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javaarraylistdoubleaverage
JBoy
JBoy

Reputation: 5755

Find the average within variable number of doubles

I have an ArrayList of doubles, and i need to find the average between all numbers. The amount of Double instances in the arraylist is not constant, could be 2, could be 90 I have been trying for hours to get the algorithm myself but could not get it to work in anyway.

do you have any advice? or maybe you can link me to an existing library to get this average?

Thank you

Upvotes: 2

Views: 17973

Answers (7)

bvdb
bvdb

Reputation: 24800

If you like streams:

List<Number> list = Arrays.asList(number1, number2, number3, number4);

// calculate the average
double average = list.stream()
                     .mapToDouble(Number::doubleValue)
                     .average()
                     .getAsDouble();

Upvotes: 5

aioobe
aioobe

Reputation: 421350

Create a sum variable:

double sum = 0;

Go through the elements in the list using a for-loop:

for (double d : yourList)

in each iteration, add to sum the current value:

    sum += d;

after the loop, to find the average, divide sum with the number of elements in the list:

double avg = sum / yourList.size();

Here's for everyone that thinks this was too simple...

The above solution is actually not perfect. If the first couple of elements in the list are extremely large and the last elements are small, the sum += d may not make a difference towards the end due to the precision of doubles.

The solution is to sort the list before doing the average-computation:

Collections.sort(doublesList);

Now the small elements will be added first, which makes sure they are all counted as accurately as possible and contribute to the final sum :-)

Upvotes: 5

Thomas
Thomas

Reputation: 5094

If you're worried about overflow as you sum the whole list together, you can use a running average. Because it has more operations, it will be slower than adding everything together and dividing once.

for (int x = 0; x < list.size(); x++)
   average = (average / (x+1)) * (x) + list.get(x) / (x+1);

Upvotes: 0

javatutorial
javatutorial

Reputation: 1944

Maybe I haven't got the question... but it's not something like this?

double sum = 0.0;
for (double element : list) {
    sum += element;
}
double average = sum / list.size();

Upvotes: 0

npinti
npinti

Reputation: 52205

Have you tried:

List<Double> list = new ArrayList<Double>();
...    
double countTotal = 0;

for (Double number : list)
{
    countTotal += number;
}

double average = countTotal / list.size();

Upvotes: 0

assylias
assylias

Reputation: 328923

If by "average between all numbers" you mean the average of all the doubles in your list, you can simply add all the doubles in your list (with a loop) and divide that total by the number of doubles in your list.

Upvotes: 0

JB Nizet
JB Nizet

Reputation: 692281

The definition of the average is the sum of all values, divided by the number of values. Loop over the values, add them, and divide the sum by the size of the list.

Upvotes: 1

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