Python | Merge two lists alternatively

Last Updated : 09 May, 2019

Given two lists, write a Python program to merge the given lists in an alternative fashion, provided that the two lists are of equal length.

Examples:

Input : lst1 = [1, 2, 3]
        lst2 = ['a', 'b', 'c']
Output : [1, 'a', 2, 'b', 3, 'c']

Input : lst1 = ['name', 'alice', 'bob']
        lst2 = ['marks', 87, 56]
Output : ['name', 'marks', 'alice', 87, 'bob', 56]

Method #1 : List comprehension

# Python3 program to merge two lists  
# alternatively 
  
def countList(lst1, lst2): 
    return [sub[item] for item in range(len(lst2)) 
                      for sub in [lst1, lst2]] 
      
# Driver code 
lst1 = [1, 2, 3] 
lst2 = ['a', 'b', 'c'] 
print(countList(lst1, lst2)) 

Output:

[1, 'a', 2, 'b', 3, 'c']

There is an alternative to use list comprehension with zip() as given below –

def countList(lst1, lst2): 
    return [item for pair in zip(lst1, lst2 + [0]) 
                                 for item in pair] 

Method #2 : Using itertools.cycle()

# Python3 program to merge two lists  
# alternatively 
from itertools import cycle 
  
def countList(lst1, lst2): 
    iters = [iter(lst1), iter(lst2)] 
    return list(iter.__next__() for iter in cycle(iters)) 
      
# Driver code 
lst1 = [1, 2, 3] 
lst2 = ['a', 'b', 'c'] 
print(countList(lst1, lst2)) 

Output:

[1, 'a', 2, 'b', 3, 'c']

Method #3 : Using reduce()

# Python3 program to merge two lists  
# alternatively 
import operator 
from functools import reduce
  
def countList(lst1, lst2): 
    return reduce(operator.add, zip(lst1, lst2)) 
      
# Driver code 
lst1 = [1, 2, 3] 
lst2 = ['a', 'b', 'c'] 
print(countList(lst1, lst2)) 

Output:

(1, 'a', 2, 'b', 3, 'c')

Method #4 : Using numpy module

# Python3 program to merge two lists  
# alternatively 
import numpy as np 
  
def countList(lst1, lst2): 
    return np.array([[i, j] for i, j in zip(lst1, lst2)]).ravel() 
      
# Driver code 
lst1 = [1, 2, 3] 
lst2 = ['a', 'b', 'c'] 
print(countList(lst1, lst2)) 

Output:

['1' 'a' '2' 'b' '3' 'c']

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