Finding the number of the nth in an unordered array-fast selection algorithm

When you visit other people's blogs, you can see this algorithm by accident, by the way, with C + + implementation.

The simplest solution is to sort the array first and return the nth number.

The following code is a quick selection algorithm (I don't know if the name is right)

1#include <vector>2#include <iostream>3#include <stdexcept>4#include <cstdio>5 6 Const intQs_eerrmsg_len = the;7 8 9 /**Ten * Quickly select the number of the nth large in the unordered array One * Because select returns a reference to an object in the array, A * So error handling has chosen exception -  */ -Template <typename t> the classQuickselect - { -  Public: -typedef TypeName Std::vector<t>:: Size_type size_type; +  -     /** + * Constructor Function A * @brief Quickselect at * @param array of type Array-vector -      */ -Quickselect (std::vector<t>&Array) - : M_array (array) -     {} -  in     /** - * Select the element of the nth large, to * Failed throws Std::out_of_range exception, + * Successful return of the resulting element - * @brief Select the * @param nth * * @return $      */Panax Notoginseng     Constt& -     Select(Size_type nth)Throw(Std::out_of_range) the     { +         //S_pos is the subscript of the nth -largest array after sorting ASize_type S_pos = m_array.size ()-nth; the  +         if(S_pos >m_array.size ()) { -             CharErrmsg[qs_eerrmsg_len]; $  $std::snprintf (ErrMsg, Qs_eerrmsg_len,"Array access Violation:{access:%ld range_length:%ld begin:%ld}.", Nth, M_array.size (),0); - std::out_of_range Oor (errmsg); -             ThrowOor; the         } - Wuyi         return  This->select_pos (S_pos,0, M_array.size ()); the     } -  Wu     /** - * @brief Select About * @param nth $ * @param begin - * @param end - * @return -      */ A     Constt& +     Select(Size_type nth, size_type begin, Size_type end)Throw(Std::out_of_range) the     { -Size_type array_size =m_array.size (); $  the         if(Begin > Array_size | | end > Array_size | | End <begin) { the             CharErrmsg[qs_eerrmsg_len]; the  thestd::snprintf (ErrMsg, Qs_eerrmsg_len,"The begin or end is not correct:{array_size:%ld begin:%ld end:%ld}.", array_size, begin, end); - std::out_of_range Oor (errmsg); in             ThrowOor; the         } the  About         //to find the length of a range theSize_type range_length = End-begin +1; the  the         //The position of the nth- largest element to find within the current range +Size_type S_pos = range_length-nth; -  the         if(S_pos >range_length) {Bayi             CharErrmsg[qs_eerrmsg_len]; the  thestd::snprintf (ErrMsg, Qs_eerrmsg_len,"Array access Violation:{access:%ld range_length:%ld begin:%ld}.", Nth, range_length, begin); - std::out_of_range Oor (errmsg); -             ThrowOor; the         } the         Else the  the         return  This-Select_pos (S_pos, begin, end); -     } the Private: the     /** the * POS represents the position of the element from begin94 * @brief Select_pos the * @param pos the * @param begin the * @param end98 * @return About      */ -     Constt&Select_pos (size_type pos, size_type begin, Size_type end)101     {102t& pivot =M_array[begin];103Size_type Swap_pos =begin;104  the         if(Begin = =end) {106             returnM_array[pos];107         }108 109         //make a quick sort the          for(Size_type i = begin +1; i < End;i + +){111             if(M_array[i] <pivot) { theStd::swap (M_array[i], M_array[swap_pos + +]);113             } the         } the  the         //multiple exchanges when the number of array elements is odd117         if(M_array.size ()%2!=0){118             if(M_array[end-1] <pivot) {119Std::swap (M_array[end-1], M_array[swap_pos + +]); -             }121         }122 123         if(Swap_pos-begin = =POS) {124             returnM_array[swap_pos]; the         }126         Else if(Swap_pos-begin <POS) {127             return  This->select_pos (POS, Swap_pos +1, end); -         }129         Else{ the             return  This-Select_pos (POS, begin, Swap_pos);131         } the     }133 134     /**135 * @brief M_array136      */137std::vector<t>&M_array;138};

Then for the sake of testing convenience, the vector output is realized, very simple output

1 namespacestd{2 3Template <typename t>4std::ostream&operator<< (std::ostream& out,Conststd::vector<t>&Array)5 {6      for(Auto E:array) {7          out<< e <<" ";8     }9      out<<Std::endl;Ten  One     return  out; A } -  -}

Here is the test case

1 intMain ()2 {3std::vector<int> some{1,2,3,5,4};4 5Std::cout <<some;6 7quickselect<int>QS (some);8 9Std::cout <<qs.Select(2) <<Std::endl;Ten  One     return 0; A}

The result output is

[Email protected] myjourney]# g++ quick_select. cpp -std=c++[[email protected] myjourney]#. /123544[[email protected] myjourney]# 

Finding the number of the nth in an unordered array-fast selection algorithm