Question

I’m completely new to Python and I’ve been trying to make a fibonacci program with it.

def fib(n):
    print 'n =', n
    if n > 1:
        return n * fib(n - 1)
    else:
        print 'end of the line'
    return 1

n = raw_input('Input number: ')
int(n)
fib(n)

When I try to run this program, I get the following error after entering the number:

Input number: 5

n = 5

Traceback (most recent call last):

File “fibonacci.py”, line 11, in

fib(n)

File “fibonacci.py”, line 4, in fib

return n * fib(n - 1)

TypeError: unsupported operand type(s)
for -: ‘str’ and ‘int’

If I run the interpreter and import just the function (without the code after it), supply the value for n and call the function with the value as the parameter, it works.

I tried converting the input to int since I thought it was a string problem but no dice. I don’t really know where I went wrong so if you could please shed some light on the subject, it’ll be much appreciated.

I’d love to change the problem title to something specific but I don’t really know what the problem is.

Answer 1

The problem is that raw_input is providing a string, not an integer. Can I suggest just putting the integer value into n in the first place:

n = int(raw_input('Input numver: '))
fib(n)

Avoid n = int(n) as in a longer section of code it would be unclear when you came back to it what type n is, and you have no need for the original string value.

The deeper understanding you need is that Python is strongly typed – it cares what type everything is, but it is also dynamically typed, so n could change from holding a string value to holding an integer value. But Python is still keeping track of what is held, so when you put the return value of raw_input into n, Python knows it is a string, so multiplying that string by a number makes no sense and returns an error.