Using index on an empty list in python

Question

For the following program, in the addEntry(self, dictKey, dictVal) function, I don't understand why the following line of code doesn't generate indexing error:

if hashBucket[i][0] == dictKey

self.buckets is initially a list of empty list: self.buckets = [[],[]...[]]
When addEntry is executed the first time, hashBucket is just an empty list, so I expect hashBucket[i][0] will generate an indexing error, but the program actually works, why? Thank you very much for your help.

Here is the program

class intDict(object):
    """A dictionary with integer keys"""

    def __init__(self, numBuckets):
        """Create an empty dictionary"""
        self.buckets = []
        self.numBuckets = numBuckets
        for i in range(numBuckets): 
            self.buckets.append([])

    def addEntry(self, dictKey, dictVal):
        """Assumes dictKey an int. Adds an entry"""
        hashBucket = self.buckets[dictKey%self.numBuckets]
        for i in range(len(hashBucket)):
            if hashBucket[i][0] == dictKey:
                hashBucket[i] = (dictKey, dictVal)
                return
        hashBucket.append((dictKey, dictVal))

    def getValue(self, dictKey):
        """Assumes dictKey an int. Returns entry associated with the key dictKey"""
        hashBucket = self.buckets[dictKey%self.numBumBuckets]
        for e in hashBucket:
            if e[0] == dictKay:
                return e[1]
        return None

    def __str__(self):
        result = '{'
        for b in self.buckets:
            for e in b:
                result = result + str(e[0]) + ":" + str(e[1]) + ','
        return result[:-1] + '}' # result[:-1] omits the last coma

for i in range(len(hashBucket)) becomes for i in range(0) which means that it never enters the loop, so only the hashBucket.append(...) is executed. — AChampion
– AChampion, Commented Sep 24, 2016 at 1:53

Karin · Accepted Answer · 2016-09-24 01:53:44Z

2

Since hashBucket is an empty list at first, for i in range(len(hashBucket)): is essentially for i in range(0):, meaning it never gets to the conditional if hashBucket[i][0] == dictKey on the first call to addEntry.

answered Sep 24, 2016 at 1:53

Karin

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danidee · Accepted Answer · 2016-09-24 01:54:54Z

2

when you try to loop over an empty list, nothing is going to happen, fire up a python interpeter and try this

>>> for i in range(len([])):
...     print(i)
... 
>>>

Nothing gets printed, so in the same way if hashBucket is empty then everything inside the for loop will never get executed

def addEntry(self, dictKey, dictVal):
    """Assumes dictKey an int. Adds an entry"""
    hashBucket = self.buckets[dictKey%self.numBuckets]
    for i in range(len(hashBucket)):
        # This is never executed if hashBucket is empty
        if hashBucket[i][0] == dictKey:
            hashBucket[i] = (dictKey, dictVal)
            return
    hashBucket.append((dictKey, dictVal))

answered Sep 24, 2016 at 1:54

danidee

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gaoithe · Accepted Answer · 2016-09-24 01:59:54Z

0

When executed for first time hashBucket is empty. So range is empty. So this for loop doesn't do anything. 'for i in range(len(hashBucket)):' I think. Is that right?

answered Sep 24, 2016 at 1:59

gaoithe

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