[erlang-questions] [patch] new float_to_list/2

Gleb Peregud <>
Fri Jan 14 16:33:46 CET 2011


Here's my patch:

http://www.erlang.org/cgi-bin/ezmlm-cgi/4/43529

It uses double sprintf invocations. Your solution is clearly better (I
didn't knew it is possible to specify precision in args).

Your benchmark shows that difference is negligible. Between integer as
a second parameter or a proplist (~0.4%). Though it might be a bit
more "stable" in terms of running time, but still probably not worth
considering it for performance reasons. It still may be useful in
terms of simpler API, but I have no strong opinion on it.

So generally +1 on including Serge's patch into Erlang. Serge, can you
put it into GitHub pull request? As described here:

https://github.com/erlang/otp/wiki/submitting-patches

Best,
Gleb

On Wed, Jan 12, 2011 at 18:13, Serge Aleynikov <> wrote:
> I did a micro-benchmark on a slightly modified version of the BIF that
> accepts an integer as its options.  The results shown below display a very
> insignificant difference between a call with no options and a call with an
> integer precision passed as the second argument:
>
> 1> test:test().
> float_to_list(123.4, [])               = {0.619512,"123.4000"}
> float_to_list(123.4, [{precision, 4}]) = {0.624895,"123.4000"}
> float_to_list(123.4, 4)                = {0.622896,"123.4000"}
>
> The majority of time is actually spent in the printf(3) function, which
> takes longer to execute when given the "%.*f" argument compared to "%g" as
> in float_to_list/1 case.
>
> Did your patch rely on printf?
>
>
> On 1/12/2011 9:23 AM, Gleb Peregud wrote:
>>
>> Some time ago I've submitted similar but simpler patch. It was a
>> float_to_list/2 with a second parameter being an integer specifying
>> precision. For me it was important to generate A LOT of floats as
>> strings as fast as possible with specified precision. Serge's version
>> has an overhead of inspecting proplist of the second parameter. So I
>> was wondering about introducing two versions of this function: with a
>> proplist as a second parameter and with a number as a second
>> parameter. Alternatively proplist version can be factored out into
>> float_to_list_opts/2.
>>
>> Just my 0.2 cents
>>
>> On Wed, Jan 12, 2011 at 15:10, Serge Aleynikov<>
>>  wrote:
>>>
>>> The reason I called it precision was to be consistent with the naming
>>> convention of the printf function.  Below is the extract from "man 3
>>> printf", which refers to the digits after the decimal point as
>>> "precision":
>>>
>>>       f, F   The  double  argument is rounded and converted to decimal
>>>              notation in the style [-]ddd.ddd, where the number of
>>>              digits after the decimal-point character is equal to the
>>>              precision specification.  If the precision is missing,
>>>              it  is taken as 6; if the precision is explicitly zero,
>>>              no decimal-point character appears.  If a decimal point
>>>              appears, at least one digit appears before it.
>>>
>>> I don't have a very strong preference for calling it precision or scale,
>>> but
>>> I do have a strong preference for including this patch in the
>>> distribution,
>>> because the default behavior of float_to_list/1 hard-coded in C is
>>> deficient.
>>>
>>>
>>> On 1/12/2011 4:58 AM, nox wrote:
>>>>
>>>> Il should be called "scale", shouldn't it?
>>>>
>>>> Le 12 janv. 2011 à 10:26, Pierpaolo Bernardi<>    a
>>>> écrit
>>>> :
>>>>
>>>>> On Wed, Jan 12, 2011 at 06:44, Serge Aleynikov<>
>>>>>  wrote:
>>>>>>
>>>>>> Attached please find a patch that adds a new float_to_list/2 BIF.  The
>>>>>> patch
>>>>>> was created off of the master branch of https://github.com/erlang/otp.
>>>>>>
>>>>>> This BIF solves a problem of float_to_list/1 that doesn't allow
>>>>>> specifying
>>>>>> the number of digits after the decimal point when formatting floats.
>>>>>>
>>>>>>        float_to_list(Float, Options) ->    string()
>>>>>>
>>>>>>        Float = float()
>>>>>>        Options = [Option]
>>>>>>        Option = {precision, Precision::integer()} | compact
>>>>>>
>>>>>>        Text representation of a float formatted using given options
>>>>>>
>>>>>>        Returns a string which corresponds to the text
>>>>>>        representation of Float using fixed decimal point formatting.
>>>>>>        When precision option is specified
>>>>>>        the returned value will contain at most Precision number of
>>>>>>        digits past the decimal point.  When compact option is provided
>>>>>>        the trailing zeros at the end of the list are truncated.
>>>>>
>>>>> I think the option is misnamed.
>>>>>
>>>>> In the usual terminology, 'precision' is the total number of
>>>>> significative digits, not only the ones past the decimal point.
>>>>>
>>>>> Cheers
>>>>> P.
>>>>>
>>>>> ________________________________________________________________
>>>>> erlang-questions (at) erlang.org mailing list.
>>>>> See http://www.erlang.org/faq.html
>>>>> To unsubscribe; mailto:
>>>>>
>>>
>>> ________________________________________________________________
>>> erlang-questions (at) erlang.org mailing list.
>>> See http://www.erlang.org/faq.html
>>> To unsubscribe; mailto:
>>>
>>>
>


More information about the erlang-questions mailing list