Flutter check if variable is NaN

Question

I have var number; and it gets assigned by some calculations.

If I do print(number); I get NaN as response;

I expected that I would be able to do something like

if (number is NaN)

But I get NaN is not defined. How to check if variable is NaN in flutter?

Answer 1

You cannot check for nan by comparing it to the double.nan constant. isNan is the way to go.

yourNumber.isNaN

Why does comparing to double.nan not work?

print(double.nan == double.nan);
// prints false
print(double.nan.isNaN);
// prints true

This is because by definition NaN is not equal to itself. In fact if you would want to implement a NaN check yourself you would do it like so:

bool isNan(double x) => x != x;

This also has the consequence that if you save a NaN into any other kind of object, the object will no longer be equal to itself (as long as the NaN is used in the comparison - like with data classes). This can lead to unexpected behavior. You could use this code to check for this case.

bool hasNan(Object? x) => x != x;

In general this is true for all languages i am aware of - not just dart - as it's a hardware thing.