Extract the nth key in a python dictionary?

Question

Given a python dictionary and an integer n, I need to access the nth key. I need to do this repeatedly many times in my project.

I have written a function which does this:

def ix(self,dict,n):
    count=0
    for i in sorted(dict.keys()):
        if n==count:
            return i
        else:
            count+=1

But the problem is that if the dictionary is huge, the time complexity increases when used repeatedly.

Is there an efficient way to do this?

Asked By: Hemanth Malla

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Source

Answer 1

I guess you wanted to do something like this, but as dictionary don’t have any order so the order of keys in dict.keys can be anything:

def ix(self, dct, n): #don't use dict as  a variable name
   try:
       return list(dct)[n] # or sorted(dct)[n] if you want the keys to be sorted
   except IndexError:
       print 'not enough keys'

Answered By: Ashwini Chaudhary

Answer 2

dict.keys() returns a list so, all you need to do is dict.keys()[n]

But, a dictionary is an unordered collection so nth element does not make any sense in this context.

Note: Indexing dict.keys() is not supported in python3

Answered By: shyam

Answer 3

For those that want to avoid the creation of a new temporary list just to access the nth element, I suggest to use an iterator.

from itertools import islice
def nth_key(dct, n):
    it = iter(dct)
    # Consume n elements.
    next(islice(it, n, n), None) 
    # Return the value at the current position.
    # This raises StopIteration if n is beyond the limits.
    # Use next(it, None) to suppress that exception.
    return next(it)

This can be notably faster for very large dictionaries compared to converting the keys into a temporary list first and then accessing its nth element.

Answered By: normanius

Answer 4

It is mentioned in multiple answers, that dictionaries were unordered. This is nonly true for python versions up to 3.6. From 3.7 ongoing, dictionaries are in fact ordered.

Answered By: Andreas

Extract the nth key in a python dictionary?

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