Question

I’m not sure how could I write code for the following recurrence:

[a, b] --> [a, a*2/3, a*1/3+b*2/3, b];

[a, b, c] --> [a, a*2/3, a*1/3+b*2/3, b, b*2/3+ c/3, b/3+c*2/3, c]

that’s it, takes a list, and expands that as in the example. I’m not sure how can I write code for that. Could someone please help me with that?

Answer 1

Pretty easy: takes a list as input, and produces a list as output.

public static <T extends Number> List<Double> expandThirds(List<T> input) {
    List<Double> output = new ArrayList<Double>();

    if(input.size() == 0)
        return output;

    output.add(input.get(0).doubleValue());

    for(int i=0; i<input.size()-1; i++) {
        double a = input.get(i).doubleValue();
        double b = input.get(i+1).doubleValue();
        output.add(a*2/3 + b/3);
        output.add(a*3 + b*2/3);
        output.add(b);
    }
    return output;
}