Automatically return output using decimals instead of fractions

Question

I have a mathematica file with an algorithm that feeds another program with data. my problem is that some times (randomly) the output is in fraction instead of decimal. The other program (java) does not read fractions, so I have to feed it with decimal numbers. One solution is to use the N[.] function before it output, but this is really painful procedure. Is there any setting in mathematica forcing it to return as output only decimals?

Answer 1

You can use $Post:

If you set $Post to N, N is applied to the output of every evaluation:

 Cos[Pi/4]

$Post = N;
Cos[Pi/4]
(* 0.707107  *)

To restore $Post to its default value, use

 $Post =.

Answer 2

Two things might help you here:

1. Make sure all your input is in floating point (Real) form, not Integers or Rational. You might find this tutorial helpful. I am willing to bet that the cases where you get a fraction are not really random.
2. If wrapping your output in N is unacceptable to you for some reason, you could instead use a rule once you have your output: data /. x_Rational :> N@x. This will almost certainly be slower than N.

For example:

{1/2, 256/225, 0.6} /. x_Rational :> N@x

(* output is {0.5, 1.13778, 0.6}*)

N is Listable, so the following also works:

N[{1/2, 256/225, 0.6}]