Re: How to read an integer from the minibuffer

[Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

Jean Louis wrote:

>> We see a problem BTW that `string-to-number' returns 0
>> on error!
>
> That problem I have solved in rcd-utilities.el with:
>
> (defun string-is-number-p (s)
>   "Return number only if string is pure number,
> otherwise NIL."

  A number is said to be pure if

    1. It has even number of digits.
    2. All the digits are either 4 or 5.
    3. And the number is a palindrome.

  First few pure numbers are 44, 55, 4444, 4554, 5445, 5555 ... [1]

Also the familiar docstring functional parameter issue.

>   (let ((s (string-trim s)))
>     (cond ((seq-empty-p s) nil)
> 	  ((string-match "[^0123456789\\.]" s) nil)

Use a char class or an interval?

> 	  ((numberp (string-to-number s)) (string-to-number s)))))

(string-is-number-p "-0")
(string-is-number-p "-5.3")

[1] https://www.geeksforgeeks.org/find-the-first-n-pure-numbers/

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