"Backward"-Iterator - Beginners question
Terry Reedy
tjreedy at udel.edu
Thu Oct 31 18:35:03 EDT 2013
On 10/31/2013 5:29 PM, Ulrich Goebel wrote:
> I'm locking for an "iterator" type with not only the .next() method, but
> with a .previous(), .first() and .last() method, so that I can through
> it from the beginning or from the end, and in both directions, even
> alternately (for example two steps forward, one backward, two steps
> forward).
You are free to write such a class, if it is appropriate for your actual
use case.
If you have a concrete sequence object seq with random access, there is
no reason to do so. First and last are seq[0] and seq[-1]. Given
'cursor' i, prev and next are 'i-=1;seq[i]' and 'i+=1;seq[i]'.
There *are* virtual sequences where first and last are known or
relatively easy to compute and for which prev and next are much easier
to compute than a random nth item. Note that if you start with first and
mostly move forward, prev might best be implemented using a list of
items already seen. The list and the prev function might be limited to
the last N items seen. It you give up the last function, the underlying
sequence does not even have to have a definite end.
A somewhat realistic (useful) example might be the following. You have a
very long sequence of bytes that represent utf-8 encoded characters. You
want to view the sequence as a sequence of sentences. The sequence is
too long to simply create a list of (decoded) sentences in memory.
--
Terry Jan Reedy
More information about the Python-list
mailing list