[Tutor] How to correct decimal addition.

[Keith Winston]

On Sat, Jan 25, 2014 at 3:57 AM, spir <denis.spir at gmail.com> wrote:
>> .009 to the price, so that people do not have to type the full amount.
>>   Example, 3.49 /gallon would return 3.499 /gallon.
>>
>> This is what I have tried and the results of it.
>>
>> def gas_price(price):
>>     price == raw_input("What is the price of gas?")  return price + .09
>>    3.49=> 3.4899999999999998

I think there's an inconsistency in your post that might confuse the
answers. You mention in the lead that you want to add 9 ONE HUNDREDTHS
of a dollar, or tenths of a cent (which is in fact how gas is priced
in the US, and yes it's crazy stupid). However in your example you add
only tenths, but then in the answer you appear to have added
hundredths, which makes me think that you didn't cut & paste, but
rather retyped (and mistyped).

This will make it a little trickier to use Denis' last idea of using
integers, since you'll have to take them out one more order of
magnitude. If this exercise is later followed by interest
calculations, or anything like that, you might regret limiting your
internal accuracy/representation.

I think that you should probably do your math in floating point (why
get complicated? And you might need the accuracy, for hundredths of
dollars and interest) and then format the output to be what you want.
Watch out for rounding.

>>> p = 3.499
>>> print('{0:.3f}'.format(p))   # format a float with 3 places after the decimal
3.499
>>> p = 3.4994
>>> print('{0:.3f}'.format(p))
3.499
>>> p = 3.4999999
>>> print('{0:.3f}'.format(p))
3.500

-- 
Keith