如何从输入的整数列表中找出哪个任意数字出现两次？ （Python）

Question

Say I'm receiving a list of arbitrary numbers from input, like

[1,2,3,4,5,6,7,8,8,9,10]

My code doesn't know what numbers these are going to be before it receives the list, and I want to return the number that appears twice automatically. How do I go about doing so?

Thank you.

Answer 1

You can use Counter By defualt Method in python 2 and 3

from collections import Counter
lst=[1,2,3,4,5,6,7,8,8,9,10]
items=[k for k,v in Counter(lst).items() if v==2]
print(items)

Answer 2

You could do:

input = [1,2,3,4,5,6,7,8,8,9,10]
list_of_duplicates = []
for i in input:
    if i not in list_of_duplicates:
        list_of_duplicates.append(i)
        input.pop(i)
print(input)

Now input will have all the numbers that were in the list multiple times.

Answer 3

Hope this helps.

input = [1,2,3,4,5,6,7,8,8,9,10]
unique = set(input)
twice = []    
for item in unique:
    if input.count(item) == 2:
        twice.append(item)

Answer 4

I've created something monstrous that does it in one line because my brain likes to think when it's time for bed I guess?

This will return a list of all duplicate values given a list of integers.

dupes = list(set(map(lambda x: x if inputList.count(x) >= 2 else None, inputList))-set([None]))

How does it work? The map() function applies a function every value of a list, in your case our input list with possible duplicates is called "inputList". It then applies a lambda function that returns the value of the integer being iterated over IF the iterated value when applied to the inputList via the .count() method is greater than or equal to two, else if it doesn't count as a duplicate it will return None. With this lambda function being applied by the map function, we get a list back that contains a bunch of None's and the actual integers detected as duplicates via the lambda function. Given that this is a list, we the use set to de-duplicate it. We then minus the set of duplicates against a static set made from a list with one item of None, stripping None values from our set of the map returned list. Finally we take the set after subtraction and convert it to a list called "dupes" for nice and easy use.

Example usage...

inputList = [1, 2, 3, 4, 4, 4, 5, 6, 6, 7, 1001, 1002, 1002, 99999, 100000, 1000001, 1000001]
dupes = list(set(map(lambda x: x if inputList.count(x) >= 2 else None, inputList))-set([None]))
print(dupes)
[1000001, 1002, 4, 6]

I'll let someone else elaborate on potential scope concerns..... or other concerns......

Answer 5

This will create a list of the numbers that are duplicated.

 x = [1, 2, 3, 4, 5, 6, 7, 8, 8, 9, 10]


 s = {}
 duplicates = []
 for n in x:
     try:
         if s[n]:
             duplicates.append(n)
             s[n] = False
     except KeyError:
         s[n] = True

print(duplicates)

Assuming the list doesn't contain 0