Precision, new prices

[downs]

Saaa wrote:
> I thought my previous precision question was so retarded it didn't need any 
> explanation; anybody here could easily tell me my fault in reasoning.
> 
> Here is my question again but this time with examples.
> 
> Is there some formatting which lets me print out a floating point in full 
> precision?
> 
> Observation: 6 digits is not full precision:
> 
> float f=3.4028234; // 0x7f7f ffff
> writefln("%.8g",f); // prints 3.4028234
> writefln(f.dig); // prints 6
> writefln(3.4028234f.dig); // prints 6
> 
> Shouldn't f.dig print out 8?
> .dig =  number of decimal digits of precision
> 
> I still think I don't get something, somewhere. 
> 
> 


Here's an incredibly simple hack.

import std.stdio, std.string;

string ftoaFull(float f) {
  if (f < 0) return "-" ~ ftoaFull(-f);
  auto start = f;
  auto res = toString(cast(int) f) ~ ".";
  while (true) {
    f -= cast(int) f;
    f *= 10;
    res ~= "0123456789"[cast(int) f];
    // The critical step
    if (res.atof() == start) return res;
    if (res.length > 32) {
      writefln("Cannot find string representation for ", start, ", parses as ", res.atof(), ", delta ", res.atof() - start);
      return res;
    }
  }
}

void main() {
  writefln(ftoaFull(0f));
  writefln(ftoaFull(-5.234));
  writefln(ftoaFull(1f / 3f)); // problematic
}

Output:

0.0
-5.2340002059936523437
Cannot find string representation for 0.333333, parses as 0.333333, delta 5.96046e-09
0.3333333492279052734375000000000

And there you have it.