How do I convert an ArrayList of integers to a double[]?

Question

I have set up the following ArrayList:

ArrayList<Integer> myIntegerValues = new ArrayList<Integer>();
myIntegerValues.add(0);
myIntegerValues.add(1);
myIntegerValues.add(0);
myIntegerValues.add(1);

I want to convert this to a double[] and run a simple piece of code like this

double[] myList = {1.9, 2.9, 3.4, 3.5, 2.9, 3.8, 10.2};        
for (int i = 0; i < myList.length; i++) {
    System.out.println(myList[i] + " ");
}

How do I convert an ArrayList to a double[]?

I found this link How to cast from List<Double> to double[] in Java? but I'm not sure if this answers my question or not.

Do you want 1 to be converted as 1.9 or 1.3 or something like that? How does anyone know what 1 in real value is unless you feed it out? You can though convert it from 1 to 1.0 — SSC, Oct 06 '16 at 21:32
Did you have a look to this: http://stackoverflow.com/questions/718554/how-to-convert-an-arraylist-containing-integers-to-primitive-int-array ? Try to add a cast — acornagl, Oct 06 '16 at 21:34
The java statistical package I am using requires `double[]` as an input. The ArrayList doesn't work as an input. So I am trying to convert the numbers in my ArrayList so that I can use them in the statistical package datastructures. — Mr.NoName, Oct 06 '16 at 21:34
Best thing to do is to create an array of matching size and then iterate over the entries, casting as you go. — Andre M, Oct 06 '16 at 21:35

Andrew Tobilko · Answer 1 · 2016-10-06T22:26:58.260

5

double[] array = myIntegerValues.stream().mapToDouble(i -> (double) i).toArray();

edited Oct 06 '16 at 22:26

answered Oct 06 '16 at 21:40

Andrew Tobilko

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1

Could also use `Integer::doubleValue` instead of the lambda. – Jorn Vernee Oct 06 '16 at 21:48
@JornVernee, yeap, but for OP it can be slightly unobvious – Andrew Tobilko Oct 06 '16 at 21:53
1

aw no fair, I like being the one to throw up the Java 8 answers. – Rogue Oct 06 '16 at 21:53
I tried this solution and couldn't get it to work. It looks more elegant than the one I accepted and I would prefer to accept yours if it is more efficient. But I don't see the ArrayList anywhere in your line of code. I'm not sure how it is therefore supposed to work. – Mr.NoName Oct 06 '16 at 22:25
@Mr.NoName, the `list` variable was your `ArrayList`, I renamed it to `myIntegerValues` – Andrew Tobilko Oct 06 '16 at 22:26

score 0 · Answer 2 · answered Oct 06 '16 at 21:33

Declare a double arry of the size of the arraylist first, since arrays aren't dynamic.

double[] doublesList = new double[myIntegerValues.size()];

then loop through and convert each one

int x = 0;
for(Integer i : myIntegerValues){
    doublesList[x] = (double)i;
    x++;
}

score 0 · Accepted Answer · answered Oct 06 '16 at 21:37

Best thing to do is to create an array of matching size and then iterate over the entries, casting as you go.

double[] myList = new double[myIntegerValues.size()];
for (int i=0; i<myIntegerValues.size(); i++) {
   myList[i] = (double) myIntegerValues.get(i);
}

Note, while I could use an iterator, using indexes makes the one to one relationship clear.

How do I convert an ArrayList of integers to a double[]?

3 Answers3