Using iterators to write in the structure being iterated through?

[Pierre Thibault]

On Wed, 26 Jul 2006 16:11:48 -0700, Paddy wrote:

> 
> Paddy wrote:
>> Pierre Thibault wrote:
>> > Hello!
>> >
>> > I am currently trying to port a C++ code to python, and I think I am stuck
>> > because of the very different behavior of STL iterators vs python
>> > iterators. What I need to do is a simple arithmetic operations on objects
>> > I don't know. In C++, the method doing that was a template, and all that
>> > was required is that the template class has an iterator conforming to the
>> > STL forward iterator definition. Then, the class would look like:
>> >
>> <SNIP>
>> > Then I discovered python and wanted to use all its goodies. I thought it
>> > would be easy to do the same thing but I can't: the iterator mechanism is
>> > read-only, right? So it does no make sense to write:
>> >
>> > io1 = iter(object1)
>> > io2 = iter(object2)
>> >
>> > try:
>> >   while 1:
>> >     io1.next() += io2.next()
>> > except StopIteration:
>> >   pass
>> >
>> > That won't work:
>> > SyntaxError: can't assign to function call
>> >
>> > Here is my question: how could I do that and retain enough generallity?
>> >
>> > Thanks!
>> >
>> > Pierre
>>
>> Pierre,
>> You should be able to write
>>   io1.next().param += io2.next().param
>> If iter(object1) and iter(object2) both return classes or instances
>> with the appropriate parameter.
>> Here is what I was thinking of:
>>
>>
>> class ParamHolder(object):
>>     def __init__(self, n):
>>         self.param = n
>>
>> class C1(object):
>>     def __init__(self,m):
>>         self.value = [ParamHolder(n) for n in range(m)]
>>     def __getitem__(self, p):
>>         return self.value[p]
>>
>> obj1 = C1(5)
>> obj2 = C1(5)
>>
>> io1 = iter(obj1)
>> io2 = iter(obj2)
>>
>> print "obj1 pre loop",[r.param for r in obj1.value]
>>
>> try:
>>     while 1:
>>         io1.next().param += io2.next().param
>> except StopIteration:
>>     pass
>>
>> print "obj1 post loop",[r.param for r in obj1.value]
>>
>> - Paddy.
> 
> I don't like the try/except code and would write something like the
> following:
> 
>>>> obj1 = C1(5)
>>>> obj2 = C1(5)
>>>> from itertools import izip
>>>> for x,y in izip(obj1, obj2):
> ... 	x.param += y.param
> ...
>>>> print "obj1 post for loop",[r.param for r in obj1.value]
> obj1 post for loop [0, 2, 4, 6, 8]
>>>> 
> 
> - Paddy.

Thanks Paddy,

This looks like the closest thing to what I wanted, though the need for
this "param" makes it not very general to my taste. Besides, this method
would not work with ndarrays anyways.

Thanks again for taking the time to answer,

Pierre