frac(3.14159265358979);
sub frac
{ my ($x,$a,$b,$c,$d,$e,$f) = (@_,0,1,1,0,1,1);
  while ($f<1000)
  { if ($e<$x*$f) { ($a,$b) = ($e,$f) }
    else          { ($c,$d) = ($e,$f) }
    ($e,$f) = ($a+$c, $b+$d);
    print "$a/$b < $x < $c/$d; => $e/$f\n";
} }