如何获得可迭代的最后n个值以外的所有值

Question

def drop(iterable, n):
    iterator = iter(iterable)
    iter_list = []
    
    try:
        while True:
            it = next(iterator)
            if len(iter_list) <= n:
                iter_list.append(it)
                yield it
    except StopIteration:
        pass

I'm suppose to produce all values from the iterable except for the last n values. Right now, I have it so that it produces at most n values. How do I do it so that it's the other way around? I'd like to keep the same structure if I can.

I'm not allowed to count the number of values the iterable produces. I also can't use itertools or call len, index, slice, etc on the iterable.

Answer 1

from collections import deque

def drop(it, n):
    queue = deque()
    for _ in range(n):
      queue.append(next(it))
    while True:
      # If next(it) throws a StopIteration, we need a little bit more
      # code here to indicate this iterable if finished.
      queue.append(next(it))
      yield queue.popleft()

I probably need to add a couple of try/except StopIteration to get it running perfectly, but this is the basic idea.

Answer 2

you have 2 options here first is to generate the whole iterable and drop the last n, and in the while loop you return every value from the start or the more clever way of using a buffer of n values. Just a slight modification to your code will do the trick

def drop(iterable, n):
   iterator = iter(iterable)
   iter_list = []

   try:
        while True:
            it = next(iterator)
            iter_list.append(it)
            if len(iter_list) > n:
                element = iter_list.pop(0)
                yield element
   except StopIteration:
        pass

Here every new element is appended in the while loop and the condition to yield elements runs after the first n elements are generated. Then the first generated element is yielded, and so on with n elements generated always. When the iterator has no more to give the exception is raised and the while loop stops withholding the last n elements that are never returned