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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.

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    Please provide meaningful title for the question next time Commented Nov 15, 2017 at 8:08

4 Answers 4

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6

raw_input() returns a string, as you've surmised. However, int(n) doesn't change the value of n to an integer, instead it returns the converted value. You need to do:

n = int(n)

instead.

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3

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.

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I'll be accepting this as the answer. I like the explanation you made along with your answer.
3

Just make a small change :

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

The int conversion takes a string which you obtained from raw_input and returns a integer. You need to pass the return value to fib(..).

int_obj = int(str_obj)
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use int(raw_input('Input number: ')) instead of raw_input('Input number: ')

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