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Python | Find elements of a list by indices

  • Last Updated : 18 Apr, 2019

Given two lists with elements and indices, write a Python program to find elements of list 1 at indices present in list 2.

Examples:

Input : lst1 = [10, 20, 30, 40, 50]
        lst2 = [0, 2, 4]
Output : [10, 30, 50]
Explanation: 
Output elements at indices 0, 2 and 4 i.e 10, 30 and 50 respectively. 

Input : lst1 = ['Hello', 'geeks', 'for', 'geeks']
        lst2 = [1, 2, 3]
Output : ['geeks', 'for', 'geeks']

 
Below are some Pythonic approaches to do the above task.

Approach #1 : Naive(List comprehension)

The first approach to find the desired elements is to use list comprehension. We traverse through ‘lst2’ and for each ith element, we output lst1[i].






# Python3 program to Find elements of a 
# list by indices present in another list
  
def findElements(lst1, lst2):
    return [lst1[i] for i in lst2]
              
# Driver code
lst1 = [10, 20, 30, 40, 50]
lst2 = [0, 2, 4]
print(findElements(lst1, lst2))
Output:
[10, 30, 50]

 
Approach #2 : Using Python map()

We can also use Python map() method where we apply lst1.__getitem__ function on lst2 which return lst1[i] for each element ‘i’ of lst2.




# Python3 program to Find elements of a 
# list by indices present in another list
  
def findElements(lst1, lst2):
    return list(map(lst1.__getitem__, lst2))
              
# Driver code
lst1 = [10, 20, 30, 40, 50]
lst2 = [0, 2, 4]
print(findElements(lst1, lst2))
Output:
[10, 30, 50]

 
Approach #3 : Using itemgetter()




# Python3 program to Find elements of a 
# list by indices present in another list
from operator import itemgetter 
  
def findElements(lst1, lst2):
    return list((itemgetter(*lst2)(lst1)))
              
# Driver code
lst1 = [10, 20, 30, 40, 50]
lst2 = [0, 2, 4]
print(findElements(lst1, lst2))
Output:
[10, 30, 50]

 
Approach #4 : Using numpy




# Python3 program to Find elements of a 
# list by indices present in another list
import numpy as np 
  
def findElements(lst1, lst2):
    return list(np.array(lst1)[lst2])
              
# Driver code
lst1 = [10, 20, 30, 40, 50]
lst2 = [0, 2, 4]
print(findElements(lst1, lst2))
Output:
[10, 30, 50]




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