How to reverse a dictionary that it has repeated values

How to reverse a dictionary that it has repeated values

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So, I have a dictionary with almost 100,000 (key, values) pairs and the majority of the keys map to the same values. For example imagine something like that:

mydict =  {'a': 1, 'c': 2, 'b': 1, 'e': 2, 'd': 3, 'h': 1, 'j': 3}

What I want to do, is to reverse the dictionary so that each value in mydict is going to be a key at the reverse_dict and is going to map to a list of all the mydict.keys that used to map to that value at the mydict. So based on the example above I would get:

reversed_dict = {1: ['a', 'b', 'h'], 2:['e', 'c'] , 3:['d', 'j']} 

I came up with a solution that is very expensive and I would really want to hear any ideas more efficient than mine.

my expensive solution:

reversed_dict = {}
for value in mydict.values():
    reversed_dict[value] = []
    for key in mydict.keys():
        if mydict[key] == value:
            if key not in reversed_dict[value]: reversed_dict[value].append(key)

Output >> reversed_dict = {1: ['a', 'b', 'h'], 2: ['c', 'e'], 3: ['d', 'j']}

I would really appreciate to hear any ideas better and more efficient than than mine. Thanks!


from collections import defaultdict
reversed_dict = defaultdict(list)
for key,value in mydict.iteritems():
    reversed_dict[value].append(key)

Please do not use dict as a variable, this collides with function dict()

Python, Given a dictionary, the task is to find keys with duplicate values. Let's discuss a few methods for the same. Method #1: Using Naive approach. In this method first,​  Given a dictionary, the task is to find keys with duplicate values. Let’s discuss a few methods for the same. Suppose you need to find keys having duplicate values. Check out this Author's contributed articles. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your


I think you're wasting a few cycles by replacing a key with the same key again and again...

reversed_dict = {}
for value in mydict.values():
    if value not in reversed_dict.keys(): #checking to be sure it hasn't been done.
        reversed_dict[value] = []
        for key in mydict.keys():
            if mydict[key] == value:
                if key not in reversed_dict[value]: reversed_dict[value].append(key)

How to strip a Python dictionary of duplicates, As Uri Granta writes in his excellent and thorough answer, a dictionary cannot by definition have duplicates among its keys… so “stripping duplicates” can only mean Undo. 4 Answers. Dan Lenski, I do all my best thinking in Python, and I'​ve  According to Capi Etheriel, there’s yet another way to reverse a dictionary with unique values. Specifically, they mentioned zipping the keys and values in their reverse order and converting the output to a dictionary: dict(zip(my_dict.values(), my_dict.keys())) When I first saw this solution, I was a bit skeptical that it would actually work.


for k,v in dict.iteritems():
    try:
      reversed_dict[v].append(k)
    except KeyError:
       reversed_dict[v]=[k]

Python: Remove duplicates from Dictionary, Write a Python program to remove duplicates from Dictionary. what the computer is doing step-by-step as it executes the said program:  The reverse dictionary uses the Datamuse API, which in turn uses several lingustic resources described in the "Data sources" section on that page. Yikes. Why are so many of my search results complete nonsense? For some types of searches only the first result or the first few results are likely to be useful.


reversed_dict = {}
for key, value in mydict.items():
    reversed_dict.setdefault(value, [])
    reversed_dict[value].append(key)

Invert a dictionary (one-liner) « Python recipes « ActiveState Code, One way to deal with duplicate values is to turn all keys with a common value into a list when you invert the dictionary. I haven't found a shortcut  Create a dictionary, using the List items as keys. This will automatically remove any duplicates because dictionaries cannot have duplicate keys.


reversed_dict = collections.defaultdict(list)
for key, value in dict_.iteritems():
  reversed_dict[value].append(key)

How to Invert a Dictionary in Python: Comprehensions, Defaultdict , Instead, we want to accumulate a list of keys for each non-unique value while performing the inversion. As it turns out, this is pretty easy to do. Note: If you're  In this article we will discuss different ways to check if a list contains any duplicate element or not. Suppose we have a list of elements i.e. Now we want to check if this list contains any duplicate element or not. There are several ways to do this, but here we will discuss 3 ways and will also analyze there performance.


Dictionaries, To see whether something appears as a value in a dictionary, you can use the Second, there is no simple syntax to do a reverse lookup; you have to search. one-syllable, five-letter word recently that has the following unique property. To correct this, the output dictionary should have the same format as the input, namely mapping each of the items in the input lists to a list of corresponding keys. If the input represents a directed graph (mapping nodes to lists of neighbours), this corresponds to computing the transposed or reversed graph.


Collections, Create Dictionary, dicts, Was in Collections until RF 2.9. Contain Sub List · List Should Contain Value · List Should Not Contain Duplicates List · Remove Values From List · Reverse List · Set List Value · Set To Dictionary  Programmers like to create a dictionary for small data source to store key value type data. Keys are unique, but dictionary values may be duplicates. Using the code . Here I use a simple LINQ statement to find duplicate values from dictionary. // initialize a dictionary with keys and values.


How to remove duplicates from a Python List, print(mylist). Create a dictionary, using the List items as keys. This will automatically remove any duplicates because dictionaries cannot have duplicate keys. In this article, we will discuss how to find duplicates in a list along with their frequency count and their index positions in the list. Suppose we have a list of strings i.e. We have created a function that accepts a list and returns a dictionary of duplicate elements in that list along with their frequency count,