快速选择算法 [英] Quick Select Algorithm

问题描述

我正在尝试对随机生成数字的数组实施快速选择算法。现在，在用代码编写算法之后，它无法将数组从最低到最高排序，也无法找到第k个最小的元素。我会很感激我得到的帮助。谢谢。

Im trying to implement the Quick Select Algorithm on a Array that has randomly generated numbers. Now after writing the algorithm in code, it does not sort the array from lowest to highest nor am i able to find the kth smallest element. I would appreciate the help i get. Thank you.

#include<iostream>
#include<ctime>
#include<cstdlib>
using namespace std;

void printArray(int *myArray, int n){
    for(int i = 0; i < n; i++){
       cout << myArray[i] << " ";
}
}

int Partition(int *myArray, int startingIndex, int endingIndex){
int pivot = myArray[endingIndex];
int partitionIndex = startingIndex;
for(int i = startingIndex; i<endingIndex; i++){
    if(myArray[i]<= pivot){
        swap(myArray[i],myArray[partitionIndex]);
        partitionIndex++;
    }
}
swap(myArray[partitionIndex],myArray[endingIndex]);
return partitionIndex;
} 

int QuickSelect(int *myArray, int startingIndex, int endingIndex, int KthElement){
/*if(startingIndex < endingIndex){
    int partitionIndex = Partition(myArray, startingIndex,endingIndex);
    QuickSelect(myArray,startingIndex,partitionIndex-1);
    QuickSelect(myArray,partitionIndex+1,endingIndex);
}*/1
if (startingIndex < endingIndex){
    int partitionIndex = Partition(myArray, startingIndex, endingIndex);
    if(KthElement == partitionIndex)
        return KthElement;
    if(KthElement < partitionIndex)
        QuickSelect(myArray, startingIndex, partitionIndex - 1, KthElement);
    else
        QuickSelect(myArray, partitionIndex + 1, endingIndex, KthElement);
}
}
int main(){

int numOfElements;
int KthElement;
srand(time(NULL));
cout<<"Enter The Amount Of Numbers You Wish To Use: ";
cin >> numOfElements;
int myArray[numOfElements];

cout << "Array Before Sorting: ";
for(int i = 0; i< numOfElements; i++){
    myArray[i] = rand() %10;
}

printArray(myArray, numOfElements);

cout << endl;
cout << endl;

cout <<"Enter The Index Of The Kth Element You Wish To Retrieve: ";
cin >> KthElement;

QuickSelect(myArray, 0,numOfElements,KthElement);
cout << "Array After Sorting: ";
printArray(myArray, numOfElements);
cout << endl;

cout <<"The " <<KthElement<<" Smallest Element Is: " <<   QuickSelect(myArray,0,numOfElements,KthElement);

}

推荐答案

对于 numOfElements 为5，数组从0扩展到4。
您的 endingIndex 假定其最后一个索引为

For numOfElements as 5, array extends from 0 to 4. Your endingIndex assumes that its the last index of the array.

修复：

QuickSelect(myArray, 0,numOfElements-1,KthElement);

您的代码存在问题：

• 您的程序访问

• Your program accesses out of bound array locations in

int pivot = myArray[endingIndex];

检查 k< 1 和 k>（num_of_elements）。

还要检查代码中的num_of_elements = 1。

Check your code for num_of_elements = 1 as well.

检查k对数组的含义，即对于 k = 1 ， arr [0] 不应返回 arr [1] ;

Check what k means for the array, i.e For k=1 , arr[0] should be returned not arr[1];

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