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POget the flagged bit index in pure python
 

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  1. POget the flagged bit index in pure python
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    <p>i involve here 2 questions: one is for 'how', and second is for 'is this great solution sounds ok?'<br> the thing is this: i have an object with int value that stores all the persons' ids that used that object. it's done using a flagging technique (person id is 0-10). </p> <p>i got to a situation where in case this value is flagged with only one id, i want to get this id. </p> <p>for the first test i used <code>value &amp; (value-1)</code> which is nice, but as for the second thing, i started to wonder what's the best way to do it (the reason i wonder about it is because this calculation happens at least 300 times in a second in a critical place). </p> <p>so the 1st way i thought about is using <code>math.log(x,2)</code>, but i feel a little uncomfortable with this solution since it involves "hard" math on a value, instead of very simple bits operation, and i feel like i'm missing something. </p> <p>the 2nd way i thought about is to count <code>value&lt;&lt;1</code> until it reaches 1, but it as you could see in the benchmark test, it was just worse. </p> <p>the 3rd way i was implemented is a non-calc way, and was the fastest, and it's using a dictionary with all the possible values for ids 0-10.</p> <p>so like i said before: is there a 'right' way for doing it in <strong>pure</strong> python?<br> is a dictionary-based solution is a "legitimate" solution? (readability/any-other-reason-why-not-to?) </p> <pre><code>import math import time def find_bit_using_loop(num,_): c=0 while num!=1: c+=1 num=num&gt;&gt;1 return c def find_bit_using_dict(num,_): return options[num] def get_bit_idx(num, func): t=time.time() for i in xrange(100000): a=func(num,2) t=time.time()-t #print a return t options={} for i in xrange(20): options[1&lt;&lt;i]=i num=256 print "time using log:", get_bit_idx(num, math.log) print "time using loop:", get_bit_idx(num, find_bit_using_loop) print "time using dict:", get_bit_idx(num, find_bit_using_dict) </code></pre> <p>output:</p> <pre><code>time using log: 0.0450000762939 time using loop: 0.156999826431 time using dict: 0.0199999809265 </code></pre> <p>(there's a very similar question here: <a href="https://stackoverflow.com/questions/5520655/return-index-of-least-significant-bit-in-python">return index of least significant bit in Python</a> , but first, in this case i know there's only 1 flagged bit, and second, i want to keep it a pure python solution)</p>
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    1. COWhy do you want to use only one value to store a set of numbers?
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    2. CObecause the flagging/flag-testing actions are both in the most intensive areas in the code, and the most efficient way i found for that is the bitwise operations.
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    3. COOK. Why are you particularly asking about the legitimacy of the `dict` solution? It seems that it is the most readable, most Pythonic, and fastest, just from seeing what you have written there. Is there some reason *you* think it is not a "legitimate" solution?
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