如何计算数组中偶数的百分比？

Question

i am beginner and here is the method I am struggling with.

Write a method called percentEven that accepts an array of integers as a parameter and returns the percentage of even numbers in the array as a real number. For example, if the array stores the elements {6, 2, 9, 11, 3}, then your method should return 40.0. If the array contains no even elements or no elements at all, return 0.0.

here is what I have so far...

public static double percentEven(int[]a){
    int count = 0;
    double percent = 0.0;
    if (a.length > 0){
        for ( int i = 0; i < a.length; i++){
            if ( a[i] % 2 == 0){
                count++;
            }
        }
            percent  =(count/a.length)*100.0;
    }
            return percent;
}

i keep returning 0.0 when array contains a mix of even and odd elements but works fine for all even element array or all odd array? i can't see where the problem is? thanks in advance.

Answer 1

count/a.length returns 0 since you are dividing two ints, and the second operand is larger than the first. Change it to (double)count/a.length in order to perform floating point division.

Alternately, change the order of operations to :

percent = 100.0*count/a.length;

Answer 2

@Bathsheba : Well said, thanks for the suggestion.

Here is sample code :

public class PercentEven {

    public static void main(String args[]){
        int count = 0;
        int[] a={2, 5, 9, 11, 0}; // this can be dynamic.I tried diff values 
        double percent = 0.0;
        if (a.length > 0){
            for ( int i = 0; i < a.length; i++){
                if ( a[i] % 2 == 0){
                    count++;
                }
            }
                percent  = (100*count/a.length);
        }
        System.out.println(percent);
    }
}

Answer 3

For a simple division like 2*100.0/5 = 40.0, the above logic would work fine but think about the situation where we have 51*100.0/83 the output would be less readable and its always advisable to truncate the percentage to a limited decimal digits.

An example:

int count = 51;
Double percent = 0.0;
int length = 83;
percent = count*100.0/length;

System.out.println(percent);

output: 61.44578313253012

When you truncate it:

Double truncatedDouble = new BigDecimal(percent ).setScale(3, BigDecimal.ROUND_HALF_UP).doubleValue();
        System.out.println(truncatedDouble);

output: 61.446

Answer 4

List<Integer> numbers = Arrays.asList(a);
int number = numbers.stream().filter(n->n%2==0).count();
int percent = number*100.0/numbers.size();

I have done this in java 8