itertools.intersect?

[Francis Carr]

On Jun 11, 6:23 pm, Mensanator <mensana... at aol.com> wrote:
> Removing the duplicates could be a big problem.

It is fairly easy to ignore duplicates in a sorted list:
<pre>
from itertools import groupby
def unique(ordered):
    """Yield the unique elements from a sorted iterable.
    """
    for key,_ in groupby(ordered):
        yield key
</pre>

Combining this with some ideas of others, we have a simple, complete
solution:
<pre>
def intersect(*ascendingSeqs):
    """Yield the intersection of zero or more ascending iterables.
    """
    N=len(ascendingSeqs)
    if N==0:
        return

    unq = [unique(s) for s in ascendingSeqs]
    val = [u.next() for u in unq]
    while True:
        for i in range(N):
            while val[i-1] > val[i]:
                val[i] = unq[i].next()
        if val[0]==val[-1]:
            yield val[0]
            val[-1] = unq[-1].next()
</pre>

This works with empty arg-lists; combinations of empty, infinite and
finite iterators; iterators with duplicate elements; etc.  The only
requirement is that all iterators are sorted ascending.

 -- FC