[Tutor] Looking for duplicates within a list

[Ken G.]

Alex Hall wrote:
> On 6/11/10, Ken G. <beachkid at insightbb.com> wrote:
>   
>> I have been working on this problem for several days and I am not making
>> any progress.  I have a group of 18 number, in ascending order, within a
>> list.  They ranged from 1 to 39.  Some numbers are duplicated as much as
>> three times or as few as none.
>>     
> FYI, Python's "set" data type will let you have a list and never have
> a repeat. I know that is not your goal now, but if you want to remove
> duplicates, it seems like a good choice.
>   
>> I started with one list containing the numbers.  For example, they are
>> listed as like below:
>>
>> a = [1, 2, 3, 3, 4]
>>
>> I started off with using a loop:
>>
>>     for j in range (0, 5):
>>     x = a[0] # for example, 1
>>
>> How would I compare '1' with 2, 3, 3, 4?
>>
>> Do I need another duplicated list such as b = a and compare a[0] with
>> either b[0], b[1], b[2], b[3], b[4]?
>>
>> Or do I compare a[0] with a[1], a[2], a[3], a[4]?
>>     
> A couple points here. First, you will want to make life easier by
> saying range(0, len(a)) so that the loop will work no matter the size
> of a.
> Second, for comparing a list to itself, here is a rather inefficient,
> though simple, way:
>
> for i in range(0, len(a)):
>  x=a[i]
>  for j in range(0, len(a)):
>   y=a[j]
>   if(x==y and i!=j): #match since a[i]==a[j] and i and j are not the
> same index of a
>   
>> In any event, if a number is listed more than once, I would like to know
>> how many times, such as 2 or 3 times.  For example, '3' is listed twice
>> within a list.
>>     
> Do not quote me here, but I think sets may be able to tell you that as well.
>   
>> TIA,
>>
>> Ken
>>     
Thank you for your contribution.  As seen here, I have much to learn.

Ken

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