetValue ($stack ; 2 ))  ;
          $stack =  Substitute( $stack ; [GetValue ( $stack; 1 )  & "¶" & GetValue ( $stack; 2 )  & "¶"  ; $result &¶] ; [GetValue ( $stack; 1 )  & "¶" & GetValue ( $stack; 2 ) ; $result]);
          $i = $i+1
          ]; 
          RPN( exp ) 
          ) // end let

     ; // else append stack

        Let ([ 
        $stack = Case( $i = 1; GetValue (exp ; 1 ) ; GetValue(exp ; $i) & ¶& $stack );
        $i = $i+1
        ]; RPN( exp )
        ) // end let
     ) // end if

) // end if


//============================

) // close let

This CF will allow you to evaluate an expression with Rerverse Polish Notation. Where entry detrmines order of operation, using a stack to push and pull from.

RPN input would be written : 10 25 2 * + 40 +
Infix input would be written : ((25 * 2) + 10) + 40

see also http://en.wikipedia.org/wiki/Reverse_polish

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RPN ( expression )

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