How do I append text to a csv/txt file in Processing?

Question

I use this simple code to write a few strings to the file called "example.csv", but each time I run the program, it overwrites the existing data in the file. Is there any way to append the text to it?

void setup(){
  PrintWriter output = createWriter ("example.csv");
  output.println("a;b;c;this;that ");
  output.flush();
  output.close();

}

Answer 1

import java.io.BufferedWriter;
import java.io.FileWriter;

String outFilename = "out.txt";

void setup(){
  // Write some text to the file
  for(int i=0; i<10; i++){
    appendTextToFile(outFilename, "Text " + i);
  } 
}

/**
 * Appends text to the end of a text file located in the data directory, 
 * creates the file if it does not exist.
 * Can be used for big files with lots of rows, 
 * existing lines will not be rewritten
 */
void appendTextToFile(String filename, String text){
  File f = new File(dataPath(filename));
  if(!f.exists()){
    createFile(f);
  }
  try {
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(f, true)));
    out.println(text);
    out.close();
  }catch (IOException e){
      e.printStackTrace();
  }
}

/**
 * Creates a new file including all subfolders
 */
void createFile(File f){
  File parentDir = f.getParentFile();
  try{
    parentDir.mkdirs(); 
    f.createNewFile();
  }catch(Exception e){
    e.printStackTrace();
  }
}    

Answer 2

You have to use a FileWriter (pure Java (6 or 7)) rather than PrintWriter from the Processing API. FileWriter has a second argument in it's constructor that allows you to set a Boolean to decide whether you will append the output or overwrite it (true is to append, false is to overwrite).

The documentation is here: http://docs.oracle.com/javase/7/docs/api/java/io/FileWriter.html Note you can also use a BufferedWriter, and pass it a FileWriter in the constructor if that helps at all (but I dont think it's necessary in your case).

Example:

try {
  FileWriter output = new FileWriter("example.csv",true); //the true will append the new data
  output.println("a;b;c;this;that ");
  output.flush();
  output.close();
}
catch(IOException e) {
  println("It Broke :/");
  e.printStackTrace();
}

As above, this will work in the PDE - and in Android - but if you need to use it in PJS, PyProcessing, etc, then you will have to hack it

• dynamically read the length of the existing file and store it in an ArrayList
• add a new line to the ArrayList
• use the ArrayList index to control where in the file you are currently writing

If you want to suggest an enhancement to the PrintWriter API (which is probably based off of FileWriter), you can do so at Processing's Issue page on GitHub:

https://github.com/processing/processing/issues?state=open

Answer 3

Read in the file's data, append your new data to that, and write the appended data back to the file. Sadly, Processing has no true "append" mode for file writing.