Sort an array of primitives with a custom comparator and without converting to objects

Question

What\'s the simplest way to sort a primitive array in Java with a custom comparator (or key) function and without converting to an array of objects (for performance †).

Answer 1

Create a list of Integer representing the indexes in the array, sort the list. You can reuse the list of indexes many times for many sorts.

Answer 2

Sorting of an array of primitive values with a custom comparator is not supported by the standard Java libraries.

You could easily implement your a simple sort ( e.g. bubblesort - O(N^2) ) from scratch, but the problem is that for large enough arrays the saving you make by not converting to boxed types is lost in the less efficient sorting algorithm.

So your choices are:

• Implement a high performance sort (mergesort, modified quicksort, etc) from scratch.

• Find an existing high performance sort for primitive types that doesn't support comparators, and modify it.

• See if you can find a suitable 3rd-party library that supports ptimitive arrays and comparators. (I haven't managed to find one ...)

(Note: the Comparator interface won't work here. It is not suitable for comparing primitive types.)

Answer 3

You can just build your own compare functions , and use one of the sort algorithms

easiest and slowest: BubbleSort ( O(N^2) ).

hardest but fastest : MergeSort( (O(Nlog(n) ) .

now in both algos you have the part that asks if A > B , in this part you should put your compare func.

boolean compare(int x, int y){
     if(/* your crazy compare condition */)
         return true;
     else return false; 
}

example in bubble sort :

    procedure bubbleSort( A : list of sortable items )
   repeat     
     swapped = false
     for i = 1 to length(A) - 1 inclusive do:
       /* if this pair is out of order */
       if compare(A[i],A[i-1]) then // Notcie the compare instead of A[i] > A[i-1]
         /* swap them and remember something changed */
         swap( A[i-1], A[i] )
         swapped = true
       end if
     end for
   until not swapped
end procedure

hope this helps

Answer 4

In Java 8 you can have your sort method take a function interface. This is modified code from OpenJDK (Copyright 1997-2007 Sun Microsystems, Inc. GPLv2):

import java.util.function.LongBinaryOperator;

public class ArraySort {
    public static void sort(long[] x, LongBinaryOperator op) {
        sort1(x, 0, x.length, op);
    }

    private static void sort1(long x[], int off, int len, LongBinaryOperator op) {
        if (len < 7) {
            for (int i=off; i<len+off; i++)
                // Use custom comparator for insertion sort fallback
                for (int j=i; j>off && (op.applyAsLong(x[j-1], x[j]) > 0); j--)
                    swap(x, j, j-1);
            return;
        }

        int m = off + (len >> 1);
        if (len > 7) {
            int l = off;
            int n = off + len - 1;
            if (len > 40) {
                int s = len/8;
                l = med3(x, l,     l+s, l+2*s);
                m = med3(x, m-s,   m,   m+s);
                n = med3(x, n-2*s, n-s, n);
            }
            m = med3(x, l, m, n);
        }
        long v = x[m];

        int a = off, b = a, c = off + len - 1, d = c;
        while(true) {
            // Use custom comparator for checking elements
            while (b <= c && (op.applyAsLong(x[b], v) <= 0)) {
                if (x[b] == v)
                    swap(x, a++, b);
                b++;
            }
            // Use custom comparator for checking elements
            while (c >= b && (op.applyAsLong(x[c], v) >= 0)) {
                if (x[c] == v)
                    swap(x, c, d--);
                c--;
            }
            if (b > c)
                break;
            swap(x, b++, c--);
        }

        int s, n = off + len;
        s = Math.min(a-off, b-a  );  vecswap(x, off, b-s, s);
        s = Math.min(d-c,   n-d-1);  vecswap(x, b,   n-s, s);

        if ((s = b-a) > 1)
            sort1(x, off, s, op);
        if ((s = d-c) > 1)
            sort1(x, n-s, s, op);
    }

    private static void swap(long x[], int a, int b) {
        long t = x[a];
        x[a] = x[b];
        x[b] = t;
    }

    private static void vecswap(long x[], int a, int b, int n) {
        for (int i=0; i<n; i++, a++, b++)
            swap(x, a, b);
    }

    private static int med3(long x[], int a, int b, int c) {
        return (x[a] < x[b] ?
                (x[b] < x[c] ? b : x[a] < x[c] ? c : a) :
                (x[b] > x[c] ? b : x[a] > x[c] ? c : a));
    }
}

And call it with lambdas or anything else implementing the LongBinaryOperator interface:

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        long x[] = {5, 5, 7, 1, 2, 5, 8, 9, 23, 5, 32, 45, 76};
        ArraySort.sort(x, (a, b) -> b - a);         // sort descending
        System.out.println(Arrays.toString(x));
    }
}

Output:

[76, 45, 32, 23, 9, 8, 7, 5, 5, 5, 5, 2, 1]

Answer 5

I copied the following from Java 6's Arrays.java and modified it to my needs. It uses insertion sort on smaller arrays so it should be faster than pure quicksort.

/**
 * Sorts the specified sub-array of integers into ascending order.
 */
private static void sort1(int x[], int off, int len) {
    // Insertion sort on smallest arrays
    if (len < 7) {
        for (int i=off; i<len+off; i++)
            for (int j=i; j>off && x[j-1]>x[j]; j--)
                swap(x, j, j-1);
        return;
    }

    // Choose a partition element, v
    int m = off + (len >> 1);       // Small arrays, middle element
    if (len > 7) {
        int l = off;
        int n = off + len - 1;
        if (len > 40) {        // Big arrays, pseudomedian of 9
            int s = len/8;
            l = med3(x, l,     l+s, l+2*s);
            m = med3(x, m-s,   m,   m+s);
            n = med3(x, n-2*s, n-s, n);
        }
        m = med3(x, l, m, n); // Mid-size, med of 3
    }
    int v = x[m];

    // Establish Invariant: v* (<v)* (>v)* v*
    int a = off, b = a, c = off + len - 1, d = c;
    while(true) {
        while (b <= c && x[b] <= v) {
            if (x[b] == v)
                swap(x, a++, b);
            b++;
        }
        while (c >= b && x[c] >= v) {
            if (x[c] == v)
                swap(x, c, d--);
            c--;
        }
        if (b > c)
            break;
        swap(x, b++, c--);
    }

    // Swap partition elements back to middle
    int s, n = off + len;
    s = Math.min(a-off, b-a  );  vecswap(x, off, b-s, s);
    s = Math.min(d-c,   n-d-1);  vecswap(x, b,   n-s, s);

    // Recursively sort non-partition-elements
    if ((s = b-a) > 1)
        sort1(x, off, s);
    if ((s = d-c) > 1)
        sort1(x, n-s, s);
}

/**
 * Swaps x[a] with x[b].
 */
private static void swap(int x[], int a, int b) {
    int t = x[a];
    x[a] = x[b];
    x[b] = t;
}

/**
 * Swaps x[a .. (a+n-1)] with x[b .. (b+n-1)].
 */
private static void vecswap(int x[], int a, int b, int n) {
    for (int i=0; i<n; i++, a++, b++)
        swap(x, a, b);
}

/**
 * Returns the index of the median of the three indexed integers.
 */
private static int med3(int x[], int a, int b, int c) {
    return (x[a] < x[b] ?
            (x[b] < x[c] ? b : x[a] < x[c] ? c : a) :
            (x[b] > x[c] ? b : x[a] > x[c] ? c : a));
}