Question

I am working with achartengine in my app. They have added a feature to handle null values but in order to handle them properly you need MathHelper.NULL_VALUE to be in the place of a null value. The graph needs a List Double[] (list of double arrays) in order to properly display the data. My code loops through a cursor, populates a list of doubles, converts the list of doubles into a Double[] then finally adds the Double[] to my List Double[].

I tried putting an if/else statement in the loop saying if column_TEMP is null then cvalue=MathHelper.NULL_VALUE but it does the calculations before it is needed and the results were quite catastrophic (it failed). The graph did not populate properly having a single small line on the very bottom of the screen and only showed 2 points instead of 10. It cannot do the calculations for MathHelper.NULL_VALUE until after it is in the List Double[]. I am trying to figure out a way to change null values when it goes from Double[] to List Double[]. Here is the code I am currently using. As-is it makes null values show as 0 on the graph. Graphs using larger numbers can be quite messed up if one null value is there. You could go from several hundred all the way across to 0 out of nowhere then back to several hundred again. Any suggestions are very appreciated.

private List<Double[]> values;
private List<Double> cValue;
private Double[] Dvalue;

        while (cursor.moveToNext()) {
            try {
                String cdate=null;
                Double cvalue=null;
                column_date = cursor.getColumnIndex(Provider.DATE);
                column_value = cursor.getColumnIndex(Provider.TEMP);
                cdate = cursor.getString(column_date);
                cvalue = cursor.getDouble(column_value);
                };
                SimpleDateFormat curFormater = new SimpleDateFormat("yyyy/MM/dd");
                Date dateObj = curFormater.parse(cdate);
                if (dateObj.getTime() >= startDate.getTime() && 
                        dateObj.getTime() <= endDate.getTime())  {
                    SimpleDateFormat postFormater = new SimpleDateFormat("yyyy,MM,dd");
                    String dateObj1 = postFormater.format(dateObj);
                    SimpleDateFormat finalFormater = new SimpleDateFormat("yyyy,MM,dd");
                    Date dateObj2 = finalFormater.parse(dateObj1);
                    cDates.add(dateObj2);
                    cValue.add(cvalue);
                }
            } catch (ParseException e) {
                e.printStackTrace();
            }
        }

    dateValues = cDates.toArray(new Date[0]);
    dates.add(dateValues);

    Dvalue = (Double[]) cValue.toArray(new Double[0]);
    values.add(Dvalue);

In theory this is what the array should look like but I have no clue how to input the MathHelper.NULL_VALUE into the existing array.

values.add(new double[] { 21.2, 21.5, 21.7, 21.5, 21.4, 21.4, 21.3, 21.1, 20.6, 20.3, 20.2,
    19.9, 19.7, 19.6, 19.9, 20.3, 20.6, 20.9, 21.2, 21.6, 21.9, 22.1, 21.7, 21.5 });
values.add(new double[] { 1.9, 1.2, 0.9, 0.5, 0.1, -0.5, -0.6, MathHelper.NULL_VALUE,
    MathHelper.NULL_VALUE, -1.8, -0.3, 1.4, 3.4, 4.9, 7.0, 6.4, 3.4, 2.0, 1.5, 0.9, -0.5,
    MathHelper.NULL_VALUE, -1.9, -2.5, -4.3 });

Edit:

I figured out one issue. In my while loop I put this code

            Double d=MathHelper.NULL_VALUE; 
            String s=Double.toString(d); 
            Log.i(getClass().getSimpleName(), s);

and it logs 0.0 but lower down by my chartfactory settings (outside the loop) I put the
same code and it returned 1.7976931348623157E308 as expected. Why
would the while loop cause it to return 0.0?

Answer 1

Ok the value showing as 0.0 was an ID10T error. stupid me had string = instead of string.equals. The main problem though was it didn’t like passing the MathHelper.NULL_VALUE in the original list as a Double. What I had to do was pass everything as a String to the list of strings then add that to the string array and finally I passed that to the list of string arrays. Once I had everything in the list of string arrays I had to come up with new code to convert the list of string arrays into a list of Double arrays and it worked perfect.

Thanks for the help Laurence 🙂