Question

I am trying to find a data structure that takes in a particular value from a range of values and map it to a key.

For example, I have the following conditions:

1. From 1 to 2.9, I want to map it to A.
2. From 4 to 6, I want to map it to B.
3. From 6.5 to 10, I want to map it to C.

I have a value of 5 and I would like to map it to a key. So based on the above conditions, I should map it to B.

Is there any data structure in Java that anyone can recommend to me to solve the problem?

Currently I am using a hashtable that can only map a value to a key. I tried to map the range of values to a particular value that exists in the hashtable. However, I got stuck in the mapping of the range of values to a particular value. So now I am trying to do another way of mapping the ranges of values to a key. Does anyone have any idea how I can solve this problem?

EDIT:

Thanks to Martin Ellis, I decided to use TreeMap to solve the problem.

Answer 1

Are your ranges non-overlapping? If so you could use a TreeMap:

TreeMap<Double, Character> m = new TreeMap<Double, Character>();
m.put(1.0, 'A');
m.put(2.9, null);
m.put(4.0, 'B');
m.put(6.0, null);
m.put(6.5, 'C');
m.put(10.0, null);

The lookup logic is a bit complicated by the fact that you probably want an inclusive lookup (i.e. 2.9 maps to ‘A’, and not undefined):

private static <K, V> V mappedValue(TreeMap<K, V> map, K key) {
    Entry<K, V> e = map.floorEntry(key);
    if (e != null && e.getValue() == null) {
        e = map.lowerEntry(key);
    }
    return e == null ? null : e.getValue();
}

Example:

mappedValue(m, 5) == 'B'

More results include:

0.9 null
1.0 A
1.1 A
2.8 A
2.9 A
3.0 null
6.4 null
6.5 C
6.6 C
9.9 C
10.0 C
10.1 null