How can I control the number format of exported data?

Question

I'm having issues while exporting a tables of numerical values to "*.dat" data files.

Here's the thing, sometimes when I export a table, I get more decimals on some random values. The problem is that they weren't even there in the first place.

Now, for the sake of illustration, here's a very simple example on the issue. I tried constructing the following table :

Table[t, {t, 0, 0.4, 0.1}]

Which provides with the following list :

{0., 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, \
0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, \
0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, \
0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4}

Now, I just tried to export it using the Exportcommand like this :

Export["t.dat", Table[t, {t, 0, 0.4, 0.1}], "Table"]

and everything should be fine, but when you try to read what's in "t.dat" using FilePrint :

FilePrint["t.dat"]

I get the following :

0. 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 0.32 0.33 0.34 0.35000000000000003 0.36 0.37 0.38 0.39 0.4

Note that the number 0.35 suddenly became 0.35000000000000003, which messes everything up. Is there something to solve this issue? I tried using the NumberForm function but it changes the structure of the exported list...

What I intend to do is to export t list presented above, alongside with another list in which I do operations using the values of the first one. For example :

time = Table[NumberForm[i, 6], {i, 0, 0.2, 0.01}];
op = Table[time[[i]]^2 + time[[i]], {i, 1, Length@time}];
Export["data.dat",Transpose@{time,op}, "Table"];
FilePrint["data.dat"];

Which returns :

0.  NumberForm[0., 6] + NumberForm[0., 6]^2
0.01    NumberForm[0.01, 6] + NumberForm[0.01, 6]^2
0.02    NumberForm[0.02, 6] + NumberForm[0.02, 6]^2
0.03    NumberForm[0.03, 6] + NumberForm[0.03, 6]^2
0.04    NumberForm[0.04, 6] + NumberForm[0.04, 6]^2
0.05    NumberForm[0.05, 6] + NumberForm[0.05, 6]^2
0.06    NumberForm[0.06, 6] + NumberForm[0.06, 6]^2
0.07    NumberForm[0.07, 6] + NumberForm[0.07, 6]^2
0.08    NumberForm[0.08, 6] + NumberForm[0.08, 6]^2
0.09    NumberForm[0.09, 6] + NumberForm[0.09, 6]^2
0.1 NumberForm[0.1, 6] + NumberForm[0.1, 6]^2
0.11    NumberForm[0.11, 6] + NumberForm[0.11, 6]^2
0.12    NumberForm[0.12, 6] + NumberForm[0.12, 6]^2
0.13    NumberForm[0.13, 6] + NumberForm[0.13, 6]^2
0.14    NumberForm[0.14, 6] + NumberForm[0.14, 6]^2
0.15    NumberForm[0.15, 6] + NumberForm[0.15, 6]^2
0.16    NumberForm[0.16, 6] + NumberForm[0.16, 6]^2
0.17    NumberForm[0.17, 6] + NumberForm[0.17, 6]^2
0.18    NumberForm[0.18, 6] + NumberForm[0.18, 6]^2
0.19    NumberForm[0.19, 6] + NumberForm[0.19, 6]^2

What I mean by "changes its structure" is the fact that I can no longer operate on the time list and its values correctly...

Answer 1

Well, as it turns out, the answer to my question was trivial...

Instead of using NumberForm on the time list to operate, I just needed to export using another NumberForm version of the time list to operate... like time2... pretty easy...

time = Table[i, {i, 0, 0.4, 0.01}];
time2 = Table[NumberForm[i, 6], {i, 0, 0.4, 0.01}];
op = Table[time[[i]]^2 + time[[i]], {i, 1, Length@time}];
Export["data.dat", Transpose@{time2, op}, "Table"];
FilePrint["data.dat"];

which finally gives what I needed :

    0.  0.
0.01    0.0101
0.02    0.0204
0.03    0.0309
0.04    0.0416
0.05    0.0525
0.06    0.0636
0.07    0.0749
0.08    0.0864
0.09    0.0981
0.1 0.11
0.11    0.1221
0.12    0.1344
0.13    0.1469
0.14    0.1596
0.15    0.1725
0.16    0.1856
0.17    0.1989
0.18    0.2124
0.19    0.2261
0.2 0.24
0.21    0.2541
0.22    0.2684
0.23    0.2829
0.24    0.2976
0.25    0.3125
0.26    0.3276
0.27    0.3429
0.28    0.3584
0.29    0.3741
0.3 0.39
0.31    0.4061
0.32    0.4224
0.33    0.4389
0.34    0.4556
0.35    0.4725
0.36    0.4896
0.37    0.5069
0.38    0.5244
0.39    0.5421
0.4 0.56

Answer 2

NumberForm seems fine for the example you give:-

Export["t1.dat", Table[t, {t, 0.3, 0.4, 0.01}], "Table"];
Export["t2.dat", Table[NumberForm[t, 2], {t, 0.3, 0.4, 0.01}], "Table"];
TableForm[ReadList["!diff t1.dat t2.dat", "String"]]

(*
4,5c4,5
< 0.32999999999999996
< 0.33999999999999997
---
> 0.33
> 0.34
*)