Hi, On Sunday 19 April 2009 18:44:30 michel paul wrote: > On Sun, Apr 19, 2009 at 2:48 PM, Gregor Lingl <gregor.li...@aon.at> wrote: > > >How do you explain the nature of range to beginners? > > How about using list(range())? Something like: > >>> # Here's how you can create a list of integers: > >>> list(range(10)) > > [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] > > >>> list(range(1, 10)) > > [1, 2, 3, 4, 5, 6, 7, 8, 9] > > >>> list(range(1, 10, 2)) > > [1, 3, 5, 7, 9] > > >>> list(range(-10, 10, 2)) > > [-10, -8, -6, -4, -2, 0, 2, 4, 6, 8] > > >>> # 'list' creates a list, and 'range' specifies its starting point, > > ending point, and interval between points.
I would say that the range specifies the sequence of objects that the list is created from. I agree that using list is a good way to approach this. > >>> # a range object produces these values when called upon to do so. > >>> # for example, in a loop: > >>> for x in range(10): (x, x**2) > > (0, 0) > (1, 1) > (2, 4) > (3, 9) > (4, 16) > (5, 25) > (6, 36) > (7, 49) > (8, 64) > (9, 81) > > Now, interesting, here I've stumbled on a question that I need some > > clarification on: > >>> a = range(10) > >>> type(a) > > <class 'range'> > > >>> next(a) > > Traceback (most recent call last): > File "<pyshell#49>", line 1, in <module> > next(a) > TypeError: range object is not an iterator > > >>> help(range) > > Help on class range in module builtins: > > class range(object) > > | range([start,] stop[, step]) -> range object > | > | Returns an iterator that generates the numbers in the range on demand. > > So is range an iterator? No, a range object is not itself an iterator. It is an object that returns an iterator when you ask it for one. You can create any number of independent iterators from a single range object: >>> x = range(1,10) >>> it1 = iter(x) >>> >>> next(it1) 1 >>> next(it1) 2 >>> it2 = iter(x) >>> next(it2) 1 >>> next(it2) 2 > A for loop always "asks" the sequence object for its iterator. If you want to loop over a collection object, it should implement the __iter__ hook that produces and iterator object. It is the iterator object that implements the __next__ hook. Of course, the usual approach is to use a generator method (uses yield) which itself hands back an iterator when it is "called." --John -- John M. Zelle, Ph.D. Wartburg College Professor of Computer Science Waverly, IA john.ze...@wartburg.edu (319) 352-8360 _______________________________________________ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
