On 11/18/2011 06:04 PM, Dave Angel wrote:
On 11/18/2011 08:16 AM, Kĩnũthia Mũchane wrote:
<snip>
Well, it doesn't count the number of occurrences correctly if the list
is empty. It should get zero, and it gets -1. But for any non-empty
list, nothing ever looks at the -1 value, so it doesn't matter what
you put there. This example diverges from traditional recursion in
many ways, but the chief reason it's not traditional recursion is that
it never uses the return value. within the function.
If the list is empty, which is the base case, s.slais([], 4) returns
-1. Now using some bush logic, in a non-empty list, in order for the
recursion to terminate it has to 'hit' the base case and return -1.
Where does this -1 go ?
Nowhere. You don't use it, so it gets discarded. if you were going
to use it, your internal calls to the method would look something like:
b = self.slais(lst[1:], x)
and then you'd do something with b.
Further, why do not I get a *TypeError* when I use a simple *return*
statement in the *if* clause?
You would if you actually used the value for arithmetic. But the
return itself would be perfectly legal. it's just a shortcut for
'return None'
The reason I am asking that is that I think(wrongly, of course :-))
it should be part of the answer and therefore I should be getting an
answer that is off by one or a *TypeError*!!
And by the way, the reason I used a *class* was that I could not get
a suitable place in the program to initialise my *count* variable
otherwise.
Using a class is fine. But you're abusing it. What happens if you
try to sum a second list? (Hint, it gives you a higher number)
That is very true, I tried it with a second list and the value was higher.
Thanks...
If you really want to recursively count, you need a structure which
uses no objects of global lifetime. The intermediate values should be
stored in local variables to that method.
def counter(mylist, val):
if len(mylist == 0):
return 0
prev = counter(mylist[1:], val) #this is actually using the
recursive return value
if mylist[0] == val:
return prev + 1
else:
return prev
I was trying to come up with something like this but I was totally
stumped. Now I understand, perfectly.
My heartfelt thanks to you, Dave, and Christian ! ;-)
Totally untested. And there are certainly many other variants
possible. But the key is you have to do something with the value
returned to you by the lower level function.
--
Kĩnũthia
S 1º 8' 24”
E 36º 57' 36”
1522m
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