Function Break Out of For Loop Unexpectedly in Python

Question

I'm still a beginner when it comes to Python. I've most recently attempted the 2nd Project Euler question regarding fibonacci numbers, and unfortunately I am stuck. On a simple for-loop, no less. Within this for-loop I attempt to assign the returned result of a function to a variable, and whether or not said variable matches a condition, a total is increased. However, this loop only runs once before it exits, and I am absolutely perplexed. I'm at the end of my wits; if any of you can help to correct this issue I would be grateful. Below is the relevant portion of my code:

def fibonacci(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        x = n - 1
        y = n - 2
        return x + y

num = input("Input your number of choice: ")
total = 0

The problem area:

for i in range(int(num)):
    val = fibonacci(i)
    if (val % 2 == 0):
        total = total + val

print(total)

Answer 1

Yes, it will only calculate total = total + val once, when val = 0 , for all other cases, your fibonacci function is returning 2*n -3 which is always odd number, but you are checking if the returned val is even (which it would never be except for when 0) , and hence the issue.

Maybe you wanted to return something like -

return fibonacci(x) + fibonacci(y) # or the simpler , fibonacci(n-1) + fibonacci(n-2)

Answer 2

Note that the sequence Fn of Fibonacci numbers is defined by the recurrence relation F_n = F_{n-1} + F_{n-2} .

Option 1: Using recursion

Instead of return (n-1) + (n-2) , it should be return fibonacci(n-1) + fibonacci(n-2) .

def fibonacci(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        x = fibonacci(n-1)
        y = fibonacci(n-2)
        return x + y

num = input("Input your number of choice: ")

print(fibonacci(num))

Here is an example:

$ python fibonacci.py
Input your number of choice: 11
89

Option 2: Using loop

def fibonacci(num):
 a,b = 1,1
 for i in range(num-1):
  a,b = b,a+b
 return a

print fibonacci(num)

Answer 3

The issue I see is you are trying to print a variable that was assigned in a for loop. Instead, you can do...

for i in range(int(num)):
     val = fibonacci(i)
     if (val % 2 == 0):
         return (total + val)