Java Array sort: Quick way to get a sorted list of indices of an array

Question

The problem: Consider the following floats[]: ``` d[i] = 1.7 -0.3 2.1 0.5 ``` What I want is an array of int[] that represents the order of the original array with indices. ``` s[i] = 1 3 0 2 d[s[i]] = -0.3 0.5 1.7 2.1 ``` Of course it could be done with a custom comparator, a sorted set of custom objects, or by simply sorting the array and then searching for the indices in the original array (shudder). What I am in fact looking for is the equivalent for the second return argument of Matlab's sort function. Is there an easy way to do that (<5 LOC)? May there be a solution that does not need to allocate a new object for each element? Update: Thanks for your responses. Unfortunately, none of what has been proposed so far resembles the simple and efficient solution I was hoping for. I therefore openened a thread in the JDK feedback forum, proposing the addition of a new class-library function to address the issue. Lets see what Sun/Oracle thinks about the issue. http://forums.java.net/ji…

Accepted Answer

I would tailor the quicksort algorithm to perform the exchange operation on multiple arrays at the same time: the index array and the value array. For example (based on this quicksort):

```
public static void quicksort(float[] main, int[] index) {
    quicksort(main, index, 0, index.length - 1);
}

// quicksort a[left] to a[right]
public static void quicksort(float[] a, int[] index, int left, int right) {
    if (right <= left) return;
    int i = partition(a, index, left, right);
    quicksort(a, index, left, i-1);
    quicksort(a, index, i+1, right);
}

// partition a[left] to a[right], assumes left < right
private static int partition(float[] a, int[] index, 
int left, int right) {
    int i = left - 1;
    int j = right;
    while (true) {
        while (less(a[++i], a[right]))      // find item on left to swap
            ;                               // a[right] acts as sentinel
        while (less(a[right], a[--j]))      // find item on right to swap
            if (j == left) break;           // don't go out-of-bounds
        if (i >= j) break;                  // check if pointers cross
        exch(a, index, i, j);               // swap two elements into place
    }
    exch(a, index, i, right);               // swap with partition element
    return i;
}

// is x < y ?
private static boolean less(float x, float y) {
    return (x < y);
}

// exchange a[i] and a[j]
private static void exch(float[] a, int[] index, int i, int j) {
    float swap = a[i];
    a[i] = a[j];
    a[j] = swap;
    int b = index[i];
    index[i] = index[j];
    index[j] = b;
}
```

Java Array sort: Quick way to get a sorted list of indices of an array

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