Specify column width while exporting

Question

I am trying to export a set of data into a file but I can not force mathematica to keep a certain width for each column. For example, when I export a table:

table={{10,2},{3,40}};
Export["file.dat",table, "Table", "FieldSeparators"->" ", Alignment->Right]

The result is:

10  2
 2 40

Mathematica automatically determines column width such that the number with largest number of digits fits in. How can I export data with arbitrary space in each column to get:

 10    2
  2   40

such that "FieldSeparators"->" " is still the same but first column has space for three digits and the second one has space for 4 digits?

Answer 1

One way would be to do the padding manually:

padToWidth[width_, n_] := StringJoin@PadLeft[Characters@ToString[n], width, " "]

newData = MapAt[padToWidth[4, #] &, data, {All, 1}]; (* Apply to first column *)
newData = MapAt[padToWidth[3, #] &, newData, {All, 2}]; (* Apply to second column *)
Export["data.dat", StringJoin[First@#, " ", Last@#] & /@ newData]

Answer 2

table = {{10, 2}, {3, 40}};

export = (Transpose@IntegerString@table)[[#]] /. 
 a_String :> Table[" ", {# + 2 - StringLength@a}] <> a & /@ {1, 2} // Transpose;

Export["file.dat", export, "Table"]