Java: an ordered index with TreeMap
A database index maps a column value to the rows that hold it. A hash index answers exact lookups quickly but cannot answer range queries such as ages between 25 and 40. An ordered index, usually a B-tree, keeps keys sorted and supports both.
Java
import java.util.*;
class Main {
public static void main(String[] args) {
TreeMap<Integer, String> byAge = new TreeMap<>();
byAge.put(31, "Cy");
byAge.put(24, "Ada");
byAge.put(45, "Di");
byAge.put(29, "Bo");
System.out.println(byAge.firstKey() + " " + byAge.lastKey());
System.out.println(byAge.subMap(25, true, 40, true));
System.out.println(byAge.floorKey(30) + " " + byAge.ceilingKey(30));
}
}