What is 0.01 here not 0.01 here 0.009999999999999?

[Jean Louis <bugs@gnu.support>]

I would like to get a number increased for 0.01:

(defun rcd-vc-increase-decimal-revision-number (nn.nn)
  (let* ((nn.nn (format "%s" nn.nn))
	 (nn.nn (format "%.2f" (string-to-number nn.nn)))
	 (nn.nn (string-to-number nn.nn)))
    (+ nn.nn 0.01)))

But result is not what I want, as I want to get 10.13, though it is
tiny loss:

(rcd-vc-increase-decimal-revision-number "10.12") → 10.129999999999999

I found how to solve it, but maybe there is better way to round the
decimals behind the dot?

(defun rcd-vc-increase-decimal-revision-number (nn.nn)
  (let* ((nn.nn (format "%s" nn.nn))
	 (nn.nn (format "%.2f" (string-to-number nn.nn)))
	 (nn.nn (string-to-number nn.nn)))
    (string-to-number (format "%.2f" (+ nn.nn 0.01)))))

(rcd-vc-increase-decimal-revision-number "10.12") → 10.13


Jean

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