How to change the Integer to a special form in an expression?

I have an expression. I want to change all the Integers to a new form. The rule is, x_Integer->x._f. But we should consider some special cases. For example,

  input = 22 +  4/5 x1 x1 x1 + (2 x2^4 + 343 Pi^4)/
Sqrt[67 - x1 x1 x3 x3 + x2^(5/4)] + (4 x1 - x2)^(-3/2 q) +
Exp[-x1 x1 + 4 x2 x2 - 3 x1];


The output expression should be,

 output = 22. _f + E^(-3. _f x1 - x1^(2. _f) + 4. _f x2^(2. _f)) + (
4. _f x1^(3. _f))/(5. _f) + (4. _f x1 - x2)^(-3. _f x3/2. _f) + (
343. _f \[Pi]^(4. _f) + 2. _f x2^(4. _f))/Sqrt[
67. _f + x2^(5. _f/4. _f) - x1^(2. _f) x3^(2. _f)];


So, how to construct the rule function? Thanks!

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Something like ReplaceAll[input, x_?IntegerQ -> f[1. x]] ? –  b.gatessucks Mar 17 '13 at 9:11
not sure if you have this in mind, but does this work: input /. {Rational[x_, y_] :> Row[{x, ".", Blank[f]}]/Row[{y, ".", Blank[f]}], x_?NumberQ :> Row[{x, ".", Blank[f]}]}? –  kguler Mar 17 '13 at 9:28

One simple way would be to do:

     Unprotect[Integer];
Integer /:
Format[r_Integer, FortranForm] := (SequenceForm[r // N, "_f"]);
Protect[Integer];
Unprotect[Rational];
Rational /:
Format[r_Rational,
FortranForm] := (SequenceForm[Numerator[r] // N, "_f", "/",
Denominator[r] // N, "_f"]);
Protect[Rational];
Protect[Integer];
tos = ToString[#, InputForm] &;
pow[a_Symbol, b_Integer] := tos[a] <> "^" <> tos[FortranForm[b]];
pow[a_Symbol, b_] := tos[a] <> "^(" <> tos[FortranForm[b]] <> ")";
pow[a_, b_] := "(" <> tos[a] <> ")^(" <> tos[FortranForm[b]] <> ")";
input = 22 +
4/5 x1 x1 x1 + (2 x2^4 + 343 Pi^4)/
Sqrt[67 - x1 x1 x3 x3 + x2^(5/4)] + (4 x1 - x2)^(-3/2 q) +
Exp[-x1 x1 + 4 x2 x2 - 3 x1];
StringReplace[
ToString[FortranForm[input /. Power -> pow], InputForm], {"\\" -> "",
"\"" -> ""}]


which gives:

"22._f + (4 x1 - x2)^((-3._f*q)/2.) + E^(-x1^2._f + 4._f*x2^2._f - \
3._f*x1) + (4._f*x1^3._f)/5. + (67 + x2^(5._f/4._f) - x1^2._f \
x3^2._f)^(-1._f/2._f)*(343._f*Pi^4._f + 2._f*x2^4._f)"

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@Mertig Thanks! If input = 22 + (23/4 x1 - x2)^(3 dd/2), then some problem happens. Is it possible to find a more simple and safe method to deal with this problem? –  Orders Mar 18 '13 at 0:55
@Orders Yes, that is possible. Try yourself: change ToString to ToString[ ..., InputForm] and then make it all into a nice function, say ToFortran. Use Module. Document everything you do and post it here (you can edit my post if you want). –  Rolf Mertig Mar 18 '13 at 1:08

Simplifications/reordering that will separate the numbers from the _f pattern in a way that you seem to not want will take place on your given output expression if it is evaluated.

Therefore you will need some kind of hold function here. I will use HoldForm.

input = 22 +
4/5 x1 x1 x1 + (2 x2^4 + 343 Pi^4)/
Sqrt[67 - x1 x1 x3 x3 + x2^(5/4)] + (4 x1 - x2)^(-3/2 q) +
Exp[-x1 x1 + 4 x2 x2 - 3 x1];

input /. {
Rational[n_, d_] :> Divide @@ HoldForm /@ {N@n f_, N@d f_},
n_Integer :> HoldForm @@ {N[n] _f}
}


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5/4 should be 5._f/4._f –  Orders Mar 17 '13 at 11:49
@Orders 5/4 is Rational not Integer. Are you aware of the difference in the context of Mathematica? –  Mr.Wizard Mar 17 '13 at 11:51
Yes, I don't know how to express this problem exactly. I mean , for the Rational 5/4, the integer 5 and 4 also should be changed. –  Orders Mar 17 '13 at 11:59
@Orders Okay, I'll start on that. –  Mr.Wizard Mar 17 '13 at 11:59
@Orders Please take a look. I do wonder what you're planning for this construct, and I hope it works. –  Mr.Wizard Mar 17 '13 at 12:05