How to implement a custom iterator from scratch in Python that only iterates over multiples of 3?

Question

I want to implement an iterator class(without using python built-in iterator) to only iterate over multiples of 3(could be other number,odd or even number but lets stick with 3 for now). The code should run on the given arraylist inside main() function and return multiples of 3.

This is my pseudocode below. Pretty new to python and don't know how iterator is implemented, would appreciate any help to make this code work:

class threeIterator:
    def __init__(self):
        self.it=it
        self.currval=currval

    def hasNext(self):
        while currval%3!=0 and it.hasNext():
               temp=it.next()
               if temp==None:
                   continue
               currval=temp
          if currval%3==0:
                 return True

     def next(self):
         if hasNext():
            res=currval
            currval=1
            return res
          else:
               raise exception("No such value")

        def main():
           arraylist=[random elements]
           arraylist.threeIter()

also, what is the best way to write a test case for it so i don't have to manually input numbers? thanks

You should use the python *iterator protocol*, i.e. implement `__iter__` (which for an iterator should simply `return self`) and `__next__` — juanpa.arrivillaga, Jan 15 '18 at 07:26
the point is to not use _iter_ and build everything from scratch. — gg777, Jan 15 '18 at 07:30
... why would you do that? So your iterator doesn't work with Python `for` statements, or anything else that expects an iterator? — juanpa.arrivillaga, Jan 15 '18 at 07:30
You should read about [the iterator protocol](https://stackoverflow.com/questions/19151/build-a-basic-python-iterator) — juanpa.arrivillaga, Jan 15 '18 at 07:33
can you clarify the requirement - you can't use any iterator class within your own class, and you can't use the iterator protocol, but what you want is to have a function called next() that returns the next multiple of 3? in which case you want the link from above by @juanpa.arrivillaga — Marcin, Jan 16 '18 at 09:02

score 0 · Answer 1 · answered Jan 15 '18 at 06:00

0

Assume there is not init vals in the class.

class THREEITER(object):
    def __init__(self):
        self.currval = 3

    def hasNext(self):
        return True

    def next(self):
        if self.hasNext():
            res = self.currval
            self.currval += 3
            return res
        else:
            raise "No such value"


ti = THREEITER()
for i in range(100):
    if(ti.hasNext()):
        print ti.next(),

answered Jan 15 '18 at 06:00

Lennon liu

39
4

hey, thanks for providing code but we are already given an array. for example: given [1,2,3,4,5,6,9] in the main() iterator should return [3,6,9] – gg777 Jan 15 '18 at 08:32
try to use the index. – Lennon liu Jan 15 '18 at 17:10

Raydel Miranda · Answer 2 · 2018-12-15T08:52:48.563

As simple as I can:

class ThreeIterator(list):

    def __init__(self, *args, **kwargs):
        super(ThreeIterator, self).__init__(*args, **kwargs)

    def threeIter(self):
        return [n for n in self if n % 3 == 0]


ti = ThreeIterator([1,2,3,4,5,6,7,8,9])

for i in ti.threeIter():
    print i

Using a generator:

class ThreeIterator(list):

    def __init__(self, *args, **kwargs):
        super(ThreeIterator, self).__init__(*args, **kwargs)

    def threeIter(self):
        for i in self:
            if i % 3 == 0:
                yield i
            else:
                continue


ti = ThreeIterator([1,2,3,4,5,6,7,8,9])
git = ti.threeIter()


for i in git:
    print i

How to implement a custom iterator from scratch in Python that only iterates over multiples of 3?

2 Answers2