[Tutor] Want to keep to two decimal places for currency

[Shelby Martin]

Thank you for your replies. I suspect the solution is a bit more advanced
than where I'm at now, which is chapter 2 of a beginner's book. Not sure
why the author chose to use examples using money calculations when other
calculations that don't need rounding would have sufficed. I will have to
revisit this issue when I'm further along.


On Wed, Oct 23, 2013 at 8:44 PM, Danny Yoo <dyoo at hashcollision.org> wrote:

>
>
>
> On Wed, Oct 23, 2013 at 4:13 PM, Alan Gauld <alan.gauld at btinternet.com>wrote:
>
>> On 23/10/13 22:39, Shelby Martin wrote:
>>
>>> I've looked online but I'm confused - I need to keep it so that the
>>> following program limits the output to two decimal places since it deals
>>> in currency.
>>>
>>
>> Its generally a bad idea to use floats when dealing with currency since
>> they can introduce small errors which can accumulate over time. It's better
>> to either use the Decimal module or to convert money to cents/.pennies and
>> then convertback to dollars for display purposes.
>>
>
>
> Hi Shelby,
>
>
> If you want to read some gory details, you can find the reasons for why
> it's unsafe to represent currency with "floating point" numbers.  There's a
> classic paper called "What Every Computer Scientist Should Know About
> Floating-Point Arithmetic":
>
>     http://www.validlab.com/goldberg/paper.pdf
>
> and you probably don't want to read the whole thing.  :P  A rough gist of
> the problem: traditional, engineering-focused computer math has a
> particular trade-off that most programmers do not realize at first: it
> trades accuracy for speed.  The native hardware of your computer does
> calculations in base-2 rather than base-10 arithmetic.  Unfortunately, that
> means that fractional quantities take a representational hit: your computer
> cannot accurately represent certain decimals in base-2.
>
> It turns out that this floating point arithmetic limitation is not so bad
> for a lot of important applications.  But it doesn't bode well at all for
> applications that deal with money.  There are libraries in Python that
> avoid using the built-in floating point hardware, such as the Decimal
> library that Alan noted.  Computations with it are slower, but that's an
> acceptable price for getting the dollars and cents right.  If we would do
> this sort of thing in the real world, we'd use something like the Decimal
> library.  You'll see equivalent kinds of libraries in other programming
> languages, like the BigDecimal class in Java.
>
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