python在列表中找到大于左和右的数字

Question

The question is use iterator to find the num from list who is greater than the nums in left and right

For example, select([0,1,0,2,0,3,0,4,0]) return [1,2,3,4]

My first try is

def select(iterable):
    answer = []
    it = iter(iterable)

    try:
        v2 = next(it)
        while True:
            v1,v2= v2,next(it)
            if v2>v1 and v2>next(it):
                answer.append(v2)
    except StopIteration:
         pass
    return answer

This code fails. I think the next(it) in the while loop would be the same iterator,but the next() still iter next one in the code.

then I change the code to below one, it works.

try:
    v1,v2,v3 = next(it),next(it),next(it)
    while True:
        if v2>v1 and v2>v3:
            answer.append(v2)
        v1,v2,v3 = v2,v3,next(it)
except StopIteration:
     pass

Can someone explain what is difference happen here?

Answer 1

There are two issues with the first snippet:

1. Every time you call next(it) , it advances the iterator. You have to store the returned value if you want to access it more than once. Calling next(it) again won't do that; it will advance the iterator yet again.

2. Even if the first point weren't an issue, the following would still be problematic:

     if v2>v1 and v2>next(it): 

   The issue here is short-circuit evaluation . Depending on whether v2>v1 , this may or may not advance the iterator.

Answer 2

The error lies in this line:

if v2>v1 and v2>next(it):

Your calling next(it), but you don't store the return value. You just compare it to the v2. So the value gets skipped.

edit:

Btw, if you compare multiple values, the following comparison is much more cleaner:

 if v1 < v2 > v3: