Echo shows incorrect data

[brad]

length = 75.6; echo(length);

produces:

ECHO: 75.59999999999999

[donbright]

i think 75.6 might not be representable exactly in binary except as a series of infinite binary fractions, so the same problem occurs with any computer language that internally stores floating point numbers using a real actual physical CPU floating point type with a limited number of bits (32, 64, whatever) (ie. in python type 'x=75.6' then 'float(x)'.)

more info:

http://stackoverflow.com/questions/1089018/why-cant-decimal-numbers-be-represented-exactly-in-binary

http://www.learncpp.com/cpp-tutorial/25-floating-point-numbers/

to solve this, you could theoretically hack Value.h/Value.cpp to use GNU GMP (stores numbers as fractions) instead of 'double', but i dont know how much that might slow down openscad's OpenCSG mode??? most languages seem to get along OK without doing this , they just round output or something (like in C++, cout << precision by default gives 75.6, but if you cout << setprecision(16) , you will get 75.5999etcetcetc )

curiouser and curiouser....

---- update

here is a very cool page - Lisp has rationals built in to the language.... anyways...

http://en.wikipedia.org/wiki/Rational_data_type

[brad]

Hmm, point taken, I have been programming a long time and don't remember ever encountering this issue. It's not a problem with Python either. I suppose this is not likely to get fixed since it is a bug in C++

[donbright]

python example

don@age:~$ python
xPython 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x=75.6
>>> float(x)
75.599999999999994
>>> 

simpler python example

don@age:~$ python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x=75.6
>>> x
75.599999999999994
>>> print x
75.6
>>> 

[brad]

I guess it must be fixed in 2.7 Python 2.7.2+ (default, Oct 4 2011, 20:06:09) [GCC 4.6.1] on linux2 Type "help", "copyright", "credits" or "license" for more information.

x = 75.6 type(x) <type 'float'> print x 75.6 x 75.6

[donbright]

don@age:~$ python3.1 
Python 3.1.2 (r312:79147, Dec  9 2011, 20:54:36) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x=75.6
>>> x
75.6
>>> '%.17f' % x
'75.59999999999999432'
>>> 

(see http://mail.python.org/pipermail/tutor/2007-December/058964.html )

i.e. python is not storing the rational number 75.6 its storing the binary number 75.599999..... (the infinite series truncated based on how many bits the machine has)

if it is just a question of you want ECHO to be 'rounded', there is probably somewhere in the source code that it could easily be changed.... ???

[brad]

OK, you got me. I guess it is an issue with Python. I guess the newer versions simply hide it better by rounding to the same number of decimal places that you input the number with.

Anyway, I really don't think this is an issue.