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PORandomised select in python, working unexpectedly
 

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  1. PORandomised select in python, working unexpectedly
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    <p>I'm trying to implement the rselect algorithm that I just learnt in class. However, cant seem to figure out where Im going wrong in the implementation. Here's my code. *<em>EDIT *</em> : I tried using the info provided in the answer by David,but my code still acts weird. Here's the revised code:</p> <pre><code>def rselect(seq,length,i):# i is the i'th order statistic. if len(seq)&lt;=1:return seq lo,pi,hi,loc_pi= random_partition(seq if loc_pi==i:return pi if loc_pi&gt;i:return rselect(lo,loc_pi-1,i) elif loc_pi&lt;i:return rselect(hi,length-loc_pi,i-loc_pi)# from random import choice def random_partition(seq): pi =choice(seq) #print 'pi',pi loc_pi=seq.index(pi) print 'Location',loc_pi lo=[x for x in seq if x&lt;=pi] hi=[x for x in seq if x&gt;pi] return lo,pi,hi,len(lo)+1 #--A def test_rselect(seq,i): print 'Sequence',seq l=len(seq) print 'Statistic', rselect(seq,l,i) </code></pre> <p>However the output is different at different times and even right at times!. I'm a noob to both algorithms and python, any help on where Im going wrong would be much appreciated. Edit: Im getting different values for the ith order <strong>statistic</strong> each time I run the code , which is my issue For instance each run of the code as below gives</p> <pre><code>Revised Output: /py-scripts$ python quicksort.py Sequence [54, -1, 1000, 565, 64, 2, 5] Statistic Location 1 -1 @ubuntu:~/py-scripts$ python quicksort.py Sequence [54, -1, 1000, 565, 64, 2, 5] Statistic Location 5 Location 1 Location 0 -1 </code></pre> <p>Expected output: Im expecting find the ith order statistic here.</p> <p>And therefore</p> <p><code>test_rselect([54,-1,1000,565,64,2,5],2)</code> should return <code>5</code> as the <strong>Statistic</strong> all the time .</p> <p>Any help in where Im going wrong with this implementation would be helpful.. Thanks!!<br/> <strong>EDIT 2</strong>: From trying to analyse the algorithm I believe the error lies in how I'm <strong>returning the pivot location(loc_pi)</strong> in line marked A. Considering the following sequence of events for the above program. </p> <pre><code>test_rselect( [ 55, 900, -1,10, 545, 250], 3) // call to input array calls rselect ([ 55, 900, -1,10, 545, 250],6,3) 1st call to random_partition: pi=545 and loc_pi=4 lo=[55,-1,10,250,545] hi=[900] return to rselect function (lo,545,hi,6) here loc_pi&gt;i: so rselect(lo,5,3)// and discard the hi part 2nd recursive call to rselect: 2nd recursive call to random_partition: call random_partition on (lo) // as 'hi' is discarded pi=55 loc_pi=0 lo=[-1,10,55] hi=[250,545] return to rselect(lo,55,hi,4) here loc_pi&gt;i: rselect(lo,3,3)// The pivot element is lost already as it is in 'hi' here!! </code></pre> <p>Any help on how I can deal with returning the location of the pivot element, in order to to gain the correct o/p would be helpful. Setting a bounty, <strong>for an answer that clearly explains where I'm doing it wrong and how I could correct it ( great tips are welcome since I'm looking forward to learn :))</strong>. Looking forward to great answers! </p>
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    1. COWhat is wrong with this? What would correct operation look like?
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    2. COIf it is random, than the results should theoretically always be different
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    3. COErr I posted before question was complete, I think. My problem is that the program is returning different values of the ith order statistic
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