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  • K-way parittion quick sort in c++
  • K-way partition quick sort in C++
  • Question: How to write a k-way partition QuickSort that satisfies the test cases shown below?
  • First thing first: I don't know this algorithm and there seems to be no information available about it on the internet (i am familiar with QuickSort and k-way partition dough)
  • Is it just the recursive dual-partition QuickSort with insertion sort when length is less than or equal to 2*k?
  • i found this code:
  • https://github.com/KGene1901/Algorithms-Data-Structures-Yr1/blob/91dc231df2a27a2e306a7599afdb0be38b3935a4/k-way%20QuickSort.py
  • and tried to translate it to c++ but there seems to be a bug with my current code:
  • the resulting array in the main.cpp is shorter than the original array more specifically it zeros at the end.
  • i think it due to the way it's handled in the python code is mixing integer and lists together in the partition array and i tried to convert that by checking the vector sizes.
  • main.cpp:
  • vector<vector<int>> partition(vector<int> A, int k,int* q){
  • vector<int> pivots;
  • std::vector<std::vector<int>>sortedA;
  • int count=0;
  • int P1=1;
  • int P2=2;
  • if (k<=0){
  • return {};
  • }
  • else{
  • for (int i=A.size()-1; i>A.size()-k-1; i--){
  • pivots.push_back(A[i]);
  • A.erase(A.begin()+i);
  • sort(pivots.begin(), pivots.end());
  • }
  • for (int m: pivots){
  • sortedA.push_back({m});
  • }
  • sortedA.insert(sortedA.begin(), {});
  • sortedA.push_back({});
  • while (sortedA[P1].size() && sortedA[P2].size()){
  • sortedA.insert(sortedA.begin()+P2, {{}});
  • P1+=2;
  • P2+=2;
  • }
  • for (int ele: A){
  • count=0;
  • if (ele<pivots[0]){
  • sortedA[0].push_back({ele});
  • }
  • else if (ele>pivots[pivots.size()-1]){
  • sortedA[sortedA.size()-1].push_back({ele});
  • }
  • else{
  • while (ele>pivots[count]){
  • count+=1;
  • }
  • int index = find(sortedA.begin(), sortedA.end(), vector<int>{pivots[count-1]}) - sortedA.begin();
  • sortedA[index+1].push_back(ele);
  • }
  • }
  • }
  • return sortedA;
  • }
  • vector<int> quicksort(vector<int> A, int k, int* q){
  • if (k<=0){
  • return {};
  • }
  • if (A.size()==1){
  • return A;
  • }
  • else if (A.size()<=(2*k)){
  • sort(A.begin(), A.end());
  • return A;
  • }
  • else{
  • vector<vector<int>> sorted = partition(A,k,q);
  • for (auto ele: sorted){
  • if (ele.size()<2){
  • continue;
  • }
  • else{
  • int index = find(sorted.begin(), sorted.end(), ele) - sorted.begin();
  • sorted[index] = quicksort(ele,k,q);
  • }
  • }
  • vector<int> SortedAFinalMatch;
  • for (auto s: sorted){
  • if (s.size()==1){
  • SortedAFinalMatch.push_back(s[0]);
  • }
  • else{
  • for (auto item: s){
  • SortedAFinalMatch.push_back(item);
  • }
  • }
  • }
  • return SortedAFinalMatch;
  • }
  • }
  • int main(){
  • int n = 30;
  • int k = 13;
  • int* q = new int[2*k];
  • int* pivots = new int[k];
  • std::vector<int>a;
  • for (int i = 0; i < 2 * k; i++) pivots[i] = 4 * i + 2;
  • srand(time(NULL));
  • cout<<endl;
  • for (int i = 0; i < n; i++) { a.push_back(rand()% (4 * k)); }
  • for (auto i:a)std::cout << i << " ";
  • cout<<endl;
  • std::vector<int>b = quicksort(a,k,q);
  • for (auto i:b)std::cout << i << " ";
  • cout <<endl;
  • int i = 0;
  • for (int j = 0; j < k; j++) {
  • while (a[i] < pivots[j]){i++;cout<<"*"<<pivots[i];};
  • assert (q[2 * j] == i);
  • cout<<";"<<i<<";";
  • while (a[i] == pivots[j]){i++;cout<<"*"<<pivots[i];};
  • assert (q[2 * j + 1] == i);
  • }
  • }
  • using namespace std;
  • #include <vector>
  • #include <iostream>
  • #include <algorithm>
  • #include "I.h"
  • #pragma once
  • template <class T>
  • class KWayPartition {
  • public:
  • /* A[p .. r]
  • pivots[0 .. (k-1)] an Aay of k ordered values (from smaller to bigger)// k+1 parts with k pivots
  • q[0 .. (2k-1)] output Aay of borders
  • At the end:
  • A[p .. q[0]-1] < pivots[0]
  • A[q[0] .. q[1]-] = pivots[0]
  • pivots[0] < A[q[1] .. q[2]-1] < pivots[1]
  • A[q[2] .. q[3]-1] = pivots[1]
  • ...
