Re: [Tutor] Which libraries for Python 2.5.2 [SOLVED]
This problem was solved when my wife noticed that there was a second install disk for the 5 year old XP zx6000 PC she had given me, which I will now give to a friend. The problem originally was a missing dll that Python wanted. All is well now. -- Wayne Watson (Watson Adventures, Prop., Nevada City, CA) (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time) Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet CE 1955 October 20 07:53:32.6 UT -- "The Date" The mystery unfolds. Web Page: ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Class vs. instance
Hi there! >>> class Sample: >>> def method(self): pass >>> Sample().method() What's the difference between class __main__.Sample and __main__.Sample instance? Why should I write "Sample().method" instead of "Sample.method"? Cheers! ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Class vs. instance
On Sun, Jan 1, 2012 at 8:40 PM, Stayvoid wrote: > Hi there! > class Sample: def method(self): pass > Sample().method() > > What's the difference between class __main__.Sample and > __main__.Sample instance? > Why should I write "Sample().method" instead of "Sample.method"? > The difference can be illustrated as such: >>> Sample().method > >>> Sample().method() >>> Sample.method >>> Sample.method() Traceback (most recent call last): File "", line 1, in TypeError: unbound method method() must be called with Sample instance as first argument (got nothing instead) >>> That is, the difference between the methods is that the accessed through the instance is also attached to that instance. It will automagically get Sample() passed to it as its first argument (that would be self). The one attached to the class is unbound, which means that you can do this: >>> Sample.method(Sample()) >>> With any Sample instance, of course. This exposes a bit of syntax sugar in python and how classes are really implemented, essentially the fact that, if "a" is a sample instance, a.method(arg1, arg2, arg3) is actually just Sample.method(a, arg1, arg2, arg3) HTH, Hugo ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Class vs. instance
Thanks. I totally get it now. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] while loop ends prematurly
Hello, Can some please explain this to me? My while loop should continue while "owed" is greater than or equal to "d" first time the function is called the loop exits as expected False: 0.00 >= 0.01 the next time it does not False: 0.01 >= 0.01 Below is the snippet of code, and the out put. Thanks! def make_change(arg): denom = [100.0, 50.0, 20.0, 10.0, 5.0, 1.0, 0.25, 0.10, 0.05, 0.01] owed = float(arg) payed = [] for d in denom: while owed >= d: owed -= d b = owed >= d print '%s: %f >= %f' % (b, owed, d) payed.append(d) print sum(payed), payed return sum(payed) if __name__ == '__main__': values = [21.48, 487.69] #, 974.41, 920.87, 377.93, 885.12, 263.47, 630.91, 433.23, 800.58] for i in values: make_change(i)) False: 1.48 >= 20.00 False: 0.48 >= 1.00 False: 0.23 >= 0.25 True: 0.13 >= 0.10 False: 0.03 >= 0.10 True: 0.02 >= 0.01 True: 0.01 >= 0.01 False: 0.00 >= 0.01 21.48 [20.0, 1.0, 0.25, 0.1, 0.1, 0.01, 0.01, 0.01] True: 387.69 >= 100.00 True: 287.69 >= 100.00 True: 187.69 >= 100.00 False: 87.69 >= 100.00 False: 37.69 >= 50.00 False: 17.69 >= 20.00 False: 7.69 >= 10.00 False: 2.69 >= 5.00 True: 1.69 >= 1.00 False: 0.69 >= 1.00 True: 0.44 >= 0.25 False: 0.19 >= 0.25 False: 0.09 >= 0.10 False: 0.04 >= 0.05 True: 0.03 >= 0.01 True: 0.02 >= 0.01 False: 0.01 >= 0.01 487.68 [100.0, 100.0, 100.0, 100.0, 50.0, 20.0, 10.0, 5.0, 1.0, 1.0, 0.25, 0.25, 0.1, 0.05, 0.01, 0.01, 0.01] ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] while loop ends prematurly
On 01/01/2012 09:48 PM, brian arb wrote: Hello, Can some please explain this to me? My while loop should continue while "owed" is greater than or equal to "d" first time the function is called the loop exits as expected False: 0.00>= 0.01 the next time it does not False: 0.01>= 0.01 Below is the snippet of code, and the out put. Thanks! def make_change(arg): denom = [100.0, 50.0, 20.0, 10.0, 5.0, 1.0, 0.25, 0.10, 0.05, 0.01] owed = float(arg) payed = [] for d in denom: while owed>= d: owed -= d b = owed>= d print '%s: %f>= %f' % (b, owed, d) payed.append(d) print sum(payed), payed return sum(payed) if __name__ == '__main__': values = [21.48, 487.69] #, 974.41, 920.87, 377.93, 885.12, 263.47, 630.91, 433.23, 800.58] for i in values: make_change(i)) False: 1.48>= 20.00 False: 0.48>= 1.00 False: 0.23>= 0.25 True: 0.13>= 0.10 False: 0.03>= 0.10 True: 0.02>= 0.01 True: 0.01>= 0.01 False: 0.00>= 0.01 21.48 [20.0, 1.0, 0.25, 0.1, 0.1, 0.01, 0.01, 0.01] True: 387.69>= 100.00 True: 287.69>= 100.00 True: 187.69>= 100.00 False: 87.69>= 100.00 False: 37.69>= 50.00 False: 17.69>= 20.00 False: 7.69>= 10.00 False: 2.69>= 5.00 True: 1.69>= 1.00 False: 0.69>= 1.00 True: 0.44>= 0.25 False: 0.19>= 0.25 False: 0.09>= 0.10 False: 0.04>= 0.05 True: 0.03>= 0.01 True: 0.02>= 0.01 False: 0.01>= 0.01 487.68 [100.0, 100.0, 100.0, 100.0, 50.0, 20.0, 10.0, 5.0, 1.0, 1.0, 0.25, 0.25, 0.1, 0.05, 0.01, 0.01, 0.01] You're using float values and pretending that they can accurately represent dollars and cents. 0.19 (for example) cannot be exactly represented in a float, and when you start adding up multiple of these, sooner or later the error will become visible. This is a problem with binary floating point, and I first encountered it in 1967, when the textbook for Fortran made an important point about never comparing floating point values for equals, as small invisible errors are bound to bite you. Easiest answer is to use integers. Scale everything up by a factor of 100, and you won't need floats at all. Just convert when printing (and even then you may get into trouble). Another answer is to use Decimal class, which CAN represent decimal values exactly. BTW, if this is supposed to represent US legal tender, you left out the fifty-cent piece as well as the two dollar bill. -- DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] while loop ends prematurly
