I want to count how often an element from ArrayList "list1" occurs in the other ArrayList "list2".
I want this output:
A 2
B 0
C 1
D 2
I get this output:
A 0
B 0
C 0
D 69
Coud you please help me to do this? Thank you!
enter code here
HashMap<Character, Integer> map = new HashMap<Character, Integer>();
ArrayList<Character> list1 = new ArrayList<Character>();
ArrayList<Character> list2 = new ArrayList<Character>();
Collections.addAll(list1, 'A', 'B', 'C', 'D');
Collections.addAll(list2, 'D', 'A', 'C', 'A', 'D');
for (int i = 0; i < list1.size(); i++) {
for (int j = 0; j < list2.size(); j++) {
if (list1.get(i) == list2.get(j)) {
map.put(list1.get(i), 1);
}
if (list1.get(i) == list2.get(j) && (map.containsKey(list1.get(i)))) {
map.replace(list1.get(i), list1.get(i) + 1);
}
if (list1.get(i) != list2.get(j)) {
map.put(list1.get(i), 0);
}
}
}
System.out.println("Map: ");
for (Map.Entry<Character, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + " " + entry.getValue());
}
Use the method frequency, from Collection, namely:
for (Character c : list1)
map.put(c, Collections.frequency(list2, c));
System.out.println("Map: ");
for (Map.Entry<Character, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + " " + entry.getValue());
}
If you are a fan of one liners :
list1.forEach(c -> map.put(c, Collections.frequency(list2, c)));
Including the printing of the elements:
list1.forEach(c -> System.out.printf("%s %d%n", c, Collections.frequency(list2, c)));
By the way, your original answer was almost correct, you just had to rethink a bite the conditionals, and their order:
for (Character c1 : list1) {
for (Character c2 : list2) {
if(map.containsKey(c1) && c1 == c2 ){
map.put(c1, map.get(c1) + 1);
}
else if (!map.containsKey(c1) && c1 != c2 ) {
map.put(c1, 0);
}
else if (!map.containsKey(c1) && c1 == c2) {
map.put(c1, 1);
}
}
}
Another advice is, if you do not explicitly need the loop index, it is better to use the idiom for (Character c1 : list1) rather than for(int i = 0; i < list1.size(); i++). The first is cleaner, and less error-prone than the second version. Moreover, you can use the variable c1 instead of having to do list1.get(i) all the time.
You can use Stream API introduced in Java 8 to accomplish this task. Here is an example code which resolves the problem:
final List<Character> list1 = new ArrayList<Character>();
final List<Character> list2 = new ArrayList<Character>();
Collections.addAll(list1, 'A', 'B', 'C', 'D');
Collections.addAll(list2, 'D', 'A', 'C', 'A', 'D');
final Map<Character, Integer> map = new HashMap<>();
for (final Character c : list1) {
final int occurrences = (int) list2.stream().filter(c::equals).count();
map.put(c, occurrences);
}
System.out.println("Map: ");
for (Map.Entry<Character, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + " " + entry.getValue());
}
Note: For this group of problems I recommend you to start the implementation with writing some unit tests.
A cleaner Java 8 solution without extra loops would be:
List<Character> list1 = Arrays.asList('A', 'B', 'C', 'D');
List<Character> list2 = Arrays.asList('D', 'A', 'C', 'A', 'D');
Map<Character, Integer> map = list1.stream()
.collect(Collectors.toMap(
c -> c,
c -> Collections.frequency(list2, c)
));
// print entire map
System.out.println(map);
// print formatted map
map.entrySet().forEach(e -> System.out.printf("%s %d%n", e.getKey(), e.getValue()));
If the specific task is to print the frequencies, the intermediate map can be skipped altogether :)
list1.stream()
.map(c -> new StringBuilder()
.append(c).append(' ')
.append(Collections.frequency(list2, c))
)
.forEach(System.out::println);