OK, i’m using Python 2.7.3 and here is my code:
def lenRecur(s): count = 0 def isChar(c): c = c.lower() ans='' for s in c: if s in 'abcdefghijklmnopqrstuvwxyz': ans += s return ans def leng(s): global count if len(s)==0: return count else: count += 1 return leng(s[1:]) return leng(isChar(s))
I’m trying to modify the variable count
inside the leng
function. Here are the things that I’ve tried:
- If I put the variable count outside the
lenRecur
function it works fine the first time, but if I try again without restarting python shell, the count (obviously) doesn’t restart, so it keeps adding. - If I change the
count += 1
line forcount = 1
it also works, but the output is (obviously) one.
So, my goal here is to get the length of the string using recursion, but I don’t know how to keep track of the number of letters. I’ve searched for information about global variables, but I am still stuck. I don’t know if i haven’t understood it yet, or if I have a problem in my code.
Thanks in advance!
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Answer
count
in lenRecur
is not a global. It is a scoped variable.
You’ll need to use Python 3 before you can make that work in this way; you are looking for the nonlocal
statement added to Python 3.
In Python 2, you can work around this limitation by using a mutable (such as a list) for count
instead:
def lenRecur(s): count = [0] # ... def leng(s): if len(s)==0: return count[0] else: count[0] += 1 return lenIter(s[1:])
Now you are no longer altering the count
name itself; it remains unchanged, it keeps referring to the same list. All you are doing is altering the first element contained in the count
list.
An alternative ‘spelling’ would be to make count
a function attribute:
def lenRecur(s): # ... def leng(s): if len(s)==0: return leng.count else: leng.count += 1 return lenIter(s[1:]) leng.count = 0
Now count
is no longer local to lenRecur()
; it has become an attribute on the unchanging lenRecur()
function instead.
For your specific problem, you are actually overthinking things. Just have the recursion do the summing:
def lenRecur(s): def characters_only(s): return ''.join([c for c in s if c.isalpha()]) def len_recursive(s): if not s: return 0 return 1 + len_recursive(s[1:]) return len_recursive(characters_only(s))
Demo:
>>> def lenRecur(s): ... def characters_only(s): ... return ''.join([c for c in s if c.isalpha()]) ... def len_recursive(s): ... if not s: ... return 0 ... return 1 + len_recursive(s[1:]) ... return len_recursive(characters_only(s)) ... >>> lenRecur('The Quick Brown Fox') 16