Floating point to string
Bill Baxter
dnewsgroup at billbaxter.com
Wed Jul 18 22:13:16 PDT 2007
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
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