1
$\begingroup$

If I create a table of this kind:

(m1 = Table[{RandomInteger[], RandomInteger[{1, 5}, {3}]}, {i, 1, 
 5}]) // MatrixForm

which looks like

enter image description here

I know how to pick the first column of the second column:

m1[[All, 2, 1]] // MatrixForm

which looks like

enter image description here

But if I first export and then again import the matrix m1

Export["data.dat", m1, "Data"]
m2 = Import["data.dat", "Data"]

I do not get the same result as previously

m2[[All, 2, 1]] // MatrixForm

which looks like

enter image description here

What is wrong?

Improve this question
$\endgroup$
3
  • 1
    $\begingroup$ If you just want to save/retrive the data use "m" or "mx" format, eq Export["data.m",m1] these are "native" mathematica formats and will safely preserve whatever expression you export. If you really want to use the "Data" format for some reason that will take a bit of work to read it back in (clarify if that's what you need.. ) $\endgroup$
    – george2079
    Commented Dec 3, 2014 at 17:09
  • $\begingroup$ @george2079 Seems I somehow misinterpreted the docs ("Data": give data in a generic form (list, string, etc.)), I thought this was the one I had to add. I did not want to export as "Data" format, so actually your comment contains the solution to my problem. Could you please put it here as answer? I will mark it then. $\endgroup$
    – wondering
    Commented Dec 3, 2014 at 21:42
  • $\begingroup$ Ok, anyway, +1. $\endgroup$
    – wondering
    Commented Dec 3, 2014 at 22:28

2 Answers 2

Reset to default
3
$\begingroup$

When you export a Table, then the array cannot be arbitrarily nested. What you should do is to export your data (which is not a matrix!) in a different format. Try:

Export["data.dat", m1, "Package"]
Import["data.dat", "Package"][[All, 2, 1]] // MatrixForm

or

Export["data.dat", m1, "MX"]
Import["data.dat", "MX"][[All, 2, 1]] // MatrixForm

and read the documentation of these formats.

Improve this answer
$\endgroup$
1
  • $\begingroup$ Thank you, this works, too +1, but the solution suggested by george2079 is sufficient for me (it's shorter). $\endgroup$
    – wondering
    Commented Dec 3, 2014 at 21:44
0
$\begingroup$

Because you are exporting as text, and when you import back into Mathematica, it will see extra commas. To get the desired default, either export as csv

Export["data.csv", m1]

or WDF

Export["data.wdx", m1, "WDX"]
Improve this answer
$\endgroup$
5
  • $\begingroup$ 'Export["data.wdf", m1, "WDF"]' gives in the case of my matrix m1 'Export::infer: Cannot infer format of file data.wdf. >>' $\endgroup$
    – wondering
    Commented Dec 3, 2014 at 21:46
  • $\begingroup$ My bad, I meant WDX. It's Wolfram's proprietary file format. Try 'Export["data.wdx", m1, "WDX"]'. Works for me. $\endgroup$
    – nqduy
    Commented Dec 4, 2014 at 1:51
  • $\begingroup$ Yes, it works, not the 'Export["data.csv", m1]', however. $\endgroup$
    – wondering
    Commented Dec 4, 2014 at 6:38
  • 1
    $\begingroup$ Ah, I see what the problem is. At a glance, it looks correct, but Mathematica actually exported the inner list as a string (without added commas, so it's not too bad). You can still use this format if you do just one extra process after import: m2 = Import["data.csv", "Data"]; m2[[1, 2]] // FullForm m2[[All, 2]] = Map[ToExpression, m2[[All, 2]]] m1 == m2 m2[[1, 2]] // FullForm $\endgroup$
    – nqduy
    Commented Dec 4, 2014 at 16:24
  • $\begingroup$ Ok, thank you for the effort! $\endgroup$
    – wondering
    Commented Dec 4, 2014 at 17:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.