Hi Arthur,

From the code, you are coming to Mathematica from C. I use both, and 99.9% Mathematica. In the code below, I convert to Mathematica syntax. (* This is a comment *) and * is not needed to invoke multiplication, a space is fine, but not needed after a leading number. You use For, but most often Table is better, at least more concise. NumberForm is a wrapper used for output, it should be used after all calculation, but before printing. There is a lot of information to be found in The Virtual Book. It can be found by searching the help system, or at this link: Virtual Book

Here is code as an example. (I made up a function for ks.) I also attach as a notebook, so it can be executed without deleting all the Out cells.

In[1]:= nwwa = {};

In[2]:= nkka = {};

In[3]:= For[j = -4, j <= 4, j++,
  (* calculate w *)
  w = 16 (0.5 + 0.1 j);
  (* append it *)
  nwwa = Append[nwwa, w];
  (* calculate ks *)
  ks = Sqrt[2] w;
  (* append it *)
   nkka = Append[nkka, ks];
  ];

In[4]:= nwwa

Out[4]= {1.6, 3.2, 4.8, 6.4, 8., 9.6, 11.2, 12.8, 14.4}

In[5]:= nkka

Out[5]= {2.26274, 4.52548, 6.78823, 9.05097, 11.3137, 13.5765, \
15.8392, 18.1019, 20.3647}

In[6]:= result = Transpose[{nwwa, nkka}]

Out[6]= {{1.6, 2.26274}, {3.2, 4.52548}, {4.8, 6.78823}, {6.4, 
  9.05097}, {8., 11.3137}, {9.6, 13.5765}, {11.2, 15.8392}, {12.8, 
  18.1019}, {14.4, 20.3647}}

In[7]:= formattedResult = 
  result /. {x_, y_} -> {NumberForm[x, {3, 1}], NumberForm[y, 6]};

In[8]:= Export["c:\\temp\\mathematica1.dat", formattedResult, "Table"];

In[9]:= check = Import["c:\\temp\\mathematica1.dat"]

Out[9]= {{1.6, 2.26274}, {3.2, 4.52548}, {4.8, 6.78823}, {6.4, 
  9.05097}, {8., 11.3137}, {9.6, 13.5765}, {11.2, 15.8392}, {12.8, 
  18.1019}, {14.4, 20.3647}}

In[10]:= ListPlot[check, PlotLabel -> "Mathematica can plot this!", 
 Frame -> True, FrameLabel -> {"w", "ks"}]

Attachments:

First, something's missing from you code: you append ks to the current list nkka, but you never define what ks is is. (And the mysterious "//calculate ks" is not Mathematica code (the // itself has meaning in Mathematica that has nothing to do with a comment).

Second, to obtain the list nwwa, there's no need to use an explicit loop. The following single line does it:

 nwwa = Prepend[16 (0.5 + 0.1 Range[-4, 4]), "#w#"]

But you want to use NumberForm, too. So do the following:

nwwa = 16 (0.0005 + 0.1 Range[-4, 4];
nwwa = NumberForm[nwwa, {3, 1}];
nwwa = Prepend[nwwa, "#w#"]

At least for nwwa, your export to a table should now be OK.