Are C doubles different to .NET doubles?

Question

Comparing some C-code and the F# I'm trying to replace it with, I observed that there were some differences in the final result.

Working back up the code, I discovered that even at there were differences - albeit tiny ones.

The code starts by reading in data from a file. and the very first number comes out differently. For instance, in F# (easier to script):

let a = 71.9497985840
printfn "%.20f" a

I get the expected (to me) output 71.94979858400000000000.

But in C:

a =  71.9497985840;
fprintf (stderr, "%.20f\n", a);

prints out 71.94979858400000700000.

Where does that 7 come from?

The difference is only tiny, but it bothers me because I don't know why. (It also bothers me because it makes it more difficult to track down where my two versions of code are diverging)

Answer 1

It's a diifference in printing. Converting that value to an IEEE754 double yields

Prelude Text.FShow.RealFloat> FD 71.9497985840
71.94979858400000694018672220408916473388671875

but the representation 71.949798584 is sufficient to distinguish the number from its neighbours. C, when asked to print with a precision of 20 digits after the decimal point converts the value correctly rounded to the desired number of digits, apparently F# uses the shortest uniquely determining representation and pads it with the desired number of 0s, just like Haskell does.

Answer 2

It's just different rounding. The numbers are the same (according to CPython, at least):

>>> '%.44f' % 71.94979858400000000000
'71.94979858400000694018672220408916473388671875'
>>> '%.44f' % 71.94979858400000700000
'71.94979858400000694018672220408916473388671875'