[erlang-questions] String representations of floating point numbers

[Richard O'Keefe]

What do you mean by "a decimal lib"?

On Sun, 17 Mar 2019 at 06:35, Bryan Paxton <starbelly@REDACTED> wrote:

>  All of this begs the question of why there is no decimal lib in otp...
> I'm sure there's a good reason... ?
>
>
> On 3/16/19 12:31 PM, Bob Ippolito wrote:
>
> While it's more code, the most straightforward way would be to parse the
> whole float for that spec into its constituent parts and reassemble (with
> defaults where necessary) to parse with float_to_list.
>
>
>
>
> On Sat, Mar 16, 2019 at 4:32 AM Hugo Mills <hugo@REDACTED> wrote:
>
>>    Hi, Bob,
>>
>> On Fri, Mar 15, 2019 at 05:32:54PM -0700, Bob Ippolito 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)
>>
>>    Thanks for the confirmation. That's more or less what I discovered
>> while playing around with list_to_float. It's the first two cases that
>> are the problems for me, because the spec I'm working to(*) says that
>> "1." and ".3" are valid floats, for example, as is "1e-1".
>>
>>    Just for the record, here's the code I'm using to convert a Turtle
>> double or decimal (the former in scientific notation; the latter
>> without the E) into a form suitable for list_to_float/1:
>>
>>     [...]
>>     % W3C's description of a float is wider than erlang's. We need to
>>     % split up the number into a few parts to add extra characters
>>     % where necessary so that list_to_float/1 will work right.
>>     F = case string:lexemes(Text, "eE") of
>>         [M, E] ->
>>             fixup_decimal(M) ++ "e" ++ E;
>>         [M] ->
>>             fixup_decimal(M)
>>     end,
>>     O = lagra_model:new_literal(list_to_float(F)),
>>     [...]
>>
>> -spec fixup_decimal(string()) -> string().
>> fixup_decimal(M) ->
>>     case string:lexemes(M, ".") of
>>         [I] ->
>>             I++".0";
>>         [I, ""] ->
>>             I++".0";
>>         ["", J] ->
>>             "0."++J;
>>         [I, J] ->
>>             M
>>     end.
>>
>>    Hugo.
>>
>> (*) W3C's Turtle recommendation.
>>
>>
>> > 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             | Nostalgia isn't what it used to be.
>> hugo@REDACTED carfax.org.uk |
>> http://carfax.org.uk/  |
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>
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> --
>
> Bryan Paxton
>
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