Obtain key of a hashmap with a range of numbers as value

Question

I have a HashMap<Integer, Float> with entries:

 1 -> 0.127
 2 -> 0.167
 3 -> 0.207
 4 -> 0.247
 5 -> 0.237
 6 -> 0.327
 7 -> 0.367
 8 -> 0.407
 9 -> 0.447
10 -> 0.487
11 -> 0.527
12 -> 0.567
13 -> 0.607
14 -> 0.647
15 -> 0.652

Let suppose that I want the key to the Float 0.465 (which is not an existing value). 0.465 is between 0.447 and 0.487, so I would like to get the key 10.

The first thought that came in my mind was achive this with 15 if/else if statements or with the switch statement. But in my view, this wouldn't be very elegant and practical.

Is there any other way to do this?

Answer 1

A Map is not the appropriate data structure. Use a TreeSet instead:

TreeSet<Float> numbers = new TreeSet<>();
// populate the set with your numbers, in any order, then

int index = numbers.headSet(n).size() + 1;

This will perform very well: TreeSet finds the insertion point in O(log n) time (like a binary search) and the list returned is just a view (a new list is not created), so the whole operation is lightweight.

Also note that the elements don't need to be added in any particular order - TreeSet internally maintains their order so searching is fast.

---

Here's some test code:

TreeSet<Float> numbers = new TreeSet<>(Arrays.asList(
    0.607F, 0.647F, 0.127F, 0.167F, 0.207F, 0.247F, 0.237F, 0.327F,
    0.367F, 0.407F, 0.447F, 0.487F, 0.527F, 0.567F, 0.652F));

Output:

10

Answer 2

Assuming data is your initial map and values are unique, you could do:

java.util.NavigableMap<Double, Integer> reverseMap = new java.util.TreeMap<>();
for(java.util.Map.Entry<Double, Integer> entry : data.entrySet()) {
    reverseMap.put(entry.getValue(), entry.getKey());
}
System.out.println(reverseMap.ceilingEntry(0.465).getValue());