Question

So I have to write a program that will take formulas from the user and calculate them.

    Age, Height, fights, (age+height)*fights <- user defined
    10, 5, 10, 150 <- I calculate this 
    1, 2, 1, 3 <- I calculate this 

But now let say I change the values around and I want the formula column to update dynamically is there anyway to go about doing that ? I am storing by each row into an array list which is array list of array list. Any advice or guidance would be really helpful thank you 🙂

Answer 1

We can revise this to make a simple engine

public interface Function<T> {
    T operate(Map<String,T> values);
}

public class Calculation<T> {

     private Map<String,T> values;

     private Function<T> function;

     public Calculation(Map<String,T> values, Function<T> function) {
         this.values = new HashMap<String,T>(values);
         this.function = function;
     }

     public T calculate() {
         return function.operate(Collections.unmodifiableMap(values));
     }

     // assume setters and getters are in place to manipulate the backing map and the functor object

}

With this in place, we can define various functions and calculations

public static void main(String[] args) {
    Function<Integer> original = new Function<Integer>() {
        public Integer operate(Map<String,Integer> values) {
            // these will throw exceptions if they don't exist, which is desired
            // granted it's NullPointerException instead of IllegalStateException, but close enough for this example
            int age = values.get("age").intValue();
            int height = values.get("height").intValue();
            int fights = values.get("fights").intValue();
            return (age+height)*fights;
        }
    };
    Map<String,Integer> map = new Map<String,Integer>();
    map.put("age",10);
    map.put("height",100);
    map.put("fights",25);
    Calculation<Integer> calc = new Calculation<Integer>(map,original);

    // later someone can replace either the values in the backing map
    // or replace the function altogether to get a new function
}

The problem comes when you try to make it not only user defined, but user entered. As in, a user could define the function as being one of a set of functions you have pre-entered. In this case, you’ll need to have a parser that turns something like this:

value + (other + (something * that)) / wazook

into this:

EquationBuilder<Integer> eb = new EquationBuilder<Integer>();
eb.append(map.get("value"));
eb.append(OPERATOR.PLUS);
// etc
return eb.evaluate();

But that may be beyond the scope of your assignment. Or maybe it’s not.

Hopefully this is a little closer to your requirements than my previous example.