  • pivots[i-1] < A[q[2i-1] .. q[2i]-1] < pivots[i] 0 < i < k-1
  • A[q[2i] .. q[2i+1]-1] = pivots[i] 0 < i < k-1
  • ...
  • pivots[k-2] < A[q[2k-3] .. q[2k-2]-1] < pivots[k-1]
  • A[q[2k-2] .. q[2k-1]-1] = pivots[k-1]
  • A[q[2k-1] .. r] > pivots[k-1]
  • */
  • void insertionSort(T* A, int r) {
  • T key;
  • int j=0;
  • for (int i=0;i<r;i++) {
  • key = A[i];
  • j = i - 1;
  • while (j >= 0 && A[j] > key) {
  • A[j+1] = A[j];
  • j--;
  • }
  • A[j+1] = key;
  • }
  • }
  • vector<vector<T>> partition(vector<T> A, int k){
  • vector<T> pivots;
  • std::vector<std::vector<T>>sortedA;
  • int count=0;
  • int P1=1;
  • int P2=2;
  • if (k<=0){
  • return {};
  • }
  • else{
  • for (int i=A.size()-1; i>A.size()-k-1; i--){
  • pivots.push_back(A[i]);
  • A.erase(A.begin()+i);
  • sort(pivots.begin(), pivots.end());
  • }
  • for (int m: pivots){
  • sortedA.push_back({m});
  • }
  • sortedA.insert(sortedA.begin(), {});
  • sortedA.push_back({});
  • while (sortedA[P1].size() && sortedA[P2].size()){
  • sortedA.insert(sortedA.begin()+P2, {});
  • P1+=2;
  • P2+=2;
  • }
  • for (int ele: A){
  • count=0;
  • if (ele<pivots[0]){
  • sortedA[0].push_back(ele);
  • }
  • else if (ele>pivots[pivots.size()-1]){
  • sortedA[sortedA.size()-1].push_back(ele);
  • }
  • else{
  • while (ele>pivots[count]){
  • count+=1;
  • }
  • int index = find(sortedA.begin(), sortedA.end(), vector<T>{pivots[count-1]}) - sortedA.begin();
  • sortedA[index+1].push_back(ele);
  • }
  • }
  • }
  • return sortedA;
  • }
  • vector<T> quicksort(vector<T> A, int k){
  • if (k<=0){
  • return {};
  • }
  • if (A.size()==1){
  • return A;
  • }
  • else if (A.size()<=(2*k)){
  • sort(A.begin(), A.end());
  • return A;
  • }
  • else{
  • vector<vector<T>> sorted = partition(A,k);
  • for (auto ele: sorted){
  • if (ele.size()<2){
  • continue;
  • }
  • else{
  • int index = find(sorted.begin(), sorted.end(), ele) - sorted.begin();
  • sorted[index] = quicksort(ele,k);
  • }
  • }
  • vector<T> SortedAFinalMatch;
  • for (auto s: sorted){
  • if (s.size()==1){
  • SortedAFinalMatch.push_back(s[0]);
  • }
  • else{
  • for (auto item: s){
  • SortedAFinalMatch.push_back(item);
  • }
  • }
  • }
  • return SortedAFinalMatch;
  • }
  • }
  • virtual void Partition (T* A, T* pivots, int* q, int p, int r, int k) {
  • std::vector<T>a;
  • for (int i = 0; i <= r; i++) { a.push_back(A[i]); }
  • std::vector<T>b = quicksort(a,3);
  • cout<<endl;
  • for (auto i:b)std::cout << i << " ";
  • /*
  • * for (int i = 0; i <= r; i++) {
  • * cout<< a[i]<<", ";
  • * }
  • */
  • /*
  • * cout<<endl;
  • * a=quicksort(a,k);
  • * for (int i = 0; i < r+1; i++) {
  • * cout<< a[i]<<", ";
  • * A[i] = a[i];
  • * }
  • */
  • }
  • };
  • How do i write a KWayPartition method such that it satisfies this testcase?
