>>>>> casevh <[EMAIL PROTECTED]> (C) wrote: >C> On Feb 25, 2:44 am, [EMAIL PROTECTED] wrote: >>> Hi I'm very much a beginner with Python. >>> I want to write a function to convert celcius to fahrenheit like this >>> one: >>> >>> def celciusToFahrenheit(tc): >>> tf = (9/5)*tc+32 >>> return tf >>> >>> I want the answer correct to one decimal place, so >>> celciusToFahrenheit(12) would return 53.6. >>> >>> Of course the function above returns 53.600000000000001. >>> >>> How do I format it correctly?
>C> That is the normal behavior for binary (radix-2) numbers. Just like it >C> is impossible write 1/3 exactly as a decimal (radix-10) number, 536/10 >C> cannot be written exactly as a binary number. If you really need >C> decimal numbers, use the Decimal class. Sorry to come in so late in this discussion. Although it is correct to say that many real numbers that have an exact decimal representation cannot be exactly represented in binary, that is no excuse to print 53.6 as 53.600000000000001. This is just lousy printing and the fact that this kind of question comes up every week shows that it is confusing to many people. Python just uses the C library for printing, I presume, and the conversion routines in the C library are rather simplistic. It is, however, possible to do better, so that 53.6 -- although internally represented as something that could be described as 53.600000000000001 -- will actually be printed as 53.6. Indeed, when reading back the printed value you get the exact representation as the internal number that was printed, and IMHO, that is what matters. Apparently there is more than one representation that has this property. I would guess (but didn't check) that 53.60000000000000100000001 also gives the same number. From all these representations it would be best to choose the simplest one, i.e. 53.6. This problem and a solution has been described in one of the classical computer science publications: http://portal.acm.org/citation.cfm?id=93559 -- Piet van Oostrum <[EMAIL PROTECTED]> URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4] Private email: [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
