In Python, how do I index a list with another list?

Question

I would like to index a list with another list like this

L = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
Idx = [0, 3, 7]
T = L[ Idx ]

and T should end up being a list containing [‘a’, ‘d’, ‘h’].

Is there a better way than

T = []
for i in Idx:
    T.append(L[i])

print T
# Gives result ['a', 'd', 'h']

Asked By: Daniel Andrén

||

Source

Answer 1

T = [L[i] for i in Idx]

Answered By: van

Answer 2

T = map(lambda i: L[i], Idx)

Answered By: mmx

Answer 3

If you are using numpy, you can perform extended slicing like that:

>>> import numpy
>>> a=numpy.array(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'])
>>> Idx = [0, 3, 7]
>>> a[Idx]
array(['a', 'd', 'h'], 
      dtype='|S1')

…and is probably much faster (if performance is enough of a concern to to bother with the numpy import)

Answered By: Paul

Answer 4

I wasn’t happy with any of these approaches, so I came up with a Flexlist class that allows for flexible indexing, either by integer, slice or index-list:

class Flexlist(list):
    def __getitem__(self, keys):
        if isinstance(keys, (int, slice)): return list.__getitem__(self, keys)
        return [self[k] for k in keys]

Which, for your example, you would use as:

L = Flexlist(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'])
Idx = [0, 3, 7]
T = L[ Idx ]

print(T)  # ['a', 'd', 'h']

Answered By: jedwards

Answer 5

L= {'a':'a','d':'d', 'h':'h'}
index= ['a','d','h'] 
for keys in index:
    print(L[keys])

I would use a Dict add desired keys to index

Answered By: user4749532

Answer 6

A functional approach:

a = [1,"A", 34, -123, "Hello", 12]
b = [0, 2, 5]

from operator import itemgetter

print(list(itemgetter(*b)(a)))
[1, 34, 12]

Answered By: Padraic Cunningham

Answer 7

You could also use the __getitem__ method combined with map like the following:

L = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
Idx = [0, 3, 7]
res = list(map(L.__getitem__, Idx))
print(res)
# ['a', 'd', 'h']

Answered By: David

Answer 8

My problem: Find indexes of list.

L = makelist() # Returns a list of different objects
La = np.array(L, dtype = object) # add dtype!
for c in chunks:
    L_ = La[c] # Since La is array, this works.

Answered By: Hunaphu

In Python, how do I index a list with another list?

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