Re: converting decimal numbers into rational in scheme functions

[David Kastrup]

Janek Warchoł <address@hidden> writes:

> Hi,
>
> i have a function that takes a moment as the argument:
>
> foo =
> #(define-music-function (parser location mom)
>    (ly:moment?)
>    #{
>      \override Score.SpacingSpanner.base-shortest-duration = #mom
>    #})
>
> \new Staff {
>   \foo #(ly:make-moment 1 50)
>   c'4 d' e' f'
> }
>
> Now, i would like to be able to supply a decimal number and have it
> converted to a rational number (i.e. a fraction, which can be easily
> converted to a moment), so that i could call the function like this
>
> \foo #0.02  % 1/50 = 0.02
>
> Is there any smart way to do this (a built-in function?  I've searched
> for one but haven't found any), or do i have to write a
> decimal->rational converter?  Would anyone want to help with this?

How did you search?  It's right in the GUILE manual where one would
expect, with rational numbers.

File: guile-1.8.info,  Node: Reals and Rationals,  Next: Complex Numbers,  
Prev: Integers,  Up: Numbers

5.5.2.3 Real and Rational Numbers
.................................

[...]

 -- Scheme Procedure: rationalize x eps
 -- C Function: scm_rationalize (x, eps)
     Returns the _simplest_ rational number differing from X by no more
     than EPS.

     As required by R5RS, `rationalize' only returns an exact result
     when both its arguments are exact.  Thus, you might need to use
     `inexact->exact' on the arguments.

          (rationalize (inexact->exact 1.2) 1/100)
          => 6/5

-- 
David Kastrup