Question

I’m trying to implement a primitive type of Polish Notation Calculator and I’m using an object to define each operation.

Such that calling

new RPNCalculator().calculate([4, 5, '+']);

will produce an answer of 9 or

new RPNCalculator().calculate([4, 5, '+', 3, 5, '+', '*']);

will produce an answer of 72.

The code is here:

function RPNCalculator(arr) {

    this.calculate = function(arr) {
        var resultArr =[];
        for(var i=0; i < arr.length; i++) {
            if(typeof(arr[i]) == 'number') {
                resultArr.push(arr[i]);
            }
            else {
                var a = resultArr.pop();
                var b = resultArr.pop();
                var c = opers[arr[i]].apply(this, [a, b]);
                resultArr.push(c);
            }
        }
        return resultArr.pop();
    }

    var opers = {
        "+": function(a, b) { return a + b; },
        "-": function(a, b) { return a - b; },
        "*": function(a, b) { return a * b; },
        "/": function(a, b) { return a / b; }
    }
}

The calculations work correctly, but what I would like to know is whether the following line

var c = opers[arr[i]].apply(this, [a, b]);

is the best way to invoke the required function contained inside the opers object based on the symbol at the current index in the array, or is there a better way to do it?

Answer 1

You don’t really need a reference to this in your code, because the functions aren’t working on any class members.

In this case, you can simply do:

var c = opers[arr[i]](a, b);

Which is a little cleaner. To be the most readable, however, I recommend this:

var operator = opers[arr[i]];
var c = operator(a, b);