[erlang-questions] String representations of floating point numbers

[Bob Ippolito]

There’s a typo in that regex, the exponent part is optional of course.

[+-]?\d+[.,]\d+([eE][+-]?\d+)?

On Fri, Mar 15, 2019 at 17:32 Bob Ippolito <bob@REDACTED> wrote:

> If you dive into the implementation it's effectively a wrapper around
> strtod from C with a validation pass that is more strict than the strtod
> standard.
>
>
> https://github.com/erlang/otp/blob/OTP-21.3/erts/emulator/sys/win32/sys_float.c#L54
>
> https://github.com/erlang/otp/blob/OTP-21.3/erts/emulator/sys/unix/sys_float.c#L732
>
> So as far as a regex goes it would be something like this:
>
> [+-]?\d+[.,]\d+([eE][+-]?\d+)
>
> The major differences between this and other popular float grammars are:
>
> * At least one digit is required in each part
> * Both integer and fractional parts are required, even if there's an
> exponent part (so "1", ".1", "1e-1" would not be valid)
> * The decimal separator is either , or . (the implementation will try the
> other if necessary to compensate for a different locale)
>
>
> On Fri, Mar 15, 2019 at 3:52 PM Hugo Mills <hugo@REDACTED> wrote:
>
>>    Where in the manual is the set of allowable string representations
>> of floating point numbers documented? I'd have expected it to be here:
>>
>> http://erlang.org/doc/reference_manual/data_types.html
>>
>> ... but apparently not.
>>
>>    Specifically, I'm trying to use list_to_float/1, and I've been
>> trying to reverse engineer it:
>>
>> 1> list_to_float("-1").
>> ** exception error: bad argument
>>      in function  list_to_float/1
>>         called as list_to_float("-1")
>> 2> list_to_float("-1.0").
>> -1.0
>> 3> list_to_float("-1.0e-23").
>> -1.0e-23
>> 4> list_to_float("-1e-23").
>> ** exception error: bad argument
>>      in function  list_to_float/1
>>         called as list_to_float("-1e-23")
>> 5> list_to_float(".3").
>> ** exception error: bad argument
>>      in function  list_to_float/1
>>         called as list_to_float(".3")
>>
>>    An actual written specification would be really handy here. Even
>> just a regex or EBNF for them. I'm writing a parser for something
>> where the definition of floating point literals isn't quite the same
>> as Erlang's, and it's a bit painful.
>>
>>    Hugo.
>>
>> --
>> Hugo Mills             | Your problem is that you've got too much taste
>> to be
>> hugo@REDACTED carfax.org.uk | a web developer.
>> http://carfax.org.uk/  |
>> PGP: E2AB1DE4          |                                          Steve
>> Harris
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