python,string,recursion,integer , Why am I able to add an integer to this string value? [closed]

## Question:

Tag: python,string,recursion,integer

I have a piece of code I am trying to understand. I need to count the number of vowels in a string using recursion in Python. I understand recursion is not the best way to do this, but this is the homework assignment. With the help of the community, I have gotten my code to function as it is written below, but I do not understand what the line "return 1 + rvowels(s[1:])" is doing, because I do not understand how you can add a number to a string. Can anybody explain this to me? All help greatly appreciated!

``````def rvowels(s):
if len(s) == 0:
return 0
letter = s[0]
if letter in 'aeiou':
return 1 + rvowels(s[1:])
return rvowels(s[1:])

print(rvowels('abe'))
``````

The idea here is that `rvowels` is a function, and it has three possible things it can return when run as `rvowels(x)`:

• `0` - an integer.
• `1 + rvowels(x[1:])` - an integer plus whatever `rvowels(x[1:])` returns.
• `rvowels(x[1:])` - whatever `rvowels(x[1:])` returns

Notice that it never returns a string. It either returns an integer by itself (0), or something that involves running it again on a shorter string. Thus it's always going to return an integer.

Note that this isn't just a horrible way to do this, by the way: it will fail owing to a stack overflow on any string longer than `sys.getrecursionlimit()`, which is only 1000 by default... deeply recursive algorithms like this are a bad idea in Python, as it has no tail call optimization, and your teacher should probably feel bad for teaching them. Even with tail call optimization, however, your code might well not be optimized because of the 1+function() bit.

Refactoring your code by hand to recurse through the string in a while loop would be better for Python; in languages with tail call optimization, you would likely need to have an accumulating argument of some sort (eg, `rvowels(remainder_of_string, vowels_so_far)` )

I'd also like to point out that the better Pythonic way of doing this with list comprehensions is four times faster than the recursive version, and absurdly simpler: `sum( x in 'aeiou' for x in s )`

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