My Java method to find mode fails when my integer array is multimodal and when there's no mode

Question

I am trying to write a java method to find the mode in an unsorted integer array. This works fine when there is only one mode within the array. But it gives out a random number when there's no mode in the array.

And also I need to make sure that my method addresses multimodal arrays as well. But I find difficulty in editing the code to return an array of modes.

public static int mode(int[] marksArray){

    int maxValue =0;int maxCount = 0;

    for (int i = 0; i < marksArray.length; ++i) {
        int count = 0;
        for (int j = 0; j < marksArray.length; ++j) {
            if (marksArray[j] == marksArray[i]) ++count;
        }
        if (count > maxCount) {
            maxCount = count;
            maxValue = marksArray[i];
        }
    }
    return maxValue;
}

RaffleBuffle · Accepted Answer · 2020-05-01 11:44:47Z

2

If there's more than one mode in the array then your code returns the first, which is not unreasonable.

To return all modes you need to maintain a list.

static List<Integer> mode(int[] array)
{
    List<Integer> mode = new ArrayList<>();

    int maxCount = 2;
    for(int i=0; i<array.length; i++)
    {               
        int count = 1;
        for(int j=i+1; j<array.length; j++)
            if(array[i] == array[j]) count++;

        if(count >= maxCount)
        {
            if(count > maxCount)
            {
                mode.clear();
                maxCount = count;
            }
            mode.add(array[i]);
        }
    }

    return mode;            
}

Note that there's no need to start the inner loop at 0. You can initialize count to 1 and start at i+1. Actually this is important as it means that the count for subsequent instances of array[i] will be less than the initial count, so they won't get added to the list as equal modes.

I've edited the code to detect the case where every element of the array is distinct, in which case there's no mode.

edited May 1, 2020 at 11:44

answered May 1, 2020 at 3:30

RaffleBuffle

5,4651 gold badge11 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

YUMI Over a year ago

In my method it always returned some number as the mode though it occurred onetime in the array.But now I get that the reason was that I returned an integer instead of an array or list. Thank you very much for your clear explanation . It was really helpful.

Andreas Over a year ago

"there has to be a mode unless the array is empty" Incorrect. If no number in a set of numbers occurs more than once, that set has no mode. E.g. 1, 2, 4, 7 has no mode. --- Easy enough to fix: Initialize maxCount to 2, not 0.

Andreas · Accepted Answer · 2020-05-01 03:14:48Z

In Java 8+, it can be done like this:

public static int[] modes(int[] marksArray) {
    Entry<Long, List<Integer>> max = Arrays.stream(marksArray).boxed()
            .collect(groupingBy(identity(), TreeMap::new, counting())).entrySet().stream()
            .filter(e -> e.getValue() > 1)
            .collect(groupingBy(Entry::getValue, TreeMap::new, mapping(Entry::getKey, toList())))
            .lastEntry();
    return (max == null ? new int[0] : max.getValue().stream().mapToInt(Integer::intValue).toArray());
}

Test

public static void main(String[] args) {
    // Samples from https://www.mathsisfun.com/mode.html
    //          and https://www.purplemath.com/modules/meanmode.htm
    test(); // []
    test(1, 2, 4, 7); // []
    test(6, 3, 9, 6, 6, 5, 9, 3); // [6]
    test(1, 3, 3, 3, 4, 4, 6, 6, 6, 9); // [3, 6]
    test(13, 18, 13, 14, 13, 16, 14, 21, 13); // [13]
    test(8, 9, 10, 10, 10, 11, 11, 11, 12, 13); // [10, 11]
}
public static void test(int... marksArray) {
    System.out.println(Arrays.toString(modes(marksArray)));
}

Output

[]
[]
[6]
[3, 6]
[13]
[10, 11]

Collectives™ on Stack Overflow

My Java method to find mode fails when my integer array is multimodal and when there's no mode

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related