I have a large data set that gets exported to a .csv file. Export[data,filename,"csv"] is extremely fast and formats the data in a nice JMP-friendly manner. However, now I want to append to that file. OpenAppend[filename] works, but I can't get the formatting to match what Export[data,filename,"csv"] would output.

Specifically, I'm looking for E-notation on items that require it, each line is a row in data, comma-delimited, and no quotations around strings.

Example of what I want, and what Export[] returns:


So far, I've come relatively close with some very convoluted, annoying workaround:

StringTrim[ StringReplace[ StringTrim[ StringReplace[ ToString[ NumberForm[exportdata, NumberFormat -> (SequenceForm[#1, If[#3 == "", "", "e" <> #3]] &)]], ", " -> ","], ("{" | "}")], "},{" -> "\r"], ("{" | "}")]

This returns an output in Mathematica, that looks almost correct (missing some digits, but those aren't significant anyway):


as the output, but OpenAppend[], Write[] still:

  1. doesn't add an EOL tag (LF or CR) after each row in data
  2. adds quotes around the entire line (I'm assuming because it's technically a string)

To fix #1, I've put the Write[] inside a Do[] loop. Now the only issue is the quotation marks. How can I get rid of these?

Here's the code:

str = OpenAppend["Z:\\temp.csv", PageWidth -> [Infinity], NumberMarks -> False]
Do[Write[str, StringTrim[ StringReplace[ ToString[ NumberForm[exportdata[[i]],NumberFormat -> (SequenceForm[#1, If[#3 == "", "", "e" <> #3]] &)]], ", " -> ","], ("{" | "}")]], {i, Length[exportdata]}];

Or, am I doing everything in a completely ass-backwards way, and there's a super simple, easy, pretty way to append to an exported .csv file and I just haven't been able to find the way with all my searching?


1 Answer 1


As @WReach has noted, Export will accept a stream as its first argument so

file = OpenAppend["out.txt"]

Export[file, data, "CSV"];
WriteString[file, "\n"];


Alternatively you could first use ExportString to write the data into a string in the desired format, then send the string to the file to be appended to using WriteString.


data = RandomInteger[100, {100, 3}]

file = OpenAppend["out.txt"]

string = ExportString[data, "CSV"];

WriteString[file, string]
WriteString[file, "\n"];

  • 3
    $\begingroup$ Awesome, that's working wonderfully. I just had to add WriteString[str, "\n"] right before closing the file, so that the next append gets placed on the correct line. $\endgroup$
    – dthor
    Feb 23, 2012 at 22:14
  • 4
    $\begingroup$ +1 Note that Export will accept a stream as its first argument, so one could write Export[stream, data, "CSV"] and avoid the temporary in-memory copy of the CSV string. $\endgroup$
    – WReach
    Feb 24, 2012 at 18:42
  • 2
    $\begingroup$ @WReach I thought I had tried this in the past and didn't work. I was wrong. I updated the answer now. Thank you! $\endgroup$
    – Szabolcs
    Feb 24, 2012 at 19:26

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