How to check if two numbers in a list are the same

Question

I was curious on how I would check if 2 numbers inside of a list are identical. For instance,

myList=[1,7,9,3,1,2,8]

In this case, "1" is repeated in 'myList'.

How would I make a program that checks to see if two numbers inside of a list are the same(repeated). Try to use loops so I can understand because I haven't yet learned complex functions.

Answer 1

Using collections.Counter:

from collections import Counter

myList = [1, 7, 9, 3, 1, 2, 8]
counter = Counter(myList)
print(counter) # prints 'Counter({1: 2, 2: 1, 3: 1, 7: 1, 8: 1, 9: 1})'

With Collections.Counter, you know how many times is repeated every element of your source list myList.

After that you can use a simple list comprehension to know which elements are repeated:

result = [i for i, j in counter.items() if j > 1]
print(result) # prints '[1]'

Answer 2

If you want to use loops, you'll have to use a list or a set of numbers which you've already seen. Then while looping you'll check, with the in operator if the number is already seen.

seen = []
for number in myList:
    if number in seen:
        print "Number repeated!"
    else:
        seen.append(number)

set does not allow duplicates in it, thus it's a good fit for this sort of an algorithm. As mentioned in the comments, the time complexity for checking if an element is in a set is constant for the average case (O(1)), so this is more efficient if you have a lot of numbers.

seen = set()
for number in myList:
    if number in seen:
        print "Number repeated!"
    seen.add(number) # won't duplicate

I'd say that the most pythonic way is to use collections.Counter, but the other answers cover this already. To use a built-in, you could generate a set of the numbers which appear more than once using a generator expression and set.

In [39]: seen = set()

In [40]: print list(set(x for x in myList if x in seen or seen.add(x)))
[1]

Here the expression will loop over all values in myList and add them to a set called seen if they have already been seen. Eventually, it will convert the resulting set into a list and print the contents.