On 12/18/2009 1:00 AM, Brendan Miller wrote:
For the benefit of those of us who aren't C++ programmers, what do its iterators do that Python's don't?
It depends on what one means by 'iterator'. Python iterators do not fit in the STL hierarchy. On the other hand, Python indexes are a form of random access iterator, the top of the hierarchy.
Python iterators basically only have one operation:
next(), which returns the next element or throws StopIteration. In C++ terminology this is a Input iterator. It is good for writing "for each" loops or map reduce operations. An input iterator can't mutate the data it points to.
For that, Python uses indexes. Random access 'iterators' are a generalization of sequence indexes.
C++ also has progressively stronger iterators: http://www.sgi.com/tech/stl/Iterators.html InputIterator<- read only, one direction, single pass ForwardIterator<- read/write, one direction, multi pass BidirectionalIterator<- read/write, can move in either direction RandomAccessIterator<- read/write, can move in either direction by an arbitrary amount in constant time (as powerful as a pointer)
An index can be incremented or decremented an arbitrary amount. Note that Python indexes are object pointers, not memory byte pointers, so that index + 1 points to the next object, not the next byte of memory in possibly the same object. They are, however, much safer than pointers in that x[i] can only retrieve objects of x and never, surprise, surprise, of some other collection.
Each only adds extra operations over the one before. So a RandomAccessIterator can be used anywhere a InputIterator can, but not vice versa. Also, this is a duck typing relationship, not a formal class inheritance. Anything that quacks like a RandomAccessIterator is a RandomAccessIterator, but there is no actual RandomAccessIterator class. So, for instance stl sort function takes pair of random access iterator delimiting a range, and can sort any datastructure that can provide that powerful of an iterator (arrays, vectors, deques).
Python, traditionally, only came with one mutable builtin sequence type, so the sort function was made a method of that type. There is also the importable array module. Apparently, there has been little demand for a generic array.sort method as there is none. One could easily write a function that took an array or other seqeunce and two indexes as args.
The generic reversed() builtin function by default uses sequence indexes in the obvious manner to return a reversed version of *any* sequence, mutable or not.
http://www.sgi.com/tech/stl/sort.html MyCollection stuff; // put some stuff in stuff sort(stuff.begin(), stuff.end()); Where begin() and end() by convention return iterators pointing to the beginning and end of the sequence.
start,stop = 0, len(seq) Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
