I have an ArrayList that I need to sort but I need to get the indexes of the sorted items too. The ArrayList often has many duplicate values as it is updated and the indexex I get are duplicate. I need the unique indexes. Is there a way to get those indexes?
indices = new int[depth.size()];
ArrayList<Float> fStore = new ArrayList(depth);
Collections.sort(depth);
for (int n=0; n<depth.size(); n++){
indices[n] = fStore.indexOf(depth.get(n));
}
The problem with indexOf() is that it returns the index of the first occurrence of the element in the list, or -1 if not present. If your list has duplicates, then, for these duplicates, you'll get the index of their first occurrence within the list.
There are many ways to accomplish what you want. One is to use a SortedSet and a helper IndexedEntry class:
public class IndexedEntry<T extends Comparable<T>>
implements Comparable<IndexedEntry<T>> {
private final Integer index;
private final T entry;
public IndexedEntry(Integer index, T entry) {
this.index = index;
this.entry = entry;
}
public Integer getIndex() {
return this.index;
}
public T getEntry() {
return this.entry;
}
@Override
public int compareTo(IndexedEntry<T> other) {
int byEntry = this.getEntry().compareTo(other.getEntry());
if (byEntry == 0) {
return this.getIndex().compareTo(other.getIndex());
}
return byEntry;
}
@Override
public String toString() {
return "(" + this.entry + ", " + this.index + ")";
}
}
The IndexedEntry class simply wraps a Comparable value and its index, and implements Comparable by first comparing the value, and if the value is equal, then it compares the index. This means that an ordered list of IndexedEntry will have its elements ordered by (value, index).
To use the IndexedEntry class, just iterate over your collection, create an IndexedEntry for each element by wrapping the element and its index, and then sort the list of IndexedEntry:
List<Float> unordered = new ArrayList<>(Arrays.asList(3.2f, 1.0f, 7.5f, 3.2f, 0.8f));
System.out.println(unordered); // [3.2, 1.0, 7.5, 3.2, 0.8]
List<IndexedEntry<Float>> ordered = new ArrayList<>();
for (int i = 0; i < unordered.size(); i++) {
IndexedEntry<Float> entry = new IndexedEntry<>(i, unordered.get(i));
ordered.add(entry);
}
Collections.sort(ordered);
System.out.println(ordered); // [(0.8, 4), (1.0, 1), (3.2, 0), (3.2, 3), (7.5, 2)]
Once sorted, the ordered list will contain the elements along with the original index.
int[] a = new int[5];a[0]0IS AN INDEX...there is no such thing as an "index" in java