check if a number already exist in a list in python

Question

I am writing a python program where I will be appending numbers into a list  but I don t want the numbers in the list to repeat  So how do I check if a number is already in the list before I do list append

User · Answer

If you want to have unique elements in your list, then why not use a set, if of course, order does not matter for you: -

>>> s = set()
>>> s.add(2)
>>> s.add(4)
>>> s.add(5)
>>> s.add(2)
>>> s
39: set([2, 4, 5])

If order is a matter of concern, then you can use: -

>>> def addUnique(l, num):
...     if num not in l:
...         l.append(num)
...     
...     return l

You can also find an OrderedSet recipe, which is referred to in Python Documentation

User · Answer

You could probably use a set object instead  Just add numbers to the set  They inherently do not replicate

User · Answer

If you want your numbers in ascending order you can add them into a set and then sort the set into an ascending list   s   set   if number1 not in s    s add number1  if number2 not in s    s add number2      s   sorted s    Now a list in ascending order

User · Answer

You could do  if item not in mylist       mylist append item    But you should really use a set  like this    myset   set   myset add item    EDIT  If order is important but your list is very big  you should probably use both a list and a set  like so   mylist      myset   set   for item in          if item not in myset          mylist append item          myset add item    This way  you get fast lookup for element existence  but you keep your ordering  If you use the naive solution  you will get O n  performance for the lookup  and that can be bad if your list is big  Or  as  larsman pointed out  you can use OrderedDict to the same effect   from collections import OrderedDict  mydict   OrderedDict   for item in          mydict item    True

[python] check if a number already exist in a list in python

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