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class MyHashTable:

    def __init__(self, capacity):
        self.capacity = capacity
        self.slots = [None] * self.capacity

    def __str__(self):
        return str(self.slots )

    def __len__(self):
        count = 0
        for i in self.slots:
            if i != None:
                count += 1
        return count

    def hash_function(self, key):
        slot = key % len(self.slots)

        if key in self.slots:
            return slot

        elif (not key in self.slots) and len(self.slots) == self.capacity:
            return slot

        else:
            for i in self.slots:
                count = 0 
                if i == None:
                    return count
                count += 1

    def insert(self, key):
        print(len(self.slots)) #Why does this show an output of 2? 
        if key in self.slots:
            return -2

        elif (not key in self.slots) and (len(self.slots) != self.capacity): #Now this cant execute
            num = hash_function(key)
            self.slots[num] = key
            return num

        elif (not key in self.slots) and len(self.slots) == self.capacity:
            return -1

Im wondering why the commented part above in the insert(self, key) the print statement gives (2) instead of (0). The elif statement underneath wont execute since its giving a result of (2) instead of (0)

A function call of 

x = MyHashTable(2)
print(len(x))

Should give: 0

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  • You should not compare to None with ==. Instead you should test as e.g. if i is None, or if i is not None Commented Oct 5, 2015 at 10:15

2 Answers 2

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You're initializing self.slots = [None] * self.capacity, so with capacity = 2, self.slots is [None, None], which is of length 2.

Your __len__ method doesn't run because len(self.slot) calls self.slot.__len__, not self.__len__. If you'd like to use your override method, you should be calling len(self) instead.

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3 Comments

But what about def __len__(self): Shouldnt this override the len() function and thus give 2?
that override is for the class/object itself, not self.slot . doing len(self.slot) , would call that list's __len__() . I am guessing you wanted to do len(self) .
@WojackO'feel As Anand noted, the __len__ override is for your MyHashTable class, not for self.slot (which is an instance of list).
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You have to call your __len__ function (by calling self.__len__() ) if you want the length of the elements which are not None. For lists None are valid entries.

By the way. It is always best to compare with None by a is None or a is not None instead of == or !=.

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3 Comments

How exactly do I call the len function? I thought len() would be overrided, by the way why is "a is None" better than the == operator?
simply use self.__len__() or len(self) instead of len(self.slots)
Ah, you have overwritten the len-function but only for your class and not for the array you use for self.slots. That one is always referring to self.slots.__len__ and not to self.__len__. About the difference between == and is: python.org/dev/peps/pep-0008 (see the second point in the section about Programming Recommendations)

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