Assuming I already have the file created.

I have a programme that,returns an output, in our case number. I want Mathematica to save this number in a file, together with results saved from previous calculations.

After having checked Mathematica commands, I found that I could do something like:

file = OpenWrite["test.dat"];
Write[file, new];

However, this doesn't work, because it overwrites the information that is already in the file. Working with this limitation, I tried a different approach:

results=Import["test.dat", "Table"];
file = OpenWrite["test.dat"];

This new approach still doesn't work since when I initialize the file test.dat with

Export["test.dat", Range[1, 10]]

and then import the list, I do not get the same list, but

{{1}, {2}, {3}, {4}, {5}, {6}, {7}, {8}, {9}, {10}}

I get the same unwanted list, even if I add the "Table" option to Export, or Import...

How do I efficiently save the progress of my programme in the file test.dat?

Any help would be appreciated.

  • 1
    $\begingroup$ See Put and PutAppend $\endgroup$ Nov 20, 2014 at 12:18
  • $\begingroup$ Related: "The best way to construct a function with memory." $\endgroup$ Nov 20, 2014 at 12:25
  • $\begingroup$ @MariusLadegårdMeyer The PutAppend also seems to overwrite the info, and doesn't solve the import data problem $\endgroup$ Nov 20, 2014 at 12:29
  • 1
    $\begingroup$ @Anoldmaninthesea, try this: (# >>> "test.dat") & /@ {1, 2, 3} followed by ReadList["test.dat"]. For me, this returns {1,2,3}. If you now want to add some new numbers, say (# >>> "test.dat") & /@ {4, 5, 6, 7, 8}, you will find that ReadList["test.dat"] gives you {1,2,3,4,5,6,7,8}. If you only need to save numbers/lists, and not expressions, then this is the simplest way I can think of. $\endgroup$ Nov 20, 2014 at 12:41
  • 1
    $\begingroup$ @Anoldmaninthesea, I can't reproduce that. Did you save it both times with >>>? Try starting from a new file name. $\endgroup$ Nov 20, 2014 at 13:25

2 Answers 2


For simply saving numbers or lists to file, I personally prefer to use Put/>> and PutAppend/>>> to store data, and ReadList to retrieve it. For instance:

(# >>> "file.dat") & /@ {1,2,3};
(* {1,2,3} *)
(# >>> "file.dat") & /@ {1,2,3,4,5};
(* {1,2,3,1,2,3,4,5} *)

You could obviously also store the lists each time and then Join them if you want:

{1, 2, 3} >>> "file.dat";
{4, 5, 6, 7} >>> "file.dat";
Join @@ ReadList["file.dat"]
(* {1, 2, 3, 4, 5, 6, 7} *)

Open the file with OpenAppend not OpenWrite. Any Writes will then append to the file not overwrite.

  • $\begingroup$ this is especially useful in combination with ExportString for text like formats where you can append line by line, e.g. CSV. $\endgroup$ Nov 21, 2014 at 8:21
  • $\begingroup$ Many thanks for your answer Ymareth $\endgroup$ Nov 21, 2014 at 15:52

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