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POPython: using a recursive algorithm as a generator
 

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  1. POPython: using a recursive algorithm as a generator
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    <p>Recently I wrote a function to generate certain sequences with nontrivial constraints. The problem came with a natural recursive solution. Now it happens that, even for relatively small input, the sequences are several thousands, thus I would prefer to use my algorithm as a generator instead of using it to fill a list with all the sequences.</p> <p>Here is an example. Suppose we want to compute all the permutations of a string with a recursive function. The following naive algorithm takes an extra argument 'storage' and appends a permutation to it whenever it finds one:</p> <pre><code>def getPermutations(string, storage, prefix=""): if len(string) == 1: storage.append(prefix + string) # &lt;----- else: for i in range(len(string)): getPermutations(string[:i]+string[i+1:], storage, prefix+string[i]) storage = [] getPermutations("abcd", storage) for permutation in storage: print permutation </code></pre> <p>(Please don't care about inefficiency, this is only an example.)</p> <p>Now I want to turn my function into a generator, i.e. to yield a permutation instead of appending it to the storage list:</p> <pre><code>def getPermutations(string, prefix=""): if len(string) == 1: yield prefix + string # &lt;----- else: for i in range(len(string)): getPermutations(string[:i]+string[i+1:], prefix+string[i]) for permutation in getPermutations("abcd"): print permutation </code></pre> <p>This code does <em>not</em> work (the function behaves like an empty generator).</p> <p>Am I missing something? Is there a way to turn the above recursive algorithm into a generator <em>without replacing it with an iterative one</em>?</p>
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