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List:
A list in Python is a collection of zero or more elements. A list element can be any type of data. It can be a string, a number, or a mixture of the two. In Python, a list is equivalent to an array in C or Java. Lists are mutable, which means you may change their content, and they contain a variety of helpful specialized methods.
When working with Python lists, we may encounter an issue in which we must iterate over two list entries. Iterating one after the other is an option, but it is more time consuming, and a one-two liner is always preferred. Let’s go over several different approaches to doing this assignment.
Given two lists, the task is to iterate two lists simultaneously.
Examples:
Printing list one after other:
Example1:
Input:
samplelistone = ["hello", "this", "is", "BTechGeeks", "Python"] samplelisttwo = [38, 23, 10, 20]
Output:
printing the samplelistone hello this is BTechGeeks Python printing the samplelisttwo 38 23 10 20 printing the two lists elements simultaneously hello this is BTechGeeks Python 38 23 10 20
Example2:
Input:
samplelistone = [27823, 9792, "hello", "this", True, 7.6] samplelisttwo = ['sky', 'is', 'blue', 122, 5.3214, False, 'hello']
Output:
printing the samplelistone 27823 9792 hello this True 7.6 printing the samplelisttwo sky is blue 122 5.3214 False hello printing the two lists elements simultaneously 27823 9792 hello this True 7.6 sky is blue 122 5.3214 False hello
Printing list in parallel:
Example1:
Input:
samplelistone = ["hello", "this", "is", "BTechGeeks", "Python"] samplelisttwo = [38, 23, 10, 20, 31]
Output:
printing the samplelistone hello this is BTechGeeks Python printing the samplelisttwo 38 23 10 20 31 printing the two lists elements simultaneously hello 38 this 23 is 10 BTechGeeks 20 Python 31
Example2:
Input:
samplelistone = [27823, 9792, "hello", "this", True, 7.6] samplelisttwo = ['sky', 'is', 'blue', 122, 5.3214, False, 'hello']
Output:
printing the samplelistone 27823 9792 hello this True 7.6 printing the samplelisttwo sky is blue 122 5.3214 False hello printing the two lists elements simultaneously 27823 sky 9792 is hello blue this 122 True 5.3214 7.6 False
Program to Iterate Through Two Lists in Parallel in Python
Below are the ways to iterate through two lists in parallel in python:
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Method #1:Using for loop and “+” operator
The combination of the above functionalities may make our job easier. The disadvantage here is that we may have to concatenate the list, which would require more RAM than desired.
Below is the implementation:
# initializing lists
samplelistone = ["hello", "this", "is", "BTechGeeks", "Python"]
samplelisttwo = [38, 23, 10, 20]
# printing the two original lists
print("printing the samplelistone ", *samplelistone)
print("printing the samplelisttwo ", *samplelisttwo)
# traversing the two lists simulatenously
print("printing the two lists elements simultaneously ")
for value in samplelistone + samplelisttwo:
print(value, end=" ")
Output:
printing the samplelistone hello this is BTechGeeks Python printing the samplelisttwo 38 23 10 20 printing the two lists elements simultaneously hello this is BTechGeeks Python 38 23 10 20
Method #2: Using chain() function
This approach is identical to the one before, but it is somewhat more memory efficient because chain() is used to execute the work and internally builds an iterator.
Chain function will be available in itertools.
Below is the implementation:
from itertools import chain
# initializing lists
samplelistone = [27823, 9792, "hello", "this", True, 7.6]
samplelisttwo = ['sky', 'is', 'blue', 122, 5.3214, False, 'hello']
# printing the two original lists
print("printing the samplelistone ", *samplelistone)
print("printing the samplelisttwo ", *samplelisttwo)
# traversing the two lists simulatenously
print("printing the two lists elements simultaneously ")
for value in chain(samplelistone, samplelisttwo):
print(value, end=" ")
Output:
printing the samplelistone 27823 9792 hello this True 7.6 printing the samplelisttwo sky is blue 122 5.3214 False hello printing the two lists elements simultaneously 27823 9792 hello this True 7.6 sky is blue 122 5.3214 False hello
Method #3:Using zip()
Zip in Python 3 returns an iterator. When any of the lists in the zip() function is exhausted, the function terminates. In other words, it continues until the smallest of all the lists is reached.
The zip method and itertools.izip are implemented below, and itertools.izip iterates over two lists:
Below is the implementation:
import itertools
# initializing lists
samplelistone = [27823, 9792, "hello", "this", True, 7.6]
samplelisttwo = ['sky', 'is', 'blue', 122, 5.3214, False, 'hello']
# printing the two original lists
print("printing the samplelistone ", *samplelistone)
print("printing the samplelisttwo ", *samplelisttwo)
# traversing the two lists simulatenously
print("printing the two lists elements simultaneously ")
for i, j in zip(samplelistone, samplelisttwo):
print(i, j)
Output:
printing the samplelistone 27823 9792 hello this True 7.6 printing the samplelisttwo sky is blue 122 5.3214 False hello printing the two lists elements simultaneously 27823 sky 9792 is hello blue this 122 True 5.3214 7.6 False
Method #4:Using itertools.zip_longest()
When all lists have been exhausted, zip longest comes to a halt. When the shorter iterator(s) have been exhausted, zip longest returns a tuple with the value None.
The itertools implementation is shown below.
zip longest iterates across two lists
Below is the implementation:
import itertools
# initializing lists
samplelistone = [27823, 9792, "hello", "this", True, 7.6]
samplelisttwo = ['sky', 'is', 'blue', 122, 5.3214, False, 'hello']
# printing the two original lists
print("printing the samplelistone ", *samplelistone)
print("printing the samplelisttwo ", *samplelisttwo)
# traversing the two lists simulatenously
print("printing the two lists elements simultaneously ")
for i, j in itertools.zip_longest(samplelistone, samplelisttwo):
print(i, j)
Output:
printing the samplelistone 27823 9792 hello this True 7.6 printing the samplelisttwo sky is blue 122 5.3214 False hello printing the two lists elements simultaneously 27823 sky 9792 is hello blue this 122 True 5.3214 7.6 False None hello
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