Thanks all! This helps a lot.
On Jul 1, 2011, at 6:13 AM, Steven D'Aprano wrote:
> Ryan Kirk wrote:
>> Is there a way to limit raw_input to the hundredth decimal point?
>
> No. raw_input is a tool that does one thing: it collects input from the user.
> It doesn't understand numbers, check for decimal places, check the input for
> spelling errors, or anything else. It's a hammer, not a combination
> hammer-screwdriver-wrench-drill-saw-axe :)
>
> One solution is to build a new tool that checks for decimal places:
>
>
> def check(text):
>try:
>x = float(text)
>except ValueError:
>print "please enter a number"
>return None
>y = x*100
>if y - int(y) != 0:
>print "please enter only two decimal places"
>return None
>return x
>
>
> def get_number(prompt):
>answer = None
>while answer is None:
>text = raw_input(prompt)
>answer = check(text)
>return answer
>
>
> At first, this seems to work well:
>
> >>> get_number("Please enter a number with two decimal places: ")
> Please enter a number with two decimal places: 77.25
> 77.25
> >>>
>
> but there's a fundamental problem. The user is entering numbers in decimal
> (base 10), but Python does calculations in binary (base 2), and something
> that has two decimal places may not be exact in binary:
>
> >>> get_number("Please enter a number with two decimal places: ")
> Please enter a number with two decimal places: 77.21
> please enter only two decimal places
>
> Huh? 77.21 does have two decimal places. But the closest float to 77.21 is in
> fact 77.204. No computer on Earth can store 77.21 *exactly* as a
> binary float, no matter how hard you try!
>
> So, what to do...? You can:
>
> (1) Give up on forcing the user to only enter two decimal places, and instead
> use the round() function to round to two places:
>
> >>> round(77.2123456, 2)
> 77.204
>
> This is still not two decimal places, but it is the closest possible float to
> 7.21, so you can't do any better.
>
> (2) Or give up on using float, and use the decimal module instead. (However
> decimals are slower and less convenient than floats.)
>
> >>> from decimal import Decimal
> >>> x = Decimal("77.21")
> >>> x
> Decimal("77.21")
>
>
> If you are working with currency, then you should use decimal, and not floats.
>
>
>
> Good luck!
>
>
>
> --
> Steven
>
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