I have an [] that has some numbers (distances from some point).
I want to create an array of indexes into the first array where the indexes are sorted by the distance.
eg
suppose double[] dist=new double[5] {3.2, 1.4, 7.3, 2.2, 9.1};
then I want to get an array like this:
int[] sortedIndexes=new int[5] {1, 3, 0, 2, 4};
so if I want the second nearest distance I can check dist[sortedIndexes[1]].
I don't want to sort the original array, just the array of indexes based on the distances.
UPDATE 1: The Code I was trying looks like this:
Collections.sort(sortedIDXs, new Comparator<Integer>() {
public int compare(int idx1, int idx2) {
return Double.compare(distances[idx1], distances[idx2]);
}
});
But I am getting several errors with it with the most "problematic" one being: " Cannot refer to a non-final variable distances inside an inner class defined in a different method "
Thanks
You're on the right track, but
Integer
array than an int
array if you're using a generic Comparator<Integer>
. Arrays.sort
instead Collections.sort
for sorting an array. You have to make the distances variable final if it's referenced in an anonymous inner class.
final double[] distances=new double[]{3.2, 1.4, 7.3, 2.2, 9.1}; Integer[] sortedIDXs = new Integer[]{0,1,2,3,4}; Arrays.sort(sortedIDXs, new Comparator<Integer>() { public int compare(Integer idx1, Integer idx2) { return Double.compare(distances[idx1], distances[idx2]); } });
Works well if you want the indicies as a primative int array then you will have to create your own binary sorter which shouldn't be to difficult.
Edit: I adapted java's mergesorter to work with int's. This should save you a little time in writing your own.
public static void main(String[] args) {
double[] dist = new double[] {3.2, 1.4, 7.3, 2.2, 9.1};
int[] indices = createIndicies(dist);
System.out.println(Arrays.toString(dist) + " " + Arrays.toString(indices));
}
public static int[] createIndicies(double[] array) {
int[] intArray = new int[array.length];
for (int j = 0; j < array.length; j++) {
intArray[j] = j;
}
int[] indicies = intArray.clone();
mergeSort(intArray, indicies, 0, intArray.length, 0, new IndiciesSorter(array));
return indicies;
}
public static class IndiciesSorter implements Comparator<Integer> {
double[] array;
public IndiciesSorter(double[] array) {
this.array = array;
}
@Override
public int compare(Integer o1, Integer o2) {
return Double.compare(array[o1], array[o2]);
}
}
private static void mergeSort(int[] src, int[] dest, int low,
int high, int off, Comparator c) {
int length = high - low;
// Insertion sort on smallest arrays
if (length < 7) {
for (int i = low; i < high; i++)
for (int j = i; j > low && c.compare(dest[j - 1], dest[j]) > 0; j--)
swap(dest, j, j - 1);
return;
}
// Recursively sort halves of dest into src
int destLow = low;
int destHigh = high;
low += off;
high += off;
int mid = (low + high) >>> 1;
mergeSort(dest, src, low, mid, -off, c);
mergeSort(dest, src, mid, high, -off, c);
// If list is already sorted, just copy from src to dest. This is an
// optimization that results in faster sorts for nearly ordered lists.
if (c.compare(src[mid - 1], src[mid]) <= 0) {
System.arraycopy(src, low, dest, destLow, length);
return;
}
// Merge sorted halves (now in src) into dest
for (int i = destLow, p = low, q = mid; i < destHigh; i++) {
if (q >= high || p < mid && c.compare(src[p], src[q]) <= 0)
dest[i] = src[p++];
else
dest[i] = src[q++];
}
}
private static void swap(int[] x, int a, int b) {
int t = x[a];
x[a] = x[b];
x[b] = t;
}
I did try it now and it works! :)
double[] dist= {3.2, 1.4, 7.3, 2.2, 9.1}; // your array
int[] sortedIndexes= new int[dist.length]; // your array
double[] temp = dist.clone(); // clone the array
Arrays.sort(temp); // Use native array sort function
for(int i = 0; i<temp.length; i++) { // iterate through sorted array
for(int j = 0; j<dist.length; j++) { // iterate through original array
if (dist[j] == temp[i]) { // if sorted value == unsorted value
sortedIndexes[i] = j; // save position of match into your sortedIndex array
}
}
}
You should customize your own Comparator and use java API.
http://docs.oracle.com/javase/tutorial/collections/interfaces/order.html
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