Quadratic equation in Factor

Example for versions Factor 0.94

Word quadratic-equation takes coefficients of the equation as input and prints the solution while returning nothing. Note that this word is declared using token :: instead of : used in most cases; this means that within it lexically scoped variables can be used, in this case parameters a, b and c as well as local variables d, x0 and sd bound by :> operator. Such variables can be loaded on the stack using their names. Words and operators which process lexically scoped variables are available in dictionary locals.

Factor provides a built-in data type for complex numbers; whenever the discriminant is negative, its square root will be of type complex. In this case complex roots are printed using words real-part and imaginary-part which extract corresponding parts of a number.

readln reads a string from input stream (till the end of line), and string>number (from math.parser dictionary) converts a string to a floating-point number. 

USING: formatting io kernel locals math math.functions math.parser ;
IN: quadratic-example

:: quadratic-equation ( a b c -- )
    a 0 =
    [ "Not a quadratic equation." printf ]
    [ b sq a c * 4 * - :> d 
      b neg a 2 * / :> x0
      d sqrt a 2 * / :> sd
      d 0 =
      [ x0 "x = %f\n" printf ]
      [ d 0 >
        [ x0 sd + x0 sd - "x1 = %f\nx2 = %f\n" printf ]
        [ x0 sd + [ real-part ] [ imaginary-part ] bi "x1 = (%f, %f)\n" printf
          x0 sd - [ real-part ] [ imaginary-part ] bi "x2 = (%f, %f)\n" printf ]
        if
      ]
      if
    ]
    if ;

readln string>number
readln string>number
readln string>number
quadratic-equation