Question

Write a method counts that accepts a List of integers and a Set of integers as parameters, and returns a map from each value in the set to the number of occurrences of that value in the list.

My solution #1:

public static Map<Integer, Integer> counts(List<Integer> list, Set<Integer> set) {
    Map<Integer, Integer> map = new TreeMap<Integer, Integer>();
    Iterator<Integer> i = list.iterator();

    for(Integer element : set) {
        int count = 0;
        for(Integer sub : list) {
            if(sub == element) {
                count++;
            }
        }
        map.put(element, count);
    }

    return map;
}

My solution #2:

public static Map<Integer, Integer> counts(List<Integer> list, Set<Integer> set) {
    Map<Integer, Integer> map = new TreeMap<Integer, Integer>();
    Iterator<Integer> i = list.iterator();

    for(Integer element : set) {
        int count = 0;
        while(i.hasNext()) {
            if(i.next() == element) {
                count++;
            }
        }
        map.put(element, count);
    }

    return map;
}

Input:

list: [4, -2, 3, 9, 4, 17, 5, 29, 14, 87, 4, -2, 100]
set: [-2, 4, 29]

Expected output:

{-2=2, 4=3, 29=1}

Output for #1:

{-2=2, 4=3, 29=1}

Output for #2:

{-2=0, 4=3, 29=0}

The first one works, but the second one does not. Why? They are essentially the same thing, or am I missing anything? Also, would using a for loop work to traverse through the list? If not, why?

Answer 1

You forget to reset the iterator after each loop, try the following:

for(Integer element : set) {
    int count = 0;
    i = list.iterator(); // resets the iterator to the first element. 
    while(i.hasNext()) {
        if(i.next() == element) {
            count++;
        }
    } // by the end of this loop the iterator has no next elements, need to reset.
    map.put(element, count);
}