在数组中查找周围的数字（java）

Question

My question is related to the fibonacci sequence of numbers (but for simplicity, you could apply it to things like square/prime numbers etc)

long f1 = 0;
long f2 = 1;
long fibonacci = 0;
long[] fibonaccinumbers = new long[52];
fibonaccinumbers[0]=0;
fibonaccinumbers[1]=1;
for(int count = 2; count<=51; count++)
{
    fibonacci = f2+f1;
    fibonaccinumbers[count] = fibonacci;
    f1 = f2;
    f2 = fibonacci;        
}

The above code generates the array for fibonacci numbers 0-51.

Now what I'm looking to do is enter a number, we'll use 30 as an example. and then find and display the numbers before and after it in the sequence, which would be 21 and 34.

What is tripping me up is getting into the array and searching above and below my given number to find a match. How could I do this?

Answer 1

Since the Fibonaccy series is a sorted array (having ascending order), you can use int index = Arrays.binarySearch(fibonaccinumbers,30); to get the index such that fibonaccinumbers[index-1] < 30 < fibonaccinumbers[index] .

Therefore fibonaccinumbers[index-1] will contain 21 and fibonaccinumbers[index] will contain 34.

Note that Arrays.binarySearch will return fibonaccinumbers.length if all the numbers in your array are smaller than the number you are searching for.

Answer 2

This should be very simple, you just need to access the array at +1 and -1 of the index you have entered, for example, if the user enters 30...

System.out.println("BEFORE: " + fibonaccinumbers[input-1]);
System.out.println("AFTER: " + fibonaccinumbers[input+1]);

Where input is what the user enters, so in the case, input-1 would be 29.

Just be sure to make sure input-1 and input+1 do not go out of the bounds od your array.

If you wanted to find out what the actual fibonnaci number is as opposed to its number in the sequence, you will need to do a search for this first and then use the index you find to again, find what is before and after.

Answer 3

If you´d don´t want to use the binary search provided by the Arrays class then you could simply loop over the array and check for the needed values. You just need an int value that indicates the index in the array.

public static void main(String[] args) {
    long[] fibs = createFib();
    long search = 30;
    int small = 0;
    for(long i : fibs) {
        if(i < search) {
            ++small;
        } else {
            break;
        }
    }
    if(small < fibs.length)
        System.out.println(fibs[small-1] + " and " + fibs[small] + " do surround the value " + search);
    else {
        System.out.println(" the value " + search +" is to high, only " + fibs[small-1] + " is in range");
    }
}

private static long[] createFib() {
    long f1 = 0;
    long f2 = 1;
    long fibonacci = 0;
    long[] fibonaccinumbers = new long[52];
    fibonaccinumbers[0]=0;
    fibonaccinumbers[1]=1;
    for(int count = 2; count<=51; count++)
    {
        fibonacci = f2+f1;
        fibonaccinumbers[count] = fibonacci;
        f1 = f2;
        f2 = fibonacci;        
    }
    return fibonaccinumbers;
}