Re: Floating Point computations in elisp

[Emanuel Berg]

Jean Louis wrote:

> * Emanuel Berg via Users list for the GNU Emacs text editor
> <help-gnu-emacs@gnu.org> [2020-12-18 20:09]:
>
>> (defun compute-space-distance (dist num-beams beam-width)
>>   (let*((spaces         (1+ num-beams))
>>         (dist-covered   (* num-beams beam-width))
>>         (dist-uncovered (- dist dist-covered))
>>         (space          (/ dist-uncovered spaces 1.0) ))
>>     space ))
>> 
>> ;; (insert (format "\n;; %.2f" (compute-space-distance 262 16 12)))
>> ;; 4.12
>
> If I wish to arrive to Tau Ceti from Earth and it is maybe 11.8 light
> years distant, how many years do I need to get there provided I trave
> 5,000 km per hour?

How about: 2 548 584 years

(defun tau-ceti (speed)
  (let*((dist       (* 11.8 9.46 (expt 10 12)))
        (time       (/ dist speed) )
        (days       (/ time 24))
        (whole-days (round days)) )
    (format-seconds "%y years"
                    (- (float-time (encode-time 0 0 0 whole-days 0 0))
                       (float-time (encode-time 0 0 0 0 0 0)) ))))
;; (tau-ceti 5000)

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