Find the indexes of unique elements of a list in python?

Question

I have a list like this:

l=[1,2,2,3,4,5,5,5]

We can see that the list list contains 5 unique values with 8 total values.

I want the indexes of the unique values from the list in list format.

So the output looks like:

indexes=[0,1,3,4,5]

How to do it most efficient way using python ?

Answer 1

why did no one mention np.unique in here??

import numpy as np
np.unique([1,6,6,2,2,3,4,5,5,5], return_index=True)

>>> (array([1, 2, 3, 4, 5, 6]), array([0, 3, 5, 6, 7, 1], dtype=int64))

the first array contains the ( sorted ) unique values and the second array (returned if return_index is set to True ) is a list of the indexes of the first occurrences

Answer 2

You can use built-in types for this.

CODE

l=[1,2,2,3,4,5,5,5]

indexes = [l.index(x) for x in set(l)]

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EXPLANATION

• set
  All unique members of the list.
• list.index
  Returns the first index of an element.

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COMMENT

As pointed out in the comments, if order is important for you, you can either use sorted on the set or on the resulting index list depending on the data that is provided. If the data is already sorted, I would suggest to do it like this:

indexes = [l.index(x) for x in sorted(set(l))]

Answer 3

You can just iterate through your list. The first time you see an item add it to a set indicating it's been seen and add the number to the result list. Skip the others. This will keep the indexes in the order the item are first seen in the list:

def uniqueIndexes(l):
    seen = set()
    res = []
    for i, n in enumerate(l):
        if n not in seen:
            res.append(i)
            seen.add(n)
    return res

l=[1,2,2,3,4,5,5,5,2]

uniqueIndexes(l)

results:

[0, 1, 3, 4, 5]

Answer 4

If this isn't a pandas-specific question, and l is just a plain old list, I'd go over it and keep an ordered map from the value to the first index holding it. Then, you can return the map values:

from collections import OrderedDict
def get_unique_indexes(l):
    # OrdedDict is used to preserve the order of the indexes
    result = OrderedDict()
    for i in range(0, len(l)):
        val = l[i]
        if not val in result:
            result[val] = i

    return result.values()

Answer 5

This can be done:

l=[1,2,2,3,4,5,5,5] # Your list
indexes=[] # Your output list
for elem in set(l):
       indexes.append(l.index(elem))

In the for loop, each element of the set is taken and the index of the element in the list is taken out using list.index() method (which returns the index of the first element of the required type) and the value is inserted into the indexes list.

This is how it works.(I like to explain, so please apologize for my lengthy writing!)

Thank You.