How to make a reverse for loop in python?
Hello I'm new to python and i can't figure out how to write a reverse for loop in python e.g. the python equivalent to the c++ loop for (i = 10; i >= 0; --i) -- http://mail.python.org/mailman/listinfo/python-list
Re: How to make a reverse for loop in python?
On Sep 20, 8:13 pm, [EMAIL PROTECTED] wrote: > Duncan Booth: > > > > e.g. the python equivalent to the c++ loop > > > for (i = 10; i >= 0; --i) > > > The exact equivalent would be: > > for i in range(10, -1, -1): print i > > I'd use xrange there. Anyway, I have always felt that Python syntax > not easy to understand at first sight, expecially when you try to > convert a bit more complex inverted for loops from/to C to/from > Python. It's one of the few cases where (for example) Pascal (loop) > syntax wins a bit over Python syntax :-) > > Bye, > bearophile That's a lot of responses guys. Thanks a lot i think i got it. Another question, are there any pointers in python (or iterators) for when i use a data structure that doesn't support random access? Thanks again, Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: How to make a reverse for loop in python?
On Sep 20, 8:13 pm, [EMAIL PROTECTED] wrote: > Duncan Booth: > > > > e.g. the python equivalent to the c++ loop > > > for (i = 10; i >= 0; --i) > > > The exact equivalent would be: > > for i in range(10, -1, -1): print i > > I'd use xrange there. Anyway, I have always felt that Python syntax > not easy to understand at first sight, expecially when you try to > convert a bit more complex inverted for loops from/to C to/from > Python. It's one of the few cases where (for example) Pascal (loop) > syntax wins a bit over Python syntax :-) > > Bye, > bearophile Another quick question please, is the List data structure just a dynamic array? If so how can you use static size array, linked list, AVL trees etcetera. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to make a reverse for loop in python?
On Sep 21, 3:47 am, Steven D'Aprano <[EMAIL PROTECTED]
cybersource.com.au> wrote:
> On Sat, 20 Sep 2008 16:27:41 -0700, Alex Snast wrote:
> > Another quick question please, is the List data structure just a dynamic
> > array? If so how can you use static size array, linked list, AVL trees
> > etcetera.
>
> Before I answer your question, I should say that you can go a LONG way
> with just the standard Python built-in data structures list, dict and
> set, plus a handful of standard modules like array and collections. It's
> often (but not always) better to modify an algorithm to use a built-in
> data structure than to try to implement your own.
>
> The underlying data structure for lists is implementation specific. Only
> the behaviour is specified by the language.
>
> In the standard Python implementation written in C (usually known as
> "Python", although sometimes people explicitly describe it as CPython),
> lists are implemented as a fixed array of pointers. The array is
> periodically resized, either up or down, but only as needed. The largest
> consequence of that is that appending to the end of a list is much faster
> than inserting at the beginning of the list.
>
> Other implementations (IronPython, Jython, PyPy, CLPython...) are free to
> implement lists whatever way they need.
>
> If you want a static list, the simplest way is to create a list and
> simply not resize it. If you want to enforce that, here's a subclass to
> get you started:
>
> class StaticList(list):
> def _resize(self):
> raise RuntimeError("list can't be resized")
> extend = append = pop = insert = remove = \
> __delitem__ = __delslice__ = _resize
>
> I haven't dealt with __setitem__ or __setslice__, because they're more
> complicated: you need to make sure the slice you're setting has the same
> size as the bit you're replacing, so that this is allowed:
>
> mylist[3:6] = [1, 2, 3]
>
> but not these:
>
> mylist[3:6] = [1, 2]
> mylist[3:6] = [1, 2, 3, 4]
>
> As for linked lists and trees, don't worry about pointers, just go ahead
> and implement them.
>
> # basic, no-frills tree
> class Node(object):
> def __init__(self, data, left=None, right=None):
> self.left = left
> self.right = right
> self.info = data
>
> tree = Node('top of the tree')
> tree.left = Node('left subtree')
> tree.right = Node('right subtree', None, Node('another subtree'))
> t = tree.right.right
> t.left = Node('yet another subtree')
>
> etc.
>
> The CPython implementation of dict is a hash table, and dicts are
> extremely fast and efficient. So long as you don't mind losing the order
> of insertion, you won't beat dicts for speed and efficiency in anything
> you write in pure Python.
>
> --
> Steven
WOW you guys are really helpful, thanks everyone for all the replies.
Last question:
What IDE do you guys recommend, I'm currently using pydev.
Thanks again, Alex
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Quick sort implementation in python
Hi guys, I've been learning python in the past week and tried to implement a q.sort algorithm in python as follows: def quick_sort(l, first, last) if first < last: q = partition(a, first, last) quick_sort(a, first, q - 1) quick_sort(a, q + 1, last) def partition(a, first, last): import random pivot = random.randomint(first, last) a[last], a[pivot] = a[pivot], a[last] i = first for j in range(first, last): if a[j] <= a[last]: a[i], a[j] = a[j], a[i] i += 1 a[i], a[last] = a[last], a[i] return i Now as you can see I'm passing my list object to both functions along with their first, last indices My question is: Is that the normal way to implement algorithms in python cause in c++ i've implemented that algo via a template function which can have a randon access data structure or not. However i have no idea how to access the values of a data structure that doesn't allow random access. Thanks, Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: Quick sort implementation in python
On Sep 25, 11:47 pm, "Martin v. Löwis" <[EMAIL PROTECTED]> wrote: > > Now as you can see I'm passing my list object to both functions along > > with their first, last indices > > I cannot really see that. More specifically, it isn't definite what the > type of the "a" argument is, nor does the specific type of "a" matter > for the algorithm. It could be a list, or it could be a different > mutable collection that is integer-indexed. > > > My question is: Is that the normal way to implement algorithms in > > python > > Yes, it is. > > > cause in c++ i've implemented that algo via a template function > > which can have a randon access data structure or not. However i have > > no idea how to access the values of a data structure that doesn't > > allow random access. > > Can you please explain how you did that in C? IOW, how did you do > the partition function (template) in case you don't have random > access to the collection? > > Regards, > Martin Why exactly do you need random access for partition function? Do can swap 2 nodes of a linked list without random access (you can swap the pointers or just swap the node values) and you traverse the list till you reach it's tail. -- http://mail.python.org/mailman/listinfo/python-list
