[Tutor] Meaning of -1 in Python

[Terry Carroll]

On Fri, 10 Dec 2010, Alan Gauld wrote:

> "Ben Ganzfried" <ben.ganzfried at gmail.com> wrote
>
>>  n = s.find('not')
>>  b = s.find('bad')
>>  if n != -1 and b != -1 and b > n:
>>    s = s[:n] + 'good' + s[b+3:]
>>  return s
>> 
>> It's clear that n!=-1 and b!=-1 means something like : "if in the
>> string 's' we find the word "not" and in string 's' we find the word
>> "bad."
>
> Exactly the oopsite in fact.
> find() returns the index where the string is fouind.
> -1 means the string was not found

I don't think it's opposite; I think Ben and Alan are saying the same 
thing, albeit in different ways.

Ben:

   n = s.find('not')
   n!=-1 ... means something like : "if in the string 's' we find the word
    "not"

Yes; "n != -1" means "not not found", that is, found.

(ditto for the discussion of "b!=-1")

Alan:
   -1 means the string was not found

Exactly.  Both of you are saying that -1 means "not found"; Ben is 
expresing that negating that is equivalent to "found."