CODEWITHSUNDEEP
pythonlistindexing
Abhishek Kulkarni
Abhishek Kulkarni

Reputation: 676

Map elements of a list to their index in another list

I am comparing two lists in Python.

list1 is a superset of list2.

For the elements of list1, I want their index in list2 (if present).

Here are two examples.

list1 = ['a','b','c','d']
list2 = ['a','b']

The solution should produce [0, 1].

list1 = ['a','b','c','d']
list2 = ['b','a']

The solution should produce [1, 0].

I attempted the following code, but it only works for the first example.

list1 = ['a','b','c','d']
list2 = ['a','b']

pairwise = zip(list1,list2)
matched_index = [idx for idx, pair in enumerate(pairwise) if pair[0] == pair[1]]

This works. However, for the second set of sample data I get the wrong output [] instead of the expected output [1, 0].

list1 = ['a','b','c','d']
list2 = ['b','a']

pairwise = zip (list1,list2)
matched_index = [idx for idx, pair in enumerate(pairwise) if pair[0] == pair[1]]
print(matched_index) # prints []

Please suggest the way forward.

Upvotes: 3

Views: 3835

Answers (3)

shantanoo
shantanoo

Reputation: 3704

Assuming there are unique elements in each list and len(list1) >= len(list2)

>>> list1 = ['a','b','c','d']                                                                                            
>>> list2 = ['d','a', 'f']
>>> print([list2.index(x) for x in list1 if x in list2])

Upvotes: 0

jpp
jpp

Reputation: 164673

Since list2 is a subset of list1, you can construct a dictionary mapping and then use dict.__getitem__ on values of list2 to extract indices:

list1 = ['a','b','c','d']
list2 = ['a','b']
list3 = ['b','a']

d = {v: k for k, v in enumerate(list1)}

res1 = list(map(d.__getitem__, list2))  # [0, 1]
res2 = list(map(d.__getitem__, list3))  # [1, 0]

Upvotes: 2

timgeb
timgeb

Reputation: 78690

I suggest using a dictionary mapping the elements of list2 to their index - assuming list2 has unique elements.

>>> list1 = ['a','b','c','d']                                                                                            
>>> list2 = ['b','a']
>>> idx = {x:i for i,x in enumerate(list2)}                                                                            
>>> idx                                                                                                                
{'a': 1, 'b': 0}

Now you can issue

>>> [idx[x] for x in list1 if x in idx]                                                                                
[1, 0]

Upvotes: 3

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