As mentioned, I wish to read the first line of a file, and if needed, overwrite it with a new string. The aim is to have a CSV with a list of possible elements. Example:

Adding the elements: 5 A, 6 B, and 7 C to a blank CSV:
5 6 7
Adding 4 A, 9 D:
5 6 7
4 0 0 9
Adding 2 B, 7 E
5 6 7
4 0 0 9
0 2 0 0 7

Rewriting the file is okay for small files, but the files are expected to grow to sizes of hundreds of MB, so repeatedly overwriting the entire file is not practiceable, since the entries are preferably done quickly.

How would one go about overwriting only the first line of the file? Reading the first line is possible from https://mathematica.stackexchange.com/questions/5179/how-to-read-data-file-quickly#=, but what about writing?

  • $\begingroup$ there is no way around rewriting the file. this is not a mathematia isue but a result of the way the os stores the file. id imagine some shell tools will do this faster than mathematica though $\endgroup$
    – george2079
    Jul 29, 2014 at 11:11
  • 1
    $\begingroup$ ... you might start by loading up the first line with blanks, then when you want to add , rewrite the line keping the length fixed. off hand i'm not certain mma can do that but its worth a try. $\endgroup$
    – george2079
    Jul 29, 2014 at 11:28
  • $\begingroup$ maybe functions 'OpenAppend', 'Write', 'SetStreamPosition' and 'StreamPosition' help you $\endgroup$ Jul 29, 2014 at 14:25

2 Answers 2


Low-level file operators like Write won't work here because OutputStreams (such as you get with OpenWrite and OpenAppend) can't have their StreamPosition set before the end of the file. In general overwriting characters in an existing file isn't terribly trivial; you can use c-functions likefile_ptr = fopen(file, "rb+") to do this, but you're overwriting character-by-character, so if you remove a character or add one it can throw things off.

Generally you're better off creating a copy of the data, along the lines of:

 replaceNthRecord[file_String, n_Integer, replaceWith_] := Module[
  {readStream = OpenRead[file], writeStream = OpenWrite[file <> "temp"], temp},
  Do[Write[writeStream, Read[readStream]], {n - 1}];
  Write[writeStream, replaceWith]; Read[readStream];
  While[temp =!= EndOfFile,
   temp = Read[readStream];
   If[UnsameQ[temp, EndOfFile], Write[writeStream, temp]];
   ]; Close /@ {readStream, writeStream}



replaceNthRecord["file.dat", 3, 99]

{"file.dat", "file.dattemp"}

Make sure that "file.dattemp" is showing what you want, and then delete the original file and replace it with the temp one:


{1, 2, 99, 4, 5, 6, 7, 8, 9, 10}

(then just add this to the end of the function:

CopyFile[file <> "temp", file, OverwriteTarget -> True];
DeleteFile[file <> "temp"]

Note that using ReadList appropriately here will be more efficient than Do[Write[_,Read[_]]] but you'll need to do a bit more work to figure out what an appropriate batch size is to do this in and make sure you stop in the right place.


Being fairly annoyed that mathematica can't do this straightforward thing..here is a solution using an external python script:

      Join[{StringJoin[Join[{"*"}, ConstantArray[" ", {80}], {"*"}]]},
           RandomInteger[100, {3, 20}], {"end of file\n"}], "Table"]
 overwrite[file_, off_, string_] := 
       Run[ "python", "replacebytes.py", file, off, "'" <> string <> "'"];
 firstline = "some header 1 2";
 overwrite["test.txt", 2, firstline];
 firstline = firstline <> " 3";
 overwrite["test.txt", 2, firstline];

Where the python script "replacebytes.py" is just this:

 # usage python replacebytes.py file offset string
 # ovewrites file with string beginning at offset
 import sys
 if len(sys.argv) != 4 : sys.exit()

enter image description here

Note this will happily overwrite your data if firstline exceeds the length of the blank padded first line. (note also the asterisks are in there just so we can see whats going on, drop those in practice )


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