Find the k smallest integer in an array

Question

Here is my code, it works for finding 1-7 smallest integers, but 8 and 9. It returns null when I find 8 smallest integer in the array. Can anyone help me where is the problem? I am using quicksort here. Thanks very much!

update: I have figured out the problem, which is the array in the main function. After I change to the following look,

int[] arr = {2, 3, 1, 7, 5, 6, 20, 8, 4, 9};

and

if(front>=end) return input;

It works now!

import java.util.Arrays;
import java.io.*;

class quicksort{
public static void main(String[] args){
    int[] arr = new int[9];
    arr[0] = 7;
    arr[1] = 2;
    arr[2] = 4;
    arr[3] = 8;
    arr[4] = 3;
    arr[5] = 5;
    arr[6] = 1;
    arr[7] = 0;
    arr[8] = 10;

    System.out.println((Arrays.toString(findKSamllest(arr,8))));
}
public static int partition(int[] input, int front, int end){
    int pivot = input[front];
    while(front < end){
        while(input[front]<pivot)
            front++;
        while(input[end]>pivot)
            end--;
        swap(input,front,end);
    }
    return front;
}
public static void swap(int[] input, int s, int l){
    int temp = input[s];
    input[s] = input[l];
    input[l] = temp;
}

public static int[] findK(int[] input, int front, int end, int k){
    if(front>=end) return null;
    int pivot = partition(input,front,end);
    //System.out.println(pivot);
    if(k==pivot){
        return Arrays.copyOfRange(input,0,pivot);
    }
    else {
        if(k<pivot) 
            return findK(input,front,pivot,k); 
        return findK(input,pivot+1,end,k);
    }
}
public static int[] findKSamllest(int[] input, int k){
    return findK(input, 0, input.length-1, k);
}

}

Answer 1

Change

if(front >= end) return null;

to

if(front > end) return null;

Answer 2

Stand on the shoulders of giants and use the libraries that are already available:

Arrays.sort(myArray);
int[] returnArray = new int(NUM_ITEMS_TO_RETURN);
for (int i=0; i < NUM_ITEMS_TO_RETURN; i++)
{
   returnArray[i] = myArray[i];
}

return returnArray;

Obviously you have to do some error checking, for example that the initial array is larger or equal to the number of items you want returned, but that's trivial.

Answer 3

You could save you a bit time and impress your teacher with the fancy new Java 8 API.

It provides streams and useful functions to solve this in one (long) line or 5, if it should be readable ;-)

final List<Integer> sortedKList = Arrays.asList(7, 2, 4, 8, 3, 5, 1, 0, 10)
    .stream()
    .sorted()
    .limit(7)
    .collect(Collectors.toList());

You can then review your result with:

sortedKList.forEach(System.out::println);