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POWhat is the fastest way to check if a string contains repeating characters in Python 3?
 

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  1. POWhat is the fastest way to check if a string contains repeating characters in Python 3?
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    <p>I need to filter strings by the criterion that they <strong>contain no character twice</strong>.</p> <ul> <li>The strings are <strong>many</strong> (say 1.4 trillion).</li> <li>The strings are <strong>short</strong> (around 8 characters).</li> <li>The strings are <strong>unique</strong> (caching won't work).</li> <li>The strings have a <strong>big character set</strong> (say any Unicode character).</li> <li>The strings <strong>usually meet the criterion</strong> (say 2/3 have no repeating characters).</li> </ul> <p>The using code would look like this:</p> <pre><code>&gt;&gt;&gt; candidate_strings = ["foobnehg", "barfnehg", "bazfnehg"] &gt;&gt;&gt; result_strings = [s if unique_chars(s) for s in candidate_strings] &gt;&gt;&gt; print(result_strings) ["barfnehg", "bazfnehg"] </code></pre> <p>I implemented a naive version, simply iterating the string:</p> <pre><code>def unique_chars_naive(string_given): """ Checks if a given string contains only unique characters. This version iterates the given string, saving all occurred characters. """ chars_seen = [] for char in string_given: if char in chars_seen: return False chars_seen.append(char) return True </code></pre> <p>My next-best idea was to use a <code>set</code>, so I implemented that:</p> <pre><code>def unique_chars_set(string_given): """ Checks if a given string contains only unique characters. This version exploits that a set contains only unique entries. """ return len(string_given) == len(set(string_given)) </code></pre> <p>Saving the functions to a file <code>UniqueCharacters.py</code>, timed them:</p> <pre><code>$ python3 -m timeit -n 100000 --setup='import UniqueCharacters; candidate_strings = ["foobnehg", "barfnehg", "bazfnehg"]' '[UniqueCharacters.unique_chars_naive(s) for s in candidate_strings]' 100000 loops, best of 3: 20.3 usec per loop $ python3 -m timeit -n 100000 --setup='import UniqueCharacters; candidate_strings = ["foobnehg", "barfnehg", "bazfnehg"]' '[UniqueCharacters.unique_chars_set(s) for s in candidate_strings]' 100000 loops, best of 3: 17.7 usec per loop </code></pre> <p>This shows that the <code>unique_chars_set</code> is faster by about 15 % for this dataset.</p> <p>Is there a faster way to do this? With regular expressions maybe? Is there some method in the standard library that does this?</p>
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    1. COMight there be repeated strings in `candidate_strings`?
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    2. COTest with longer words.
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      1. USSteven Rumbalski
    3. CODo all the strings contain only lowercase alphabetical characters? If not, do they only contain ASCII characters?
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      1. USKevin
 

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