On Mon, Jan 2, 2012 at 3:48 AM, brian arb wrote: > Hello, > Can some please explain this to me? > My while loop should continue while "owed" is greater than or equal to "d" > > first time the function is called > the loop exits as expected > False: 0.00 >= 0.01 > the next time it does not > False: 0.01 >= 0.01 > > Below is the snippet of code, and the out put. > > Thanks! > > def make_change(arg): > denom = [100.0, 50.0, 20.0, 10.0, 5.0, 1.0, 0.25, 0.10, 0.05, 0.01] > owed = float(arg) > payed = [] > for d in denom: > while owed >= d: > owed -= d > b = owed >= d > print '%s: %f >= %f' % (b, owed, d) > payed.append(d) > print sum(payed), payed > return sum(payed) > > if __name__ == '__main__': > values = [21.48, 487.69] #, 974.41, 920.87, 377.93, 885.12, 263.47, > 630.91, 433.23, 800.58] > for i in values: > make_change(i)) > > > False: 1.48 >= 20.00 > False: 0.48 >= 1.00 > False: 0.23 >= 0.25 > True: 0.13 >= 0.10 > False: 0.03 >= 0.10 > True: 0.02 >= 0.01 > True: 0.01 >= 0.01 > False: 0.00 >= 0.01 > 21.48 [20.0, 1.0, 0.25, 0.1, 0.1, 0.01, 0.01, 0.01] > True: 387.69 >= 100.00 > True: 287.69 >= 100.00 > True: 187.69 >= 100.00 > False: 87.69 >= 100.00 > False: 37.69 >= 50.00 > False: 17.69 >= 20.00 > False: 7.69 >= 10.00 > False: 2.69 >= 5.00 > True: 1.69 >= 1.00 > False: 0.69 >= 1.00 > True: 0.44 >= 0.25 > False: 0.19 >= 0.25 > False: 0.09 >= 0.10 > False: 0.04 >= 0.05 > True: 0.03 >= 0.01 > True: 0.02 >= 0.01 > False: 0.01 >= 0.01 > 487.68 [100.0, 100.0, 100.0, 100.0, 50.0, 20.0, 10.0, 5.0, 1.0, 1.0, 0.25, > 0.25, 0.1, 0.05, 0.01, 0.01, 0.01] > What happened is that you ran into the weirdness that we call the IEEE 754-2008 standard, otherwise known as floating point numbers. in quite simple terms, the way the computer represents floating point numbers means that inaccuracies sneak in when performing math on them, and some numbers can't even be represented correctly, like 0.1. You can notice this in some of the simplest calculations: >>> 0.1 0.1 >>> # seems normal? Well, python is actually tricking you. Let's force it to >>> show us this number with some more accuracy: >>> "%.32f" % 0.1 # force it to show 32 digits after the period '0.1555111512312578' >>> # whoops! that's not quite 0.1 at all! let's try some more: >>> 9 * 0.1 0.9 >>> 0.9 0.9 >>> 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 0.8999 >>> "%.32f" % 0.9 '0.90002220446049250313' >>> # what?! those aren't even the same numbers!! >>> 0.1 + 0.2 0.30004 >>> # what the hell? Usually this doesn't really matter, because we don't really care about what happens after you get way far into the decimal spaces. But when you compare for equality, which is what you're doing here, this stuff can bite you in the ass real ugly. If you replace the %f with %.32f in that debugging statement, you'll see why the loop bails: False: 0.0077 >= 0.0100 That kinda sucks, doesn't it? floating point errors are hard to find, especially since python hides them from you sometimes. But there is a simple solution! Multiply all numbers by 100 inside that function and then simply work with integers, where you do get perfect accuracy. HTH, Hugo P.S.: this problem is not in inherent to python but to the IEEE standard. The sacrifice in accuracy was made deliberately to keep floating point numbers fast, so it's by design and not something that should be "fixed." Pretty much all languages that use floats or doubles have the same thing. If you really want decimal numbers, there is a Decimal class in Python that implements 100% accurate decimal numbers at the cost of performance. Look it up. P.P.S.: for more information you should read these. The first link is a simple explanation. The second is more complicated, but obligatory reading material for every programmer worth his salts: the floating point guide: http://floating-point-gui.de/ what every computer scientist should know about floating-point arithmetic: http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] while loop ends prematurly
Dave Angel wrote: Easiest answer is to use integers. Scale everything up by a factor of 100, and you won't need floats at all. Just convert when printing (and even then you may get into trouble). Another answer is to use Decimal class, which CAN represent decimal values exactly. That only applies to decimal values which can be represented using a fixed number of decimal places. So 1/5 is fine, and is 0.2 exactly, but 1/3 is not, since it would require an infinite number of decimal places. BTW, if this is supposed to represent US legal tender, you left out the fifty-cent piece as well as the two dollar bill. http://kowb1290.com/our-two-cents-on-the-two-dollar-bill/ -- Steven ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
