Question

I need to calculate the middle point (average) between two real numbers in JavaScript. The range of the numbers can vary widely, generally between 10000 and 0.0001.

The naive approach

(parseFloat(first) + parseFloat(second)) / 2

gives me unwanted precission errors, i.e.

(1.1 + 0.1) / 2 = 0.6000000000000001

How can I ensure that the result does not have extra decimal spaces? I guess, since there are two and only two inputs, that the result will need to have maximum one more decimal place than the inputs. So, I need:

 1000 and 3000 to return 2000 (without decimal spaces)
 1234.5678 and 2468.2468 to return 1851.4073
 0.001 and 0.0001 to return 0.00055
 10000 and 0.0001 to return 5000.00005
 0.1 and 1.1 to return 0.6

---

To clarify: I know all about precision errors and why this happens. What I need is a simple workaround, and I have not been able to find a previous solution on SO.

Answer 1

You’ll want to use the function toFixed(). Here’s what your code could look like ((1.1 + 0.1) / 2).toFixed(4);. And 4 is the number of decimal spaces.

Here’s a sample

var num1 = "123.456";
var num2 = "456.1235";
if (num1.split('.')[1].length > num2.split('.')[1].length)
  var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num1.split('.')[1].length+1);
else
  var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num2.split('.')[1].length+1);

And remember inputs are strings, so that’s why num1 and num2 are strings.

Update: Here’s the correct if statements for your desired results.

var num1 = "123.456";
var num2 = "456.1235";
if (num1.split('.').length == 1) {
  if (num2.split('.').length == 1)
    var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(0);
  else
    var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num2.split('.')[1].length);
} else if (num2.split('.').length == 1)
  var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num2.split('.')[1].length);
else if (num1.split('.')[1].length == num2.split('.')[1].length)
  var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num2.split('.')[1].length);
else if (num1.split('.')[1].length > num2.split('.')[1].length)
  var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num1.split('.')[1].length+1);
else if (num1.split('.')[1].length < num2.split('.')[1].length)
  var x = ((parseFloat(num1) + parseFloat(num2)) / 2).toFixed(num2.split('.')[1].length+1);

EDIT I had to change the order it checked for numbers with no decimal space.
Here’s a jsFiddle I created to help with the testing: link