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/jive/thread.jspa?threadID=62657&tstart=0
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;
}