ios-数组中的NSNumber

Question

I am trying to learn about how to put numbers into an array with nsnumber. The exact thing I'm stuck with is, To build the sequence in the array, we're going to need a loop. Between creating the sequence array and returning it, declare a for loop whose counter is limited by index + 1 and increments by one. Since the sequence requires the two previous numbers to calculate the next one, we need to prime the sequence. We're going to need to manually pass in @0 and @1 on the first two iterations of the loop. This is what I have so far.

(NSArray *)arrayWithFibonacciSequenceToIndex:(NSUInteger)index
{
NSMutableArray *sequence = [NSMutableArray array];

for(NSUInteger i = 0; i < 1; i++)
{
    index = i+1;

}

return sequence;
}

Am I on the right track? I'm not sure if my for loop is correct. Do I put sequence into the for loop and add the nsnumber @0 and @1 there or do I put those numbers into the sequence outside the loop?

Answer 1

To insert a number in an NSArray, you have to wrap them in a NSNumber:

NSInteger a = 5;
NSNumber number = @(a); // ou @5;

to perform mathematical operations on 2 NSNumbers, you have to convert them to integer (or double, float...) before

NSNumber * number1 = @1;
NSNumber * number2 = @6;
NSInteger sum = [number1 integerValue] + [number2 integerValue];

for the fib problem, youre loop is correct. The way I would think of this is : I add my value in the for loop, and if I'm adding the 1st or 2nd element, then I put a 0, else I sum the last 2 elements:

- (NSArray *) fibbonacciSequenceWithSize:(NSInteger)size
{
    NSMutableArray * result = [NSMutableArray new];

    for(NSInteger idx = 0; i < size ; i ++)
    {
        // first 2 numbers of fib sequence are 1
        if(idx == 0 || idx == 1)
        {
             [result addObject:@1];
        }
        else
        {
            // Add the 2 previous number
            // F2 = F1 + F0
            NSinteger next = [result[idx - 2] integerValue] + [result[idx - 1] integerValue];

            [result addObject:@(next)];
        }
    }
    return [result copy]; // copy the NSMutableArray in a NSArray
}

Answer 2

You can clean up the code by having a Fibonacci function that provides the sum of the last two elements.

- (NSNumber *)nextFibInArray:(NSArray *)array {
    if (array.count < 2) return @1;
    NSInteger lastIndex = array.count - 1;
    return @([array[lastIndex-1] intValue] + [array[lastIndex] intValue]);
}

Then the loop is cleaner, too.

- (NSArray *)fibonacciWithLength:(NSInteger)length {
    NSMutableArray *result = [@[] mutableCopy];
    for (NSInteger i=0; i<length; i++) {
        [result addObject:[self nextFibInArray:result]];
    }
    return result;
}

We could trim some execution time fat from this, but for short enough sequences, this should be clear and quick enough.