Calculate average or take in an ArrayList as a parameter to a function

问题

Is there a built-in method to calculate the average of an integer ArrayList?

If not, can I make a function that will do that by taking in the name of the ArrayList and returning its average value?

回答1:

It's really simple:

// Better use a `List`. It is more generic and it also receives an `ArrayList`.
public static double average(List<Integer> list) {
    // 'average' is undefined if there are no elements in the list.
    if (list == null || list.isEmpty())
        return 0.0;
    // Calculate the summation of the elements in the list
    long sum = 0;
    int n = list.size();
    // Iterating manually is faster than using an enhanced for loop.
    for (int i = 0; i < n; i++)
        sum += list.get(i);
    // We don't want to perform an integer division, so the cast is mandatory.
    return ((double) sum) / n;
}

For even better performance, use int[] instead of ArrayList<Integer>.

回答2:

If you want to computer later one more than the average I propose Colt library developed at CERN which supports many statistic functions. See BinFunctions1D and DoubleMatrix1D. An alternative (with a recent code basis) may be commons-math:

DescriptiveStatistics stats = new DescriptiveStatistics();
for( int i = 0; i < inputArray.length; i++)
{
    stats.addValue(inputArray[i]);
}
double mean = stats.getMean();

回答3:

Comming soon, using lambda expressions and method references in JDK 8:

DoubleOperator summation = (a, b) -> a + b;
double average = data.mapReduce(Double::valueOf, 0.0,  summation) / data.size();
System.out.println("Avergage : " + average);