How to compare values within an array in Python - find out whether 2 values are the same

Question

I basically have an array of 50 integers, and I need to find out whether any of the 50 integers are equal, and if they are, I need to carry out an action.

How would I go about doing this? As far as I know there isn't currently a function in Python that does this is there?

Answer 1

If you mean you have a list and you want to know if there are any duplicate values, then make a set from the list and see if it's shorter than the list:

if len(set(my_list)) < len(my_list):
    print "There's a dupe!"

This won't tell you what the duplicate value is, though.

Answer 2

If you have Python 2.7+ you can use Counter.

>>> import collections
>>> input = [1, 1, 3, 6, 4, 8, 8, 5, 6]
>>> c = collections.Counter(input)
>>> c
Counter({1: 2, 6: 2, 8: 2, 3: 1, 4: 1, 5: 1})
>>> duplicates = [i for i in c if c[i] > 1]
>>> duplicates
[1, 6, 8]

Answer 3

If your actions need to know the number or how many times that number gets repeated over your input list then groupby is a good choice.

>>> from itertools import groupby
>>> for x in groupby([1,1,2,2,2,3]):
...     print x[0],len(list(x[1]))
... 
1 2
2 3
3 1

The first number is the element and the second the number of repetitions. groupby works over a sorted list so make sure you sort your input list, for instance.

>>> for x in groupby(sorted([1,1,2,4,2,2,3])):
...     print x[0],len(list(x[1]))
... 
1 2
2 3
3 1
4 1