[Tutor] simple list query

[Gonçalo Rodrigues]

Patrick Hall wrote:
> Hi Dave,
> 
> 
>>I have a list consisting of about 250 items, I need to know if a
>>particular item is in the list. I know this is better suited to  a
>>dictionary but thats not the way it ended up ;-)
> 
> 
>>I could do a for loop to scan the list & compare each one, but I have a
>>suspission that there is a better way ?
> 
> 
> Indeed there is: just use the built-in "in":
> 
> 

[text snipped]

> 
> A list of 250 words is no problem -- "Moby Dick" has a couple hundred thousand:
> 
> 
>>>>len(mobywords)
> 
> 214112
> 
> I'm not sure I understand why you think a dictionary would be better
> in this case, a list seems fine to me.
> 

The statement

my_object in my_list

does under the hood a for-loop and sequentially compares every object in 
my_list with my_object, so, potentially it does len(my_list) comparisons 
(it's O(n) in computerese speak). With a dictionary, that is

my_object in my_dict

it is O(1), that is, it is (asymptotically) constant and independent of 
the number of items in the dictionary.

So if you are going to do a lot of in tests, it does pay to put your 
objects in a dictionary.

With my best regards,
G. Rodrigues