Java: calculating the amount of pairs in array-CodePudding

I have a program ( it is a bonus game for lottery ) that calculates the amount of pairs among generated numbers, it works good, however, I have a problem that - if there is 3 exact numbers it will count that there is already 2 pairs of number, but I need it to only count pairs, not calculate all possible probability to make pairs between numbers:

int randomChance = r.nextInt(100);
for (int newNum = 0; newNum<5 ;newNum  ) {
    int chanceGen = r.nextInt(100);
    if (chanceGen <= 50) {
        bonusGame[newNum] = 10;
    }
    else if (chanceGen <= 77) {
        bonusGame[newNum] = 20;
    }
    else if (chanceGen <= 92) {
        bonusGame[newNum] = 50;
    }
    else if (chanceGen <= 98) {
        bonusGame[newNum] = 200;
    }
    else {
        bonusGame[newNum] = 1000;
    }
    
    }
for (int z: bonusGame) {
    System.out.println(z);
}
for (int f = 0; f < bonusGame.length - 1;   f) {
    for (int j = f   1; j < bonusGame.length;   j) {
        if (bonusGame[j] == bonusGame[f] & bonusGame[j] == 10 ) {
            System.out.println(" You won 10 euro ");
            bonusGameSum  =10;
        }
        else if (bonusGame[j] == bonusGame[f] & bonusGame[j] == 20 ) {
            System.out.println(" You won 20 euro ");
            bonusGameSum  =20;
        }
        else if (bonusGame[j] == bonusGame[f] & bonusGame[j] == 50 ) {
            System.out.println(" You won 50 euro ");
            bonusGameSum  =50;
        }
        else if (bonusGame[j] == bonusGame[f] & bonusGame[j] == 200 ) {
            System.out.println(" You won 200 euro ");
            bonusGameSum  =200;
        }
        else if (bonusGame[j] == bonusGame[f] & bonusGame[j] == 1000 ) {
            System.out.println(" You won 1000 euro ");
            bonusGameSum  =1000;
        }
    }

What should i fix in order for program to calculate right amount of exact pairs?

CodePudding user response：

The following should do it and is even simpler that your nested for solution. You just need to use some Java features (Collections.frequency() for example):

int[] bonusGame = {10, 20, 20, 30, 30, 30, 30, 50, 50, 50, 200};
for (int z: bonusGame) {
    System.out.println(z);
}

List<Integer> bonusGameList = Arrays.stream(bonusGame).boxed().collect(Collectors.toList());
Set<Integer> bonusGameUnique = new HashSet<>(bonusGameList);
Map<Integer, Integer> numbersCount = new HashMap<>();

for (Integer number : bonusGameUnique) {
    int count = Collections.frequency(bonusGameList, number);
    numbersCount.put(number, count);
}

Map<Integer, Integer> numbersPairsCount = numbersCount.entrySet().stream()
        .collect(Collectors.toMap(Entry::getKey, entry -> entry.getValue()/2));

System.out.println(numbersPairsCount);

CodePudding user response：

Here's an efficient one-liner:

long numPairs = Arrays.stream(bonusGame).boxed() // stream Integers
  .collect(groupingBy(identity(), counting()))   // collect to map of freqs
  .values().stream()                             // stream frequency counts
  .mapToLong(n -> n / 2)                         // integer halve
  .sum();                                        // sum

It works by first getting the frequencies of all numbers, then sums all frequencies after halving then using integer arithmetic, which discards fractions - ie 5 / 2 results in 2.

As an example, given 10, 20, 10, 30, 10, 50, 10, 50, 30, 30, the result is 4, comprising 2 from 10,10,10,10, 1 from 30,30,30 and 1 from 50,50.