[erlang-questions] Re: floating point syntax

Hynek Vychodil <>
Wed Apr 21 11:12:52 CEST 2010


On Wed, Apr 21, 2010 at 3:08 AM, Richard O'Keefe <> wrote:
>
> On Apr 20, 2010, at 9:47 PM, Anthony Shipman wrote:
>
>> On Tue, 20 Apr 2010 07:16:02 am Robert Virding wrote:
>>>
>>> IIRC it was probably more that we/I just didn't see the need to be
>>> able to write the shortened forms. It isn't that it is much extra you
>>> have to write. :-)
>>>
>>> Robert
>>
>> When parsing numbers for other languages it would be nice to be able to
>> use
>> fread() to parse them instead of writing my own. Some sort of ~N form to
>> read
>> a number, integer or floating point, in the usual syntax would be nice.
>
> What _is_ the "usual syntax"?
> Here's a floating point number:            (Burroughs Algol)
> Here's a floating point number: 1.23E45_DOUBLE    (Fortran)
> Here's a floating point number: 1.23D45           (Fortran, Lisp, Smalltalk)
> Here's a floating point number: 1.23e45L          (C)
> Here's a floating point number: ~12.34            (ML)
> Here's a floating point number: 1_234_567.0       (Ada, OCaml)
> Here's a floating point number: 1 234 567.0       (Fortran, Algol 60, Algol
> 68)
> Here's a floating point number: 0x12.34p45        (C)
> Here's a floating point number: 101.101E2B        (PL/I)
> Here's a floating point number: 1.23Q45           (ANSI Smalltalk)

Here's a floating point number: _12.34              (J)

to your impressive collection ;-)

>
> For what it's worth, here's the "usual syntax" for floats from the
> ANSI Smalltalk standard:
>
> float ::= mantissa [exponentLetter exponent]
> mantissa ::= digits ’.’ digits
> digits ::= digit+
> exponent ::= [’-’]decimalInteger
> exponentLetter ::= ’e’ | ’d’ | ’q’
>
> Except for letting the exponent letter vary to indicate the
> precision of the number (single, double, double extended),
> this is pretty traditional.  (Even "d" is traditional.)
> Erlang is not alone in requiring a digit each side of the dot.
>
> There is so much variation between programming languages that
> it is extremely hard to cover all of them (and believe me, I've
> tried).  True, Fortran allows numbers with a digit on only one
> side of the dot, but it also allowed spaces inside numbers; is
> _that_ "usual syntax"?
>
> One really important issue these days is allowing numbers written
> by people or programs using the comma as the radix point.
>
> There are obvious advantages to "being generous in what you accept",
> but we cannot just appeal to "usual syntax" to determine what that
> should be.  We need something more explicit.
>
>
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-- 
--Hynek (Pichi) Vychodil

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