[Tutor] oops - resending as plain text

[Joel Goldstick]

On Tue, Apr 16, 2013 at 1:48 PM, Jim Mooney <cybervigilante at gmail.com>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?
>
> 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?
>

because binary computers convert decimal numbers to binary numbers.  There
is often a rounding error.

>   print(twenties, ' twenties')
>   print(change)
>
> #and so forth for the rest of the prog
>
> Jim Mooney
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-- 
Joel Goldstick
http://joelgoldstick.com
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