# [Tutor] A recursion question

Dave Angel d at davea.name
Sat Nov 19 14:59:58 CET 2011

```On 11/19/2011 01:36 AM, Kĩnũthia Mũchane wrote:
> On 11/19/2011 06:03 AM, Asokan Pichai wrote:
>> Another way to do that is to avoid any intermediate variables altogether
>> That may be easier to understand YMMV
>>
>> def counter(mylist, val):
>>      if len(mylist == 0):
>>             return 0
>>      if mylist[0] == val:
>>            return  1 + counter(mylist[1:], val)
>>      else:
>>            return counter(mylist[1:])
> The intermediate variable explanation by Dave actually clinched it for
> me. Actually, the one I wrote is suspiciously similar to yours ;-).
> Anyway, thanks Asokan!

FWIW, Asokan's code looks exactly right to me. But I figured the version
I supplied would make it clearer to you what was happening.

The key to thinking recursively is to figure out exactly what the
function as a whole does, then figure out how to use exactly such a
function that  solves a somewhat smaller problem, to solve the whole
thing.  Since the function as a whole takes in a list, and returns a
count, that's the way to use the "smaller problem" function.  Both
Asokan's answer and mine do that.  But his "local variable" is implied
in the expressions, where I made it explicit so you could see what was
happening.

If you're familiar with the mathematical proof by induction, this is
very analogous.

--

DaveA

```

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