Re: Rounding off double precision

On Mar 29, 12:48 am, Bamm <bamms...@xxxxxxxxx> wrote:
when I run the complete program

    write (*,*) COS(90 * 3.141592653589793238d0 / 180.)
    end

using g95 on this Mac, I get

    6.123256244561421E-17

which is about the expected accuracy for double precision.

Here's the answer I get for the same program above using g77 on Linux:

  6.12303177E-17

Am I doing anything wrong?

Yes! You are expecting calculations done with floating point numbers
to be EXACT. Such calculations are actually approximate. Each
operation has a small quantity of error built in. That's the nature of
floating point.

To start with, you can not represent the EXACT value of PI with a
finite number of bits. It only gets worse from there. :-).

- e

.

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