Question

If I execute the following expression in C#:

double i = 10*0.69;

i is: 6.8999999999999995. Why?

I understand numbers such as 1/3 can be hard to represent in binary as it has infinite recurring decimal places but this is not the case for 0.69. And 0.69 can easily be represented in binary, one binary number for 69 and another to denote the position of the decimal place.

How do I work around this? Use the decimal type?

Answer 1

Because you’ve misunderstood floating point arithmetic and how data is stored.

In fact, your code isn’t actually performing any arithmetic at execution time in this particular case – the compiler will have done it, then saved a constant in the generated executable. However, it can’t store an exact value of 6.9, because that value cannot be precisely represented in floating point point format, just like 1/3 can’t be precisely stored in a finite decimal representation.

See if this article helps you.