kicad-developers team mailing list archive

[Ruth Ivimey-Cook <ruth@xxxxxxxxxx>]

On 28/12/2019 13:37, Mark Roszko wrote:
> But that 1.600 is not a double/floating number, it  truncated from the
> double of 1.600000000000000088817841970012523233890533447265625

The value you quote here is illusory: the ieee-754 double format is 
incapable of storing more than 17 decimal (53 bimary) digits, and that 
value has over 30 digits.

I wrote a program (below) to demonstrate this, showing that printing 
using the generic float format "%g" you do indeed get 1.6, and you can 
only get the 'additional' precision by forcing the formatter to 'find' 
it using the precision options of printf. As well as this gcc/Linux 
version, I checked with both python 3.8 on windows 10 and php 7.3 on 
Linux and they all have the same behaviour (unsurprisingly).

The "Hex" printout shows that assigning a simple '1.6' double has the 
same encoded representation as the longer value, backing up my 
'illusory' comment. Those other digits are not there:

=====================

double = 8 bytes, long long = 8 bytes
For: v = 1.600000000000000088817841970012523233890533447265625
  Natural:  %g    v=  1.6
  30 digit: %f    v=  1.600000000000000088817841970013
  Hex:            v=  3ff999999999999a
For: w = 1.6
  Natural:  %g    w=  1.6
  30 digit: %f    w=  1.600000000000000088817841970013
  Hex:            w=  3ff999999999999a

======================

#include <stdio.h>

void main() {
  double v = 1.600000000000000088817841970012523233890533447265625;
  double w = 1.6;

  printf("double = %lu bytes, long long = %lu bytes\n", sizeof(double), 
sizeof(long long));

  long long *vx = (long long *)&v;
  printf("For: v = 
1.600000000000000088817841970012523233890533447265625\n");
  printf("  Natural:  %%g    v=  %g\n", v);
  printf("  30 digit: %%f    v=  %30.30f\n", v);
  printf("  Hex:            v=  %llx\n", *vx);

  long long *wx = (long long *)&w;
  printf("For: w = 1.6\n");
  printf("  Natural:  %%g    w=  %g\n", v);
  printf("  30 digit: %%f    w=  %30.30f\n", v);
  printf("  Hex:            w=  %llx\n", *wx);
}

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