5 ms·

> printing the number might just cut off the digits at some point. I'm not sure, but I think it's not really about cutting off the digits (although that's the

by philh 11y ago

> printing the number might just cut off the digits at some point. I'm not sure, but I think it's not really about cutting off the digits (although that's the effect it has). At least with python, I think what happens is: * repr gives you the decimal number with the exact value of the floating-point number, i.e. 0.30000000000000004 * print gives you the least-precise decimal number whose closest floating-point representation is equal to the floating-point number That is, there are (infinitely) many decimal numbers such that the closest floating-point representation has the value 0.30000000000000004. One of these decimal numbers is 0.3, and of all such decimal numbers, this has the fewest digits. So that's the one that gets printed.

deleted 11y ago
[deleted]
philh 11y ago
Someone pointed out that this isn't correct, and then deleted their comment. (?) I now think the behiviour I ascribed to print, is actually what repr does. I'm not sure about print, but I guess it is just rounding to a certain number of places. >>> from decimal import Decimal >>> Decimal(0.1+0.2) Decimal('0.3000000000000000444089209850062616169452667236328125') >>> Decimal(0.3) Decimal('0.299999999999999988897769753748434595763683319091796875') >>> Decimal(0.300000000000000000000000000001) Decimal('0.299999999999999988897769753748434595763683319091796875') >>> print 0.300000000000000000000000000001 0.3 >>> repr(0.300000000000000000000000000001) '0.3' >>> D(0.30000000000000004) Decimal('0.3000000000000000444089209850062616169452667236328125') >>> repr(0.299999999999999988897769753748434595763683319091796875) '0.3' >>> repr(0.30000000000000004) '0.30000000000000004' >>> print 0.30000000000000004 0.3
- jamesdutc 11y ago
  I believe this is relevant: https://bugs.python.org/issue1580 https://bugs.python.org/issue1580
- evanpw 11y ago
  There's some discussion here: https://docs.python.org/3.1/tutorial/floatingpoint.html https://docs.python.org/3.1/tutorial/floatingpoint.html. Basically: eval(repr(x)) == x is guaranteed, and some attempt is made to give the shortest such string representation for Python 3.1+ (and 2.7?), but it's not guaranteed on all platforms.
- indosauros 11y ago
  It also matters whether you're using Python 2 or 3. This is one of the things they changed in 3: >>> print(0.1 + 0.2) 0.30000000000000004
- _ZeD_ 11y ago
  you are using Decimal "wrong". If you want it to handle exact values, you need to pass a string with the decimal interpretation Python 3.5.0 (v3.5.0:374f501f4567, Sep 13 2015, 02:27:37) [MSC v.1900 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> from decimal import Decimal >>> Decimal('0.1') + Decimal('0.2') - Decimal('0.3') Decimal('0.0') >>> Decimal(0.1) + Decimal(0.2) - Decimal(0.3) Decimal('2.775557561565156540423631668E-17') >>> Decimal(0.1) == Decimal('0.1') False
  - philh 11y ago
    In this case, that was deliberate. I was using it to show the exact decimal value of the floating point approximations to decimal numbers, not to do decimal arithmetic.

by philh 11y ago

deleted 11y ago

[deleted]

philh 11y ago

Someone pointed out that this isn't correct, and then deleted their comment. (?) I now think the behiviour I ascribed to print, is actually what repr does. I'm not sure about print, but I guess it is just rounding to a certain number of places. >>> from decimal import Decimal >>> Decimal(0.1+0.2) Decimal('0.3000000000000000444089209850062616169452667236328125') >>> Decimal(0.3) Decimal('0.299999999999999988897769753748434595763683319091796875') >>> Decimal(0.300000000000000000000000000001) Decimal('0.299999999999999988897769753748434595763683319091796875') >>> print 0.300000000000000000000000000001 0.3 >>> repr(0.300000000000000000000000000001) '0.3' >>> D(0.30000000000000004) Decimal('0.3000000000000000444089209850062616169452667236328125') >>> repr(0.299999999999999988897769753748434595763683319091796875) '0.3' >>> repr(0.30000000000000004) '0.30000000000000004' >>> print 0.30000000000000004 0.3

jamesdutc 11y ago
I believe this is relevant: https://bugs.python.org/issue1580 https://bugs.python.org/issue1580
evanpw 11y ago
There's some discussion here: https://docs.python.org/3.1/tutorial/floatingpoint.html https://docs.python.org/3.1/tutorial/floatingpoint.html. Basically: eval(repr(x)) == x is guaranteed, and some attempt is made to give the shortest such string representation for Python 3.1+ (and 2.7?), but it's not guaranteed on all platforms.
indosauros 11y ago
It also matters whether you're using Python 2 or 3. This is one of the things they changed in 3: >>> print(0.1 + 0.2) 0.30000000000000004
_ZeD_ 11y ago
you are using Decimal "wrong". If you want it to handle exact values, you need to pass a string with the decimal interpretation Python 3.5.0 (v3.5.0:374f501f4567, Sep 13 2015, 02:27:37) [MSC v.1900 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> from decimal import Decimal >>> Decimal('0.1') + Decimal('0.2') - Decimal('0.3') Decimal('0.0') >>> Decimal(0.1) + Decimal(0.2) - Decimal(0.3) Decimal('2.775557561565156540423631668E-17') >>> Decimal(0.1) == Decimal('0.1') False
- philh 11y ago
  In this case, that was deliberate. I was using it to show the exact decimal value of the floating point approximations to decimal numbers, not to do decimal arithmetic.