bug#59531: 29.0.50: An alternative to `string-to-number` which throws an error (or returns a NIL value) when input is non-parseable as number

bug#59531: 29.0.50: An alternative to `string-to-number` which throws an error (or returns a NIL value) when input is non-parseable as number

[Ramesh Nedunchezian]

`string-to-number` returns ZERO if the input is not a number.  


This return value is not very helpful.  The choice of a number ZERO as "Not A Number" doesn't help one to distinguish between the following two cases

(1) Input was a valid number, and it parses to number zero

(2) Input was NOT a valid number, and it was forcibly reported as ZERO

Consider amending `string-to-number` to throw an error (or return NIL) when the input is not parseable as a number, or providing an alternative API to validate numbers.  I am trying to parse some fields in an org table, and see if the field value is a number or not;

If there is already an alternative to what I am trying to accomplish, I would appreciate a recipe.

bug#59531: 29.0.50: An alternative to `string-to-number` which throws an error (or returns a NIL value) when input is non-parseable as number

[Eli Zaretskii]

tags 59531 wishlist
thanks

> Date: Thu, 24 Nov 2022 11:44:43 +0530
> From: Ramesh Nedunchezian <rameshnedunchezian@outlook.com>
> 
> `string-to-number` returns ZERO if the input is not a number.  
> 
> 
> This return value is not very helpful.  The choice of a number ZERO as "Not A Number" doesn't help one to distinguish between the following two cases
> 
> (1) Input was a valid number, and it parses to number zero
> 
> (2) Input was NOT a valid number, and it was forcibly reported as ZERO
> 
> Consider amending `string-to-number` to throw an error (or return NIL) when the input is not parseable as a number, or providing an alternative API to validate numbers.  I am trying to parse some fields in an org table, and see if the field value is a number or not;

Thanks.

Changing the default behavior to signal an error is out of the question,
since this is used in the Lisp reader and elsewhere, all over the place.  It
is very useful there.

However, as an enhancement, we could have an additional optional argument to
request that the function signal an error if the string cannot be parsed as
a number.

> If there is already an alternative to what I am trying to accomplish, I would appreciate a recipe.

You could match the string to a regexp that validates its numerical
appearance, before calling string-to-number.

bug#59531: 29.0.50: An alternative to `string-to-number` which throws an error (or returns a NIL value) when input is non-parseable as number

[Jean Louis]

* Ramesh Nedunchezian <rameshnedunchezian@outlook.com> [2022-11-24 09:22]:
> `string-to-number` returns ZERO if the input is not a number.  
> 
> 
> This return value is not very helpful.  The choice of a number ZERO as "Not A Number" doesn't help one to distinguish between the following two cases
> 
> (1) Input was a valid number, and it parses to number zero
> 
> (2) Input was NOT a valid number, and it was forcibly reported as ZERO
> 
> Consider amending `string-to-number` to throw an error (or return NIL) when the input is not parseable as a number, or providing an alternative API to validate numbers.  I am trying to parse some fields in an org table, and see if the field value is a number or not;
> 
> If there is already an alternative to what I am trying to
> accomplish, I would appreciate a recipe.

I had the same problem, so I have solved it this way.

(defun string-is-number-p (s)
  "Return number only if string is actual number, otherwise NIL."
  (let* ((s (string-trim s)))
    (cond ((seq-empty-p s) nil)
	  ((string-match "[^0123456789\\.-]" s) nil)
	  ((string-match "-" s 1) nil)
	  ((numberp (string-to-number s)) (string-to-number s)))))

As in my case I liketo know that string is really representing number
and nothing else.

(string-is-number-p "Hello") ➜ nil
(string-is-number-p "") ➜ nil
(string-is-number-p "0") ➜ 0
(string-is-number-p "0a") ➜ nil
(string-to-number "0a") ➜ 0
(string-is-number-p "0.1") ➜ 0.1
(string-is-number-p "-2.5121212") ➜ -2.5121212

My function may not be perfect.

--
Jean

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bug#59531: 29.0.50: An alternative to `string-to-number` which throws an error (or returns a NIL value) when input is non-parseable as number

[Robert Pluim]

>>>>> On Thu, 24 Nov 2022 10:00:06 +0200, Eli Zaretskii <eliz@gnu.org> said:

    >> This return value is not very helpful.  The choice of a number ZERO
    >> as "Not A Number" doesn't help one to distinguish between the
    >> following two cases
    >> 
    >> (1) Input was a valid number, and it parses to number zero
    >> 
    >> (2) Input was NOT a valid number, and it was forcibly reported as ZERO
    >> 
    >> Consider amending `string-to-number` to throw an error (or return
    >> NIL) when the input is not parseable as a number, or providing an
    >> alternative API to validate numbers.  I am trying to parse some
    >> fields in an org table, and see if the field value is a number or
    >> not;

    Eli> Thanks.

    Eli> Changing the default behavior to signal an error is out of the question,
    Eli> since this is used in the Lisp reader and elsewhere, all over the place.  It
    Eli> is very useful there.

    Eli> However, as an enhancement, we could have an additional optional argument to
    Eli> request that the function signal an error if the string cannot be parsed as
    Eli> a number.

`cl-parse-integer' already has such an argument, but it only works for
integers. Alternatively, you could use `read-from-string' and check if
it returns something of type `integer' or `float'.

Robert
-- 

bug#59531: 29.0.50: An alternative to `string-to-number` which throws an error (or returns a NIL value) when input is non-parseable as number

[Jean Louis]

(defun string-is-number-p (s)
  "Return number only if string is actual number, otherwise NIL."
  (let* ((s (string-trim s)))
    (cond ((seq-empty-p s) nil)
	  ((string-match "[^0123456789\\.-]" s) nil)
	  ((string-match "-" s 1) nil)
	  ((numberp (string-to-number s)) (string-to-number s)))))


I use this function to make sure string is number and it
returns either the number of nil, and function is not
exhaustive.


Jean

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