Floating point to string

[Joe]

Bill Baxter wrote:
> Joe wrote:
>> Derek Parnell wrote:
>>> On Wed, 18 Jul 2007 21:28:47 -0500, Joe wrote:
>>>
>>>> How to do get a full string representation of a floating point?
>>>>
>>>> I've tried toString() on a floating point and the default precision 
>>>> for numbers after the decimal point is 6.  I've also tried using 
>>>> format strings, but I can't seem to get a full textual 
>>>> representation of a floating point.  Is there some trick I'm missing?
>>>
>>> Try this ...
>>> <dcode>
>>> import std.stdio;
>>> import std.string;
>>>
>>> string FullTextValue(T)(T x)
>>> {
>>>     string s;
>>>
>>>     static if(is (T:long))
>>>         const fmt = "%-52d";
>>>     else
>>>     static if(is (T:real))
>>>         const fmt = "%52.52f";
>>>     else
>>>         const fmt = "%-s";
>>>
>>>     s = std.string.format( fmt, x);
>>>     for (int i = s.length-1; i >= 0; i--)
>>>     {
>>>         if (s[i] == '.')
>>>         {
>>>             s.length = i+2;
>>>             break;
>>>         }
>>>         if ((s[i] != '0' && s[i] != ' ') || (i == 0))
>>>         {
>>>             s.length = i+1;
>>>             break;
>>>         }
>>>     }
>>>     return s;
>>> }
>>> void main()
>>> {
>>>     real x;
>>>     double y;
>>>     float z;
>>>     long a;
>>>
>>>     x = 12345678901234567890.1234567890123456789;
>>>     y = x;
>>>     z = x;
>>>     a = cast(typeof(a))x;
>>>
>>>     writefln("real   '%s'", FullTextValue(x));
>>>     writefln("double '%s'", FullTextValue(y));
>>>     writefln("float  '%s'", FullTextValue(z));
>>>     writefln("long   '%s'", FullTextValue(a));
>>>
>>>     x = 0.1234567890123456789;
>>>     y = x;
>>>     z = x;
>>>     a = cast(typeof(a))x;
>>>
>>>     writefln("real   '%s'", FullTextValue(x));
>>>     writefln("double '%s'", FullTextValue(y));
>>>     writefln("float  '%s'", FullTextValue(z));
>>>     writefln("long   '%s'", FullTextValue(a));
>>>
>>> }
>>> </dcode>
>>>
>>
>> Thanks.
>>
>> Out of curiosity (note: I haven't run this example yet), is 52 a magic 
>> number for the floating point types in D?  Is there a way at runtime 
>> to get at the number of digits available after the decimal point 
>> depending on whether it's a float, double or real?
> 
> I think he's taking that from the number of mantissa bits in IEEE double 
>  precision floating point.  There certainly can't be more meaningful 
> decimal digits of accuracy than there are binary digits.  Don could 
> probably tell it much better than me, but it works out to be about 16 
> decimal digits of accuracy.  Goes something like this: Smallest thing 
> you can represent in an IEEE double with the mantissa only is 2^-52 (or 
> maybe 2^-53?).  Which works out to about 10^-16, so that means with 
> decimal point, only about 16 digits can have meaning.  So use 20 and you 
> should be safe -- for doubles anyway.  For floats it's about 8 decimal 
> digits.
> 
> Oh, about your actual question -- float_var.dig tells you the # of 
> decimal digits of precision:
> http://www.digitalmars.com/d/property.html - "Properties for Floating 
> Point Types"
> 
> 
> --bb

I appreciate the explanation.  While I still don't grasp all the 
details, this helps a lot.