  • ```c++
  • #include <iostream>
  • #include <cassert>
  • #include <time.h>
  • #include "../src/KWayPartition.h"
  • #include "I.h"
  • using namespace std;
  • int main() {
  • int n = 30;
  • int k = 13;
  • I *A = new I[n];
  • I* pivots = new I[k];
  • int* q = new int[2*k];
  • KWayPartition<I>* fwp = new KWayPartition<I>();
  • srand(time(NULL));
  • for (int i = 0; i < 2 * k; i++)
  • pivots[i] = 4 * i + 2;
  • cout << "Initial Array:" << endl;
  • for (int i = 0; i < n; i++) {
  • A[i] = rand() % (4 * k);
  • cout << A[i] << ", ";
  • }
  • fwp->Partition(A, pivots, q, 0, n-1, k);
  • int i = 0;
  • for (int j = 0; j < k; j++) {
  • while (A[i] < pivots[j])
  • i++;
  • assert (q[2 * j] == i);
  • while (A[i] == pivots[j])
  • i++;
  • assert (q[2 * j + 1] == i);
  • }
  • cout << "\nFinal Array:" << endl;
  • for (int i = 0; i < n; i++) {
  • cout << A[i] << ", ";
  • }
  • cout << "q values:" << endl;
  • for (int i = 0; i < k; i++) {
  • cout << "Pivot " << pivots[i] << ", q< is:"<< q[2*i] << ", q= is:" << q[2*i+1] << endl;
  • }
  • int d;
  • cin >> d;
  • }
  • ```
  • What is this test cases about?
  • I don't know this algorithm and there seems to be no information available about it on the internet (I am familiar with quicksort and k-way partition though).
  • I'm not sure if it's just the recursive dual-partition quicksort with insertion sort when length <= 2*k
  • I found [an implementation](https://github.com/KGene1901/Algorithms-Data-Structures-Yr1/blob/91dc231df2a27a2e306a7599afdb0be38b3935a4/k-way%20QuickSort.py) and tried to translate it to C++ but there seems to be a bug with my current code.
  • - [main.cpp](https://paste.rs/qfI)
  • - [main3.cpp](https://paste.rs/pta)
  • - [KWayPartition.h](https://paste.rs/WI0)
  • - [I.h, I.cpp, Test.cpp](https://paste.rs/ZCp)
  • I need to write a KWayPartition method such that it satisfies this testcase:
  • ```cpp
  • int i = 0;
  • for (int j = 0; j < k; j++) {
  • while (A[i] < pivots[j])
  • i++;
  • assert (q[2 * j] == i);
  • while (A[i] == pivots[j])
  • i++;
  • assert (q[2 * j + 1] == i);
  • }
  • ```
c++ data-structures algorithms sorting
c++ data-structures algorithms sorting

Suggested almost 2 years ago by trichoplax‭