How do I generate multi-word terms recursively?

Question

Say I have a string of words: 'a b c d e f'. I want to generate a list of multi-word terms from this string.

Word order matters. The term 'f e d' shouldn't be generated from the above example.

Edit: Also, words should not be skipped. 'a c', or 'b d f' shouldn't be generated.

What I have right now:

doc = 'a b c d e f'
terms= []
one_before = None
two_before = None
for word in doc.split(None):
    terms.append(word)
    if one_before:
        terms.append(' '.join([one_before, word]))
    if two_before:
        terms.append(' '.join([two_before, one_before, word]))
    two_before = one_before
    one_before = word

for term in terms:
    print term

Prints:

a
b
a b
c
b c
a b c
d
c d
b c d
e
d e
c d e
f
e f
d e f

How would I make this a recursive function so that I can pass it a variable maximum number of words per term?

Application:

I'll be using this to generate multi-word terms from readable text in HTML documents. The overall goal is a latent semantic analysis of a large corpus (about two million documents). This is why keeping word order matters (Natural Language Processing and whatnot).

Answer 1

This isn't recursive, but I think it does what you want.

doc = 'a b c d e f'
words = doc.split(None)
max = 3          


for index in xrange(len(words)):    
    for n in xrange(max):
        if index + n < len(words):           
            print ' '.join(words[index:index+n+1])   

And here's a recursive solution:

def find_terms(words, max_words_per_term):       
    if len(words) == 0: return []
    return [" ".join(words[:i+1]) for i in xrange(min(len(words), max_words_per_term))] + find_terms(words[1:], max_words_per_term)


doc = 'a b c d e f'
words = doc.split(None) 
for term in find_terms(words, 3):
    print term

Here's the recursive function again, with some explaining variables and comments.

def find_terms(words, max_words_per_term):   

    # If there are no words, you've reached the end. Stop.    
    if len(words) == 0:
        return []      

    # What's the max term length you could generate from the remaining 
    # words? It's the lesser of max_words_per_term and how many words 
    # you have left.                                                         
    max_term_len = min(len(words), max_words_per_term)       

    # Find all the terms that start with the first word.
    initial_terms = [" ".join(words[:i+1]) for i in xrange(max_term_len)]

    # Here's the recursion. Find all of the terms in the list 
    # of all but the first word.
    other_terms = find_terms(words[1:], max_words_per_term)

    # Now put the two lists of terms together to get the answer.
    return initial_terms + other_terms