## digitalmars.D.learn - Precision, new prices

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.

Jul 25 2009

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?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 (cast(float) res.atof() == start) return res; } } void main() { writefln(ftoaFull(0f)); writefln(ftoaFull(-5.234)); writefln(ftoaFull(1f / 3f)); // problematic } Output: 0.0 -5.234 0.33333334 And there you have it.

Jul 25 2009

Ok it finally hit me :( The 24 fraction bits in a float aren't used like an integer (2^24 giving max 7 decimal digits precision) they are used by halving the value of the previous bit: 1, 0.5, 0.25 ... like fractions !!! this of course has a much wider decimal range (something like 20 or so) because not all numbers are represented. Thus, the formatting %.100g and downs code both just give the correct answer . Now, there are 2^24 different floats being represented (ignoring exponent) so the question for me now is: What is the minimal (decimal) representation of a float for which the following holds: float f; //for all possible floats string s = format(f); float f2 = to!(float)(s); assert(f == f2); Does your second code hold, downs?

Jul 25 2009