[Tutor] oops - resending as plain text

[Jim Mooney]

Further question. If I round the input right at the beginning,
round(paid,2) does that mean  I still have the original error from
using .76 even before math, or does the rounding kill it? I would
guess not if it's binary, although Python must have a way to handle
money amounts. I'm only on Chapter 2 ;')

I assume Python has some automatic way to filter input, so that if
someone entered three decimals instead of two for a money amount, they
could get a wrist slap. Can you direct me to that functionality?
Thanks.

Jim

On 16 April 2013 11:55, Sander Sweers <sander.sweers at gmail.com> wrote:
> On 04/16/2013 07:48 PM, Jim Mooney wrote:
>> I accidentally sent as HTML so this is a resend in case that choked
>> the mailing prog ;')
>>
>> I was doing a simple training prog to figure monetary change, and
>> wanted to avoid computer inaccuracy by using only two-decimal input
>> and not using division or mod where it would cause error. Yet, on a
>> simple subtraction I got a decimal error instead of a two decimal
>> result, as per below. What gives?
>
> Floats can not be represented accurately in binary and will have small
> rounding errors. See
> https://en.wikipedia.org/wiki/Floating_point#Accuracy_problems.
>
>> cost = float(input('How much did the item cost?: '))
>> paid = float(input('How much did the customer give you?: '))
>> change = paid - cost
>>
>> #using 22.89 as cost and 248.76 as paid
>>
>> twenties = int(change / 20)
>> if twenties != 0:
>>   twentiesAmount = 20 * twenties
>>   change = change - twentiesAmount
>>   #change is 5.8700000000000045, not 5.87 - how did I get this decimal
>> error when simply subtracting an integer from what  should be a
>>   #two-decimal amount?
>>   print(twenties, ' twenties')
>>   print(change)
>>
>> #and so forth for the rest of the prog
>
> You can use string formatting to show as much precision you want. Example:
>
>>>> n = 5.8700000000000045
>>>> print '%.2f' % n
> 5.87
>>>> print '%.4f' % n
> 5.8700
>
> Or use round(), example:
>
>>>> round(n,2)
> 5.87
>
> In your case I would just use string formatting to hide the rounding error.
>
> Greets
> ~sander



-- 
Jim Mooney

If you shoot a child you're a bad guy. If you splatter forty children
across a wall with a bomb, you're a heroic, manly Top Gun with
gleaming Tom Cruise teeth. The moral is you'll get laid more if you
snuff a lot of children than if you only snuff a few.