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Date: Sun, 23 Oct 2016 21:13:12 +0200
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Subject: Re: [PHP-DEV] strtod and NaN vs. zero
From: dev@mabe.berlin (Marc Bennewitz)



Am 23.10.2016 um 13:49 schrieb Niklas Keller:
> 2016-10-23 12:55 GMT+02:00 Marc Bennewitz <dev@mabe.berlin
> <mailto:dev@mabe.berlin>>:
>
>     Hi internals,
>
>     On casting a non numeric value to a float in PHP the result will be
>     float(0).
>
>     In PHP-7.0 an exception was introduced that on casting a string
>     "/\s*NaN\s*/i" will result in float(NaN).
>
>     https://3v4l.org/2Xakm
>
>     Wouldn't it be more logical and expected to return NaN in all cases
>     of casting a non numeric string to a floating point number?
>
>     Thanks
>     Marc
>
>     --
>     PHP Internals - PHP Runtime Development Mailing List
>     To unsubscribe, visit: http://www.php.net/unsub.php
>
>
> +1

I just noticed that the behavior change in PHP-7 has been "fixed" 
already by https://bugs.php.net/bug.php?id=73329.

So the old behavior will be presented soon.

But I'm still curious why casting any non numeric string results in the 
valid number float(0) where there is a special value in floating point 
numbers declared to represent not a number values.

I also did a small research on other languages and found the following:

C strtod returns float(0)
   -> but supports converting the strings "INFINITY" or "NAN" 
(case-insensitive)

JavaScript returns NaN

Python throws ValueError: could not convert string to float
   -> but supports the string +/- "NaN", "Inf", "Infinity" 
(case-insensitive)

Perl returns float(0)
   -> but supports the string +/- "NaN", "Inf", "Infinity" 
(case-insensitive)

So there is a nice mix of how other languages will handle this problem.


In my opinion it still makes much more sense to return NaN in any case a 
not numeric string will be casted to float incl. "Inf" and "Infinity".
(If spaces should be ignored or suffixes allowed is another question)

= The beginning of a string must contain at least one number by ignoring 
spaces to be casted to a valid floating point number. In any other 
situations NaN must be returned because this represents what it is = Not 
a Number.

So for me the following example table makes sense:

In -> Out
"" -> NaN
" " -> NaN
"+" -> NaN
"-" -> NaN
"." -> NaN
".5" -> 0.5
"0.5" -> 0.5
" 0.5" -> 0.5
"0" -> 0
" 0" -> 0
"Nan" -> NaN
"Inf" -> NaN
"Info" -> NaN
"foo" -> NaN
".foo" -> NaN
".2foo" -> 0.2
" .2foo" -> 0.2
"foo1" -> NaN
"foo." -> NaN
"foo0.1" -> NaN


Thoughts
Marc

>
> Regards, Niklas