Question

I’m quite new at android and SQL. I’m making an app, which can get data form the accelerometer, and the I’m storing them in a SQLLite database. Afterwards I intend to get the database out so I can plot the data. I have two questions:

1. How can I save the data as a usable file on the SD-card? I’ve seen some topics, but I couldn’t get any off it to work. I think I need some examples/tutorials.

2. Secondly, after some collection, the app starts to lag. I guess it is the storing method, which is like this in the DB-class:

   public long createEntry(float x, float y, float z, float t) {
   ContentValues cv = new ContentValues();
   cv.put(X_DATA,x);
   cv.put(Y_DATA,y);
   cv.put(Z_DATA,z);
   cv.put(TIME_DATA,t);
   return ourDatabase.insert(DATABASE_TABLE, null, cv);
   }
   

I hope you’ll help me.

Answer 1

For the first part of your question, performing all the inserts in a transaction should make it a lot faster. To do this, before you start inserting, call the following code:

ourDatabase.beginTransaction()

Then once you’ve finished inserting data, call:

ourDatabase.setTransactionSuccessful();
ourDatabase.getDb().endTransaction();

To answer the second part of question, you need to retrieve the data from the database and create a CSV file from it:

        StringBuilder csv = new StringBuilder();

        Cursor cursor = this.db.query("Data", new String[] { "x","y", "z", "t" }, null, null, null, null, null);

        while (cursor.moveToNext()) {
            csv.append(cursor.getFloat(0))
            .append(",")
            .append(cursor.getFloat(1))
            .append(",")
            .append(cursor.getFloat(2))
            .append(",")
            .append(cursor.getFloat(3))
            .append("\n");
        }

cursor.close();

        File outputFile = new File("/sdcard/mycsv.csv");

        FileWriter writer;
        try {
            writer = new FileWriter(outputFile);

            writer.write(csv.toString());

            writer.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

If you’re collecting a lot of rows (tens of thousands perhaps), you might need to stream this CSV out to the disk at the same time as writing it.