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Function "round" in python3 not working

  • Python 3.6.0 (default, Mar 27 2018, 01:18:40)
    [GCC 5.4.0] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import AdcExp
    >>> adc_1 = AdcExp.AdcExp(address=0x48)
    >>> accel_volt_x = adc_1.read_voltage(0)
    >>> g_x = (accel_volt_x - 1.51) / 0.35
    >>> g_x
    >>> round(g_x, 1)

    I want to round the number to tenths, but 'round' does not work correctly.

  • @CAP-33 python rounding is a pain point for many users for quite some time, using your value for g_x try this:

    print (round(g_x))

    result is 0

    Now try this: print (round (g_x,2))

    result is 0.34000000000000002

    and now this: print (round (g_x,2) * 2)

    result is 0.68000000000000005

    Why is the final 5 not a 4 which is 2 x 2?

    Now try this:

    print("% 0.2f" %(g_x))

    the result is 0.34 which is what you would expect.

    So we can better understand how rounding in python performs, set g_x to
    g_x = 0.34567999999999

    So a round up to 2 decimal places would be 0.35 while a truncation would be 0.34.

    round (g_x,2) produces a result of 0.34999999999999998 which if you rounded to 2 decimal places would be 0.35. You can see it is modifying the storage in order to produce a correctly rounded result.

    Now try this:
    print("% 0.2f" %(g_x)) outputs 0.35 which is what you would expect.

    Another reason why I consider python to be junk. junk. junk.

    But this is not a question for the Onion forums, it's really a question for a Python forum because it's a quirk/junk in python

  • @crispyoz
    Many thanks. I like python, but it, like other languages, has its drawbacks.

  • @CAP-33 You are correct this is a "drawback" of python. I find python has a lot of drawbacks šŸ™‚ That's why I develop my apps using C.

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