Question

I’m attempting to perform the following calculation in Javascript:

e^x / (1 + e^x)

where x is a long floating point number.

In this instance, I require accuracy to at least the 10th decimal place.

I’ve been using BigDecimal to accurately handle the floating point numbers, as suggested at The Floating Point Guide.

My first attempt of:

var foo = new BigDecimal(myVeryLongFloatingPoint)
var bar = Math.exp(foo);
var spam = bar.divide(bar.add(new BigDecimal("1")));

led to the error (where xxxxx is the floating point number):

TypeError: Object xxxxx has no method 'add'

So I attempted to tried convert bar into a BigDecimal object:

var foo = new BigDecimal(myVeryLongFloatingPoint)
var bar = new BigDecimal(Math.exp(foo));
var spam = bar.divide(bar.add(new BigDecimal("1")));

which in turn leads to the error (where xxxxx is the floating point number):

BigDecimal(): Not a number: xxxxx

Where am I going wrong?

Is this a sensible approach to handling this kind of calculation with floating points where a high degree of precision is required?

Answer 1

You should pass strings to BigDecimal:

var bar = new BigDecimal(Math.exp(foo).toString());