[erlang-questions] String representations of floating point numbers

[Bryan Paxton]

 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
> <mailto: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
>     <mailto: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> |
>     http://carfax.org.uk/  |
>     PGP: E2AB1DE4          |
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>
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--

Bryan Paxton

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