Sunday, June 17, 2007

quicksort - phrase book

http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Sort/Quick/ is a short but real code.

On P 146 [[introduction to algorithms]], the procedure partition(A,p,r) does the following on the Section A[p,r] inclusive.

* it moves the rightmost object from r to the grave/anchor position within the section
* it keeps the rightmost object value as the benchmark value throughout the procedure.
* it returns the new index of this object. The index is defined in the entire array A[].
* it shuffles 0 or more elements within the section
* it doesn't try to sort any subsection


Upon receiving an unsorted section, the procedure simply puts the rightmost thingy into the grave position within the section. 


Corollary: first scene in the quicksort movie actually completes the job of putting the rightmost object into its final resting place as an anchor within the entire array. After that, we focus on sorting the "left-section" and the "right-section" (in separate threads) without worrying about the first anchor object. Within the left-section, first scene completes the job of putting the rightmost object into its grave, a final resting place within the entire array.

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