如何在列表中查找重复项及其索引？

Question

I have a list

l=['a','b','c','c','a','d']

The output should return all the duplicate elements and their indices in the list

Output:

out = {a:['0','4'],c:['2','3']}

I have tried

def nextDuplicates(c):
    dupl_c = dict()
    sorted_ind_c = sorted(range(len(c)), key=lambda x: c[x])
    for i in xrange(len(c) - 1):
        if c[sorted_ind_c[i]] == c[sorted_ind_c[i+1]]:
            dupl_c[ sorted_ind_c[i] ] = sorted_ind_c[i+1]
    return dupl_c

Answer 1

A dict comprehension coupled with a list comprehension would work (even for more than 2 occurences) :

l = ["a", "b", "c", "c", "a", "d"]
out = {el: [i for i, x in enumerate(l) if x == el] for el in l if l.count(el) > 1}

I saw in your expected output that indexes are strings. I don't understand why, but if you really want them as strings, replace i for i, x with str(i) for i, x .

More on list comprehensions

Answer 2

Try this:

l=['a','b','c','c','a','d']
o = {}
for i in range(len(l)):
    if (l[i] in o):
        o[l[i]].append(i)
    else:
        o[l[i]] = [i]
print({key:val for key, val in o.items() if len(val) > 1})

Answer 3

Use collections.defaultdict + a set iteration for a faster lookup for counts greater than 1:

from collections import defaultdict

l = ['a','b','c','c','a','d']

result = defaultdict(list)

for x in set(l):
    if l.count(x) > 1:
        result[x].extend([i for i, y in enumerate(l) if y == x])

print(result)
# defaultdict(<class 'list'>, {'a': [0, 4], 'c': [2, 3]})

Answer 4

You can use this dict comprehension

l = ["a", "b", "c", "c", "a", "d"]
out = {ele: [str(i) for i, x in enumerate(l) if x == ele] for ele in set(l) if l.count(ele) > 1}

# Output : {'c': ['2', '3'], 'a': ['0', '4']}

Rather than iterating over the list itself using the set will give a performance improvement especially if there are many duplicates.

In your expected output you wanted a list of str as the value. If you need int, you can use i instead of str(i)

Answer 5

l=['a','b','c','c','a','d']

result = {}

for element in l:
    if element not in result:
         indexes = [i for i, x in enumerate(l) if x == element]

         if len(indexes) > 1:
              result[element] = indexes

print(result)

Iterate through list and check if element already exista in dictionary. If it doesn't then get all the indexes for that element and append the element in dictionary.