Calculating average of an array list?

Question

I\'m trying to use the below code to calculate the average of a set of values that a user enters and display it in a jTextArea but it does not work properly. Sa

Answer 1

Why use a clumsy for loop with an index when you have the enhanced for loop?

private double calculateAverage(List <Integer> marks) {
  Integer sum = 0;
  if(!marks.isEmpty()) {
    for (Integer mark : marks) {
        sum += mark;
    }
    return sum.doubleValue() / marks.size();
  }
  return sum;
}

Answer 2

Use a double for the sum, otherwise you are doing an integer division and you won't get any decimals:

private double calculateAverage(List <Integer> marks) {
    if (marks == null || marks.isEmpty()) {
        return 0;
    }

    double sum = 0;
    for (Integer mark : marks) {
        sum += mark;
    }

    return sum / marks.size();
}

or using the Java 8 stream API:

    return marks.stream().mapToInt(i -> i).average().orElse(0);

Answer 3

Using Guava, it gets syntactically simplified:

Stats.meanOf(numericList);

Answer 4

Correct and fast way compute average for List<Integer>:

private double calculateAverage(List<Integer> marks) {
    long sum = 0;
    for (Integer mark : marks) {
        sum += mark;
    }
    return marks.isEmpty()? 0: 1.0*sum/marks.size();
}

This solution take into account:

• Handle overflow
• Do not allocate memory like Java8 stream
• Do not use slow BigDecimal

It works coorectly for List, because any list contains less that 2^31 int, and it is possible to use long as accumulator.

PS

Actually foreach allocate memory - you should use old style for() cycle in mission critical parts

Answer 5

Here a version which uses BigDecimal instead of double:

public static BigDecimal calculateAverage(final List<Integer> values) {
    int sum = 0;
    if (!values.isEmpty()) {
        for (final Integer v : values) {
            sum += v;
        }
        return new BigDecimal(sum).divide(new BigDecimal(values.size()), 2, RoundingMode.HALF_UP);
    }
    return BigDecimal.ZERO;
}

Answer 6

With Java 8 it is a bit easier:

OptionalDouble average = marks
            .stream()
            .mapToDouble(a -> a)
            .average();

Thus your average value is average.getAsDouble()

return average.isPresent() ? average.getAsDouble() : 0;