125.py

#Let's implement the Fibonacci function we saw in the
#previous video in Python!
#
#Like our Factorial function, our Fibonacci function
#should take as input one parameter, n, an integer. It
#should calculate the nth Fibonacci number. For example,
#fib(7) should give 13 since the 7th number in
#Fibonacci's sequence is 13.

#So, our function definition will basically be the same:

def fib(n):
    #What do we want to do inside the function? Once again,
    #there are really only two cases: either we're looking
    #for the first two Fibonacci numbers, or we're not.
    #What happens if we're looking for the first two? Well,
    #we already know that the 1st and 2nd Fibonacci numbers
    #are both 1, so if n == 1 or n == 2, we can go ahead
    #and return 1.
    
    if n == 1 or n == 2:
        return 1
    
    #What if n doesn't equal 1? For any value for n greater
    #than 2, the result should be the sum of the previous
    #two numbers. The previous Fibonacci number could then
    #be calculated with the same kind of function call,
    #decrementing n by 1 or 2.
    
    else:
        return fib(n - 1) + fib(n - 2)
    
    #If n is greater than 2, then it returns the sum of the
    #previous two fibonacci numbers, as calculated by the
    #same function.

#Now let's test it out! Run this file to see the results.
print("fib(5) is", fib(5))
print("fib(10) is", fib(10))

#Want to see more about how this works? Select the other
#file, FibonacciwithPrints.py, from the drop-down in the
#top left to see a version of this that traces the output.