Converting ArrayList<Double> to Double Array and vice-versa for JAVA/Android

Question

I've been trying to convert a Double ArrayList to a Double Array and vice-versa.

This is the code I have for converting Double ArrayList to Double Array, but the program keeps on crashing. Am I missing something? or is the logic wrong all together?

ArrayList<Double> numsy = new ArrayList<Double>();
ArrayList<Double> numsx = new ArrayList<Double>();  
for (int j = 0; j < 100; j++) {
    numsy.add((double)generator.nextFloat());
    numsx.add((double)j);
}
double[] arrayX = new double[numsx.size()];     
double[] arrayY = new double[numsy.size()];
//assigns array x and y the values in the list.
for (int k1 = 0; k1 < numsy.size(); i++) {
    double f = numsy.get(k1);
    arrayY[k1] = f;
    f = numsx.get(k1);
    arrayX[k1] = f;
}
FFT doFFT = new FFT(4);
doFFT.fft(arrayX, arrayY);
//adding the FFT numbers back to the Lists
for (int j = 0; j < 100; j++) {
    numsy.add(arrayY[j]);
    numsx.add(arrayX[j]);
}

Can you please share your generator?

Yogendra Singh
– Yogendra Singh

2012-10-19 03:11:15 +00:00
Commented Oct 19, 2012 at 3:11 — Yogendra Singh
– Yogendra Singh, Commented Oct 19, 2012 at 3:11
@Yogendra this is possibly a Random object.

Mordechai
– Mordechai

2012-10-19 03:13:40 +00:00
Commented Oct 19, 2012 at 3:13 — Mordechai
– Mordechai, Commented Oct 19, 2012 at 3:13
What type of exception or error?

Mordechai
– Mordechai

2012-10-19 03:15:27 +00:00
Commented Oct 19, 2012 at 3:15 — Mordechai
– Mordechai, Commented Oct 19, 2012 at 3:15
@YogendraSingh the FFT class? its taken from here

Jonathan
– Jonathan

2012-10-19 03:16:13 +00:00
Commented Oct 19, 2012 at 3:16 — Jonathan
– Jonathan, Commented Oct 19, 2012 at 3:16

Russell Gutierrez · Accepted Answer · 2012-10-19 03:18:21Z

4

You are instantiating your Arrays before adding values in your ArrayLists:

double[] arrayX = new double[numsx.size()];     
double[] arrayY = new double[numsy.size()];

This will create arrays with length: 0. Then you'll have a null pointer in your loop below.

UPDATE

Then you have to edit your loop:

for (int k1 = 0; k1 < numsy.size(); i++) {

to this

for (int k1 = 0; k1 < numsy.size(); k1++) {

edited Oct 19, 2012 at 3:18

answered Oct 19, 2012 at 2:57

Russell Gutierrez

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4 Comments

Russell Gutierrez Over a year ago

Updated my answer. Try again.

Jonathan Over a year ago

I am such a fool! Thank you so much. I have even added the remaining piece of code that adds the manipulated numbers back to the list. does that logic seem right? It works just fine, but is it there a way to make it even better?

Russell Gutierrez Over a year ago

I don't see any problems with it. :)

Jonathan Over a year ago

okay thanks :) I was just wondering if I could make it more dynamic and dont relay on that 100 value again and again.

Ram kiran Pachigolla · Accepted Answer · 2012-10-19 03:12:08Z

0

ArrayList<Double> numsy = new ArrayList<Double>();
ArrayList<Double> numsx = new ArrayList<Double>();

 for (int j = 0; j < 100; j++) {
     numsy.add((double)generator.nextFloat());
     numsx.add((double)j);
 }

//Declare arrayX and arrayY after adding values to numX and numY

 double[] arrayX = new double[numsx.size()];     
 double[] arrayY = new double[numsy.size()];     
    //assigns array x and y the values in the list.
 for (int k1 = 0; k1 < numsy.size(); k1++) {
   double f = numsy.get(k1);
   arrayY[k1] = f;
   f = numsx.get(k1);
   arrayX[k1] = f;
 }

edited Oct 19, 2012 at 3:12

answered Oct 19, 2012 at 3:00

Ram kiran Pachigolla

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1 Comment

Jonathan Over a year ago

Hi Ram, even then the program is not working, its still crashing the program. saying that the application is not responding.

Mordechai · Accepted Answer · 2012-10-19 03:18:57Z

-1

Why not try to use the built-in method:
double[] arrayX = numsx.toArray(new double[0]);
double[] arrayY = numsY.toArray(new double[0]);

answered Oct 19, 2012 at 3:18

Mordechai

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5 Comments

Jonathan Over a year ago

I did not know a method like this existed in java. I've worked with python for most of my life so far. so my experience with java is very little. let me read more about it

Jonathan Over a year ago

it gives me the following error, the method toArray(T[]) in the type ArrayList<Double> is not applicable for the arguments (double[])

Mordechai Over a year ago

Oh, my mistake, use toArray(new Double[0]);

Jonathan Over a year ago

it says cannot convert from Double[] to double[]

Mordechai Over a year ago

@Jonathan well, so you'll have to use an array of Doubles, but this won't make problems when retrieving the actual values. You can say: double value = aDoubleArray[2]; and as well, double d = 2.4; aDoubleArray[2] = d; the autoboxing and unboxing of java are really wonderful.

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