Floating point to string

[Bill Baxter]

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