如何在 foreach java 8 中执行算术运算

Question

I am new to java 8 and I tried to write the below code snippet in java 8 but did work.

static int getMissingNo(int a[], int n) { 
    int i, total; 
    total = (n + 1) * (n + 2) / 2; 
    for (i = 0; i < n; i++) 
        total -= a[i]; 
    return total;
} 

// this is waht i tried in java 8
int  total = (n+1) * (n+2) /2;
unicArray.stream().forEach(e->{
    total  = total - e;
});

The code snippet gives me a compilation error saying local variable should be final but if it is final how can I return that value back

Answer 1

You can simply return the difference between the total and the sum of array as:

int expectedSum = (n + 1) * (n + 2) / 2;
int actualSum = Arrays.stream(a).sum();  // .distinct().sum() for sum of unique numbers
return expectedSum - actualSum;

Answer 2

Please find the code below:

static List<Integer> getMissingNo(int[] a, int n) {

        List<Integer> listOfDistinctElements = Arrays.stream(a)
                .boxed()
                .distinct().collect(Collectors.toList());

        List<Integer> elemntsLeftOut = new ArrayList<>();

        IntStream.range(1, listOfDistinctElements.get(listOfDistinctElements.size()-1)).forEach(index ->
                        {
                            if(!listOfDistinctElements.contains(index)){
                                 elemntsLeftOut.add(index);
                        }
                    });

        return elemntsLeftOut;
        }