Question

I’m writing an application that needs to read a list of strings from a file, save them in a data structure, and then look up those strings by prefixes. The list of strings is simply a list of words in a given language. For example, if the search function gets “stup” as a parameter, it should return [“stupid”, “stupidity”, “stupor”…]. It should do so in O(log(n)*m) time, where n is the size of the data structure and m is the number of results and should be as fast as possible. Memory consumption is not a big issue right now. I’m writing this in python, so it would be great if you could point me to a suitable data structure (preferably) implemented in c with python wrappers.

Answer 1

You want a trie.

http://en.wikipedia.org/wiki/Trie

I’ve used them in Scrabble and Boggle programs. They’re perfect for the use case you described (fast prefix lookup).

Here’s some sample code for building up a trie in Python. This is from a Boggle program I whipped together a few months ago. The rest is left as an exercise to the reader. But for prefix checking you basically need a method that starts at the root node (the variable words), follows the letters of the prefix to successive child nodes, and returns True if such a path is found and False otherwise.

class Node(object):
    def __init__(self, letter='', final=False):
        self.letter = letter
        self.final = final
        self.children = {}
    def __contains__(self, letter):
        return letter in self.children
    def get(self, letter):
        return self.children[letter]
    def add(self, letters, n=-1, index=0):
        if n < 0: n = len(letters)
        if index >= n: return
        letter = letters[index]
        if letter in self.children:
            child = self.children[letter]
        else:
            child = Node(letter, index==n-1)
            self.children[letter] = child
        child.add(letters, n, index+1)

def load_dictionary(path):
    result = Node()
    for line in open(path, 'r'):
        word = line.strip().lower()
        result.add(word)
    return result

words = load_dictionary('dictionary.txt')