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I have the following concise-ish (and working) approach to getting all the forward permutations of a list of string. So with the list:

w = ["Vehicula", "Sem", "Risus", "Tortor"]

the results should be:

['Vehicula', 'Vehicula Sem', 'Vehicula Sem Risus', 'Vehicula Sem Risus Tortor', 'Sem', 'Sem Risus', 'Sem Risus Tortor', 'Risus', 'Risus Tortor', 'Tortor']

To do this, we loop through each element, and perform an inner loop that looks ahead and saves slices of the remaining elements to a result array.

w = ["Vehicula", "Sem", "Risus", "Tortor"]
results = []

i = 0
l = len(w)
while i < l:
    j, k = i, i + 1
    while k <= l:
        results.append(" ".join(w[j:k]))    
        k = k + 1
    i = i + 1
print results

With Python, I always feel like I'm missing a trick, so I'm curios to see if there are any native Python functions that will make this more efficient?

Edit

I tested all three with timeit and mine is definitely the slowest. I've got to get to grips with itertools - it is very powerful.

Verbose (Mine): 3.61756896973
Comprehensions: 3.02565908432
Itertools: 2.83112883568
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2 Answers 2

up vote 4 down vote accepted

Here's an alternative approach using itertools.combinations to generate the (start, end) pairs:

from itertools import combinations
w = "Vehicula Sem Risus Tortor".split()
results = [' '.join(w[i:j]) for i, j in combinations(range(len(w) + 1), 2)]
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This should work. I don't know if it is more readable than your approach though.

w = ["Vehicula", "Sem", "Risus", "Tortor"]
results = [' '.join(w[i:i+j+1]) for i in range(len(w)) for j in range(len(w)-i)]
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