[erlang-questions] String representations of floating point numbers
Richard O'Keefe
raoknz@REDACTED
Sun Mar 17 09:40:53 CET 2019
Me, I'd have taken Cowlishaw's reference implementation in C or the
Intel library
https://software.intel.com/en-us/articles/intel-decimal-floating-point-math-library
and wrapped a NIF around it.
If I needed it. z/Series and POWER have decimal FP in their
instruction sets, but on other machines it's emulation or nothing.
Erlang/OTP is very much influenced by its users' needs.
(It doesn't always move *fast*, you understand, but when you
want to develop long-lived systems that can be a good thing.)
Why not write up an EEP?
On Sun, 17 Mar 2019 at 21:18, Bryan Paxton <starbelly@REDACTED> wrote:
> A library that implements IEEE-754: http://speleotrove.com/decimal/
>
> Example implementations in Erlang:
>
> https://github.com/tim/erlang-decimal
>
> https://github.com/egobrain/decimal
>
>
> On 3/17/19 1:29 AM, Richard O'Keefe wrote:
>
> 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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>
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