Getting index of item while processing a list using map in python

Question

While processing a list using map(), I want to access index of the item while inside lambda. How can I do that?

For example

ranked_users = ['jon','bob','jane','alice','chris']
user_details = map(lambda x: {'name':x, 'rank':?}, ranked_users)

How can I get rank of each user in above example?

Asked By: Jayesh

||

Source

Answer 1

Use enumerate:

In [3]: user_details = [{'name':x, 'rank':i} for i,x in enumerate(ranked_users)] 

In [4]: user_details
Out[4]: 
[{'name': 'jon', 'rank': 0},
 {'name': 'bob', 'rank': 1},
 {'name': 'jane', 'rank': 2},
 {'name': 'alice', 'rank': 3},
 {'name': 'chris', 'rank': 4}]

PS. My first answer was

user_details = map(lambda (i,x): {'name':x, 'rank':i}, enumerate(ranked_users))

I’d strongly recommend using a list comprehension or generator expression over map and lambda whenever possible. List comprehensions are more readable, and tend to be faster to boot.

Answered By: unutbu

Answer 2

Alternatively you could use a list comprehension rather than map() and lambda.

ranked_users = ['jon','bob','jane','alice','chris']
user_details = [{'name' : x, 'rank' : ranked_users.index(x)} for x in ranked_users]

Output:

[{'name': 'jon', 'rank': 0}, {'name': 'bob', 'rank': 1}, {'name': 'jane', 'rank': 2}, {'name': 'alice', 'rank': 3}, {'name': 'chris', 'rank': 4}]

List comprehensions are very powerful and are also faster than a combination of map and lambda.

Answered By: Prydie

Answer 3

In my opinion the question was about map function and preferred answer is partly correct due to syntax error caused by putting tuple argument to lambda lambda (i,x)

idea of enumerate is nice and proper solution would be:

map(lambda x: {'name':x[1], 'rank':x[0]}, enumerate(ranked_users))

and some timing to compare speed with comprehension:

def with_map():
    ranked_users = range(10 ** 6)
    list(map(lambda x: {'name': x[1], 'rank': x[0]}, enumerate(ranked_users)))

def by_comprehension():
    ranked_users = range(10 ** 6)
    [{'name': x, 'rank': i} for i, x in enumerate(ranked_users)]

from timeit import timeit
time_with_map = timeit(with_map, number=10)
time_with_comprehension = timeit(by_comprehension, number=10)

print('list comprehension is about %.2f x faster than map in this test case' % (time_with_map/time_with_comprehension))

test result: list comprehension is about 1.31 x faster than map in this test case

Answered By: cheshire cat

Answer 4

Actually here is a more elegant, verbose solution than using an enumerate tuple in the map (because of tuple indexing). Map can take more iterables as arguments so let’s use it.

map(lambda user, user_id: (user_id, user), ranked_users, range(ranked_users.__len__()))

Answered By: xbalaj

Getting index of item while processing a list using map in python

Question:

Answers: