Martin:

Please bear with me. I am also learning about compiled functions. And you taught me how to pass in lists and matrices to a compiled function. THANKS

I managed to get this to work:

cIterReal = Compile[
   {
    {vecPosAct, _Real, 2},
    {boxWidth, _Real},
    {boxWidth2, _Real},
    {c, _Integer}
    }
   ,
   Module[
    {
     vecPosActNew,
     check
     }
    ,
    vecPosActNew = vecPosAct;
    Do[
     vecPosActNew[[check]] -= 
      Round[(vecPosAct[[check]] - boxWidth2)/boxWidth]*boxWidth2*1.
     ,
     {check, 1, c, 1}
     ];
    vecPosActNew
    ],

   CompilationTarget -> "C"
   , CompilationOptions -> {"InlineExternalDefinitions" -> True}
   ];

Now,

cIterReal[{{3.2, 4., 9.}, {2.3, 4., 9.}, {9.2, 3.2, 5.0}}, 2.2, 1.1, 3]

Returns:

{{2.1, 2.9, 4.6}, {1.2, 2.9, 4.6}, {4.8, 2.1, 2.8}}

Remember that Do does not return anything. The docs say:

Unless an explicit Return is used, the value returned by Do is Null.

I hope that helps. I'd be interested to hear how much of a performance boost this gives you.

Have a great rest of your week.

Thank you very much Mike fyi its a part of an Verlet Algorithm that I'm trying to speed up.

So the argument vecPosAct is in reality a long nested list and thats pretty much the problem, I don't know how to tell the compiler that its a nested list with reals.

For my understanding {_Real,2} should work, but it does not and I don't know why since vecPosAct is a rank 2 argument?

Again thank you very much for helping me out.