Fibonacci-Numbers-Python

Generally, we can use iterative and recursive ways to get the Fibonacci numbers.

1. Iterative

    def fibonacci1(num):
        result=[0,1]
        if num==1:
            return [0]
        elif num==2:
            return [0,1]
        for i in range (2,num):
            result.append(result[i-1]+result[i-2])
        return result
   

If we call as fibonacci1(20)

It will display [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181]

2. Recursive

We need to get the nth Fibonacci number in recursive way first.

def fibonacci(num):
    if num==1 or num==0:
        return num
    else:
        return fibonacci(num-1)+fibonacci(num-2)

If we run fibonacci(20) and it will show the 21th fibonacci number - 6765

Second, we can write a loop to add them to the list.

def fibonacci2(num):
    res=[]
    for i in range(0,num):
        res.append(fibonacci(i))
        num-=1
    return res

If we call fibonacci2(20), it will display the same result as fibonacci1(20).

3. Recursive with a dictionary:

If we use keys in dictionary, the code will be shorter:

    previous = {0:1, 1:1} 

    def fibonacci(n): 
        if n in previous.keys(): 
            return previous[n] 
        else: 
            newValue = fibonacci(n-1) + fibonacci(n-2) 
            previous[n] = newValue 
            return newValue