polish.t

RPN calculator

%
% "polish.t" performs reverse polish arithmetic
%

var R : array( 64 ) of real             % stack

var expn : string                       % for keyboard input
var i, len, n : int                     % global values

program
    
    var c : string

    loop
        polish
        put "another? [y|n] "
        get c
        exit when c(0) = 'n'
    end loop

end program


procedure polish

    var ch : string( 2 )
    
    put "enter an expression using reverse polish notation"
    put ": "...
    get expn : *
    expn := expn & " "                  % must terminate line with space
    len := length( expn )

    i := -1
    n := 0
    R( n ) := 0.0                       % push zero for unary operations

    loop

        i := i + 1
        exit when i >= len              % at end of line

        continue when expn(i) = ' '     % skip white spaces

        ch(0) := expn(i)                % for use in index
        ch(1) := chr(0)
        
        if index( "0123456789", ch ) >= 0 then
            get_number
        elsif expn(i) = '+' then        % add and pop stack
            if n < 1 then
                put "error"
            else
                R( n - 1 ) := R( n - 1 ) + R( n )
                n := n - 1
            end if
        elsif expn(i) = '-' then        % subtract and pop stack
            if n < 1 then
                put "error"
            else
                R( n - 1 ) := R( n - 1 ) - R( n )
                n := n - 1
            end if
        elsif expn(i) = '*' then        % multiply and pop stack
            if n < 1 then
                put "error"
            else
                R( n - 1 ) := R( n - 1 ) * R( n )
                n := n - 1
            end if
        elsif expn(i) = '/' then        % divide and pop stack
            if n < 1 then
                put "error"
            else
                R( n - 1 ) := R( n - 1 ) / R( n )
                n := n - 1
            end if
        else
            exit
        end if

    end loop

    put "result: ", R( n )              % end of main program

end polish

procedure get_number

    var ch : string
    var j : int := 0                    % start of number string
    var number : string                 % buffer for conversion

    i := i - 1
    loop                                % get integer part

        i := i + 1
        ch(0) := expn(i)                % for use in index
        ch(1) := chr(0)
        number(j) := expn(i)
        j := j + 1
      
        continue when index( "0123456789", ch ) >= 0

        if expn(i) = '.' then          

            loop                        % get fractional part

                i := i + 1
                ch(0) := expn(i)        % for use in index
                ch(1) := chr(0)
                number(j) := expn(i)
                j := j + 1

                continue when index( "0123456789", ch ) >= 0

                exit

            end loop

        end if

        number(j) := chr( 0 )           % null terminate string

        exit

    end loop
    
    n := n + 1                          % push number onto stack
    R( n ) := strreal( number )
    watch( number )
    watch( n )
    watch( R(n) )

end get_number