Question

I have the below code somewhere in my app

float private myMethod(float c){
    result = (float) (c+273.15);
}

When “c” gets the value something like -273.1455 the result is something very near to zero like 0.0044.

But when it gets the value -273.15 i get this instead of zero: 6.1035157E-6

Why does this happen?

Answer 1

The problem is that 273.15 is a double, not a float, and neither of them can represent 273.15 exactly. However, since they have different precision they will round actually store different numbers. When the addition is done the c is converted to a double which will be able store the float representation of 273.15. So now you have two doubles with almost the same value and the difference will be non zero.

To get “more predictable” result, use 273.15f to ensure you have floats through the calculations. That should solve this problem but what you need to do is to read up on binary floating point arithmetics and how that differs from decimal arithmetic that we are taught in school.

Wiki on floating point is a good place to start.