[Tutor] Re: Recursive list checking

[Kent Johnson]

Jeffrey Maitland wrote:
> joe_schmoe writes:
> 
>> Dear Pythonites
>> I am looking for a more elegant solution to a piece of code that is 
>> too unwieldy and reptitive. The purpose of the code is for a new 
>> addition to a list to check whether it is a duplicate of a list 
>> element already a member of that list, and if so to regenerate itself 
>> randomly and to perform the same check again until such time as it is 
>> unique.
>> For example, this is what I am currently doing:
>> =============code block ========================
>>    # generate unique numbers and append to list
>>    nmbr01 = random.randrange( 1, 20 )
>>    nmbr_list.append( nmbr01 )
>>    nmbr02 = random.randrange( 1, 20 )
>>    # check for duplicates and re-generate a number if needed
>>    while nmbr02 in nmbr_list:
>>        nmbr02 = random.randrange( 1, 20 )
>>    nmbr_list.append( nmbr02 )
>>    nmbr03 = random.randrange( 1, 20 )
>>    while nmbr03 in nmbr_list:
>>        nmbr03 = random.randrange( 1, 20 )
>>    nmbr.append( nmbr03 )
>> ================================================
>> This method works, but increasing the numbers to be appended makes the 
>> code excessively long. I can't see anything in list methods that seems 
>> to do the trick, so anybody want to make a suggestion please?
>> TIA
>> /j
>> _______________________________________________
>> Tutor maillist  -  Tutor at python.org
>> http://mail.python.org/mailman/listinfo/tutor
> 
> 
> Well I would start by doing something like.
> nmbr_list = []
> value = int(raw_input("Input the number of items you wish to generate"))
> for i in range(value):
>  if i == 0:
>    nmbr = random.randrange( 1, 20 )
>    nmbr_list.append( nmbr01 )
>  else:
>     nmbr = random.randrange( 1, 20 )
>     # check for duplicates and re-generate a number if needed
>     while nmbr in nmbr_list:
>        nmbr = random.randrange( 1, 20 )
>     nmbr_list.append( nmbr )
> I hope that helps. or gives you an idea.

The special case for i==0 is not needed, in this case the test for nmbr in nmbr_list will fail and 
nmbr will be added to the list.

But if you are trying to get n random elements from range(m) you are probably better off using 
random.sample(), I think it does exactly what you want:
  >>> random.sample(xrange(10000000), 10)
[274075, 2925710, 7715591, 8236811, 1161108, 5804222, 2385884, 9236087, 5603149, 8473299]

If you actually want *all* elements of the range in random order, use random.shuffle():
  >>> l=range(20)
  >>> random.shuffle(l)
  >>> l
[13, 7, 6, 9, 3, 10, 1, 8, 4, 0, 18, 12, 11, 17, 19, 5, 16, 15, 2, 14]

You might also want to look at random.choice()...

Kent