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Tries

Tries#

A trie is a special kind of tree-shaped data structure. We’ll meet more such data structures soon, but we’ll be using tries here to represent a set of words.

The gist of a trie is that it represents words by their prefixes, which makes it very easy and efficient to lookup, insert, and remove words from the set.

You’ll implement three operations: trie_insert, trie_lookup, and trie_delete. First, let’s understand what a trie is.

Suppose we wanted to represent a set of five words: he, hey, hem, his, and we. Our trie will model them pictorially as:

../../../_images/image.png

Each letter represents a part of the trie; a 👍 sign means that the path from the root to that letter is a word in the trie. (Notice that ‘hi’ is a real word, but it’s not recorded as such in our trie.) As a Python data structure, we have:

t = {'h': {'e': {'is_word': True,
                'y': {'is_word': True},
                'm': {'is_word': True}},
         'i': {'s': {'is_word': True}}},
   'w': {'e': {'is_word': True}}}

That is, a trie is a series of nested dictionaries, where each key is a letter or 'is_word'. At a given level, is_word means that the path from the root to that point is a word, i.e., 'he' is a word because t['h']['e']['is_word'] is True. We don’t bother recording 'is_word' as False, since it would make the output noisy.

Notice that every key in the outer dictionary of a trie is either 'is_word' or a letter; if it’s 'is_word', it maps to a boolean. If it’s a letter, it maps to… another trie! We call this other trie the child and the outer one is the parent.

For all of the following functions, both recursion and loops work. It’s up to you which you use!

Adding a word to a trie with trie_insert#

Let’s get a feel for tries by writing trie_insert. The function should take a string word and a trie t; it should update t so that the path to word is in t and 'is_word' is set.

Here are some examples:

>>> t = {}
>>> trie_insert('hem', t)
>>> t
{'h': {'e': {'m': {'is_word': True}}}}
>>> trie_insert('hey', t)
>>> t
{'h': {'e': {'m': {'is_word': True}, 'y': {'is_word': True}}}}
>>> trie_insert('he', t)
>>> t
{'h': {'e': {'m': {'is_word': True}, 'y': {'is_word': True}, 'is_word': True}}}
>>> trie_insert('we', t)
>>> t
{'h': {'e': {'m': {'is_word': True}, 'y': {'is_word': True}, 'is_word': True}}, 'w': {'e': {'is_word': True}}}

Write four asserts as tests; don’t just copy what we have here.

Looking words up in the trie with trie_lookup#

Now that we can build tries, lets write a function to see whether a given word is in a trie. Write the function trie_lookup, which takes a string word and trie t, returning True if the word is in the trie (i.e., the path to it exists and is_word is present and True).

Here are some examples:

>>> t
{'h': {'e': {'is_word': True, 'm': {'is_word': True}, 'y': {'is_word': True}}, 'i': {'s': {'is_word': True}}}, 'w': {'e': {'is_word': True}}}
>>> trie_lookup('he', t)
True
>>> trie_lookup('head', t)
False
>>> trie_lookup('wed', t)
False
>>> trie_lookup('we', t)
True
>>> trie_lookup('', t)
False

Write four asserts as tests; don’t just copy what we have here.

Removing words from the trie with trie_remove#

You might notice that trie_remove above leaves some empty dictionaries lying around. How unsightly! As a completely optional challenge problem, write a new function trie_remove_cleanup that makes sure these empty dictionaries are cleared out. (This function is a bit easier to write using recursion than loops—with loops, it’s harder to cleanup without re-traversing the trie again.)

Here are some examples—though this isn’t for credit and won’t be graded.

>>> t
{'h': {'e': {'is_word': True, 'm': {'is_word': True}, 'y': {'is_word': True}}, 'i': {'s': {'is_word': True}}}, 'w': {'e': {'is_word': True}}}
>>> trie_remove_cleanup('hey', t)
>>> t
{'h': {'e': {'is_word': True, 'm': {'is_word': True}}, 'i': {'s': {'is_word': True}}}, 'w': {'e': {'is_word': True}}}
>>> trie_remove_cleanup('hem', t)
>>> t
{'h': {'e': {'is_word': True}, 'i': {'s': {'is_word': True}}}, 'w': {'e': {'is_word': True}}}
>>> trie_remove_cleanup('his', t)
>>> t
{'h': {'e': {'is_word': True}}, 'w': {'e': {'is_word': True}}}
>>> trie_remove_cleanup('he', t)
>>> t
{'w': {'e': {'is_word': True}}}
>>> trie_remove_cleanup('we', t)
>>> t
{}
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