The List is equivalent to arrays in other languages, with the extra benefit of being dynamic in size.
In Python, the list is a type of container in Data Structures, which is used to store multiple data at the same time. Unlike Sets, lists in Python are ordered and have a definite count.
Ways to Iterate over a List in Python
Let’s see all the different ways to iterate over a list in Python and the performance comparison between them.
- Using for loop
- Using for loop and range()
- Using a while loop
- Using list comprehension
- Using enumerate() method
- Using NumPy module
- Using the iter function and the next function
- Using the map() function
Iterate over a list using For loop
We can iterate over a list in Python by using a simple For loop.
Python3
# Python3 code to iterate over a listlist = [1, 3, 5, 7, 9] # Using for loopfor i in list: print(i) |
Output:
1
3
5
7
9
Time complexity: O(n) – where n is the number of elements in the list.
Auxiliary space: O(1) – as we are not using any additional space.
Iterate through a list using for loop and range()
In case we want to use the traditional for loop which iterates from number x to number y.
Python3
# Python3 code to iterate over a listlist = [1, 3, 5, 7, 9] # getting length of listlength = len(list) # Iterating the index# same as 'for i in range(len(list))'for i in range(length): print(list[i]) |
Output:
1
3
5
7
9
Time complexity: O(n), where n is the length of the input list.
Auxiliary space: O(1), which is constant space
Iterate through a List in Python using a while loop
We can also iterate over a Python list using a while loop.
Python3
# Python3 code to iterate over a listlist = [1, 3, 5, 7, 9] # Getting length of listi = 0 # Iterating using while loopwhile i < len(list): print(list[i]) i += 1 |
Output:
1
3
5
7
9
Time complexity: O(n) where n is the length of the list.
Auxiliary space: O(1) as only a constant amount of extra space is used for variables i and length.
Iterate through a list using list comprehension
We can use list comprehension(possibly the most concrete way) to iterate over a list in Python.
Python3
# Python3 code to iterate over a listlist = [1, 3, 5, 7, 9] # Using list comprehension[print(i) for i in list] |
Output:
1
3
5
7
9
Iterate through a List in Python using enumerate()
If we want to convert the list into an iterable list of tuples (or get the index based on a condition check, for example in linear search, you might need to save the index of minimum element), you can use the enumerate() function.
Python3
# Python3 code to iterate over a listlist = [1, 3, 5, 7, 9] # Using enumerate()for i, val in enumerate(list): print (i, ",",val) |
Output:
0 , 1
1 , 3
2 , 5
3 , 7
4 , 9
Note: Even method 2 can be used to find the index, but method 1 can’t (Unless an extra variable is incremented every iteration) and method 5 gives a concise representation of this indexing.
Iterate over a list in Python Using NumPy
For very large n-dimensional lists (for example an image array), it is sometimes better to use an external library such as numpy.
Python3
import numpy as geek # creating an array using # arrange methoda = geek.arange(9) # shape array with 3 rows # and 4 columnsa = a.reshape(3, 3) # iterating an arrayfor x in geek.nditer(a): print(x) |
Output:
0
1
2
3
4
5
6
7
8
We can use np. enumerate() to mimic the behavior of enumerating. The extra power of NumPy comes from the fact that we can even control the way to visit the elements (Fortran order rather than C order, say :)) but the one caveat is that the np.nditer treats the array as read-only by default, so one must pass extra flags such as op_flags=[‘readwrite’] for it to be able to modify elements.
Iterate through a List in Python using the iter function and the next function
Here is an additional approach using the iter function and the next function:
Python3
# Python3 code to iterate over a listlist = [1, 3, 5, 7, 9]# Create an iterator object using the iter functioniterator = iter(list)# Use the next function to retrieve the elements of the iteratortry: while True: element = next(iterator) print(element)except StopIteration: pass |
1 3 5 7 9
Time complexity: O(n), where n is the length of the list because it iterates through the elements of the list once and performs a constant number of operations on each element.
Auxiliary space: O(1), because it does not create any additional data structures to store intermediate results or the final result.
Iterate over a list in Python using the map() function
Use the map() function to apply a function to each element of a list.
Python3
# Define a function to print each elementdef print_element(element): print(element)# Create a listmy_list = [1, 3, 5, 7, 9]# Use map() to apply the print_element() function to each element of the listresult = map(print_element, my_list)# Since map() returns an iterator, we need to consume# the iterator in order to see the outputfor _ in result: pass |
1 3 5 7 9
Time complexity: O(n), where n is the length of the list.
Auxiliary space: O(1)
