An introduction to the use of Iteration in Python
Loops can be nested in Python, as they can with other programming languages.
A nested loop is a loop that occurs within another loop, structurally similar to nested if statements. These are constructed like so:
for [first iterating variable] in [outer loop]: # Outer loop [do something] # Optional for [second iterating variable] in [nested loop]: # Nested loop [do something]
The program first encounters the outer loop, executing its first iteration. This first iteration triggers the inner, nested loop, which then runs to completion. Then the program returns back to the top of the outer loop, completing the second iteration and again triggering the nested loop. Again, the nested loop runs to completion, and the program returns back to the top of the outer loop until the sequence is complete or a break or other statement disrupts the process.
Let’s implement a nested for loop so we can take a closer look. In this example, the outer loop will iterate through a list of integers called num_list, and the inner loop will iterate through a list of strings called alpha_list.
num_list = [1, 2, 3] alpha_list = ['a', 'b', 'c'] for number in num_list: print(number) for letter in alpha_list: print(letter)
When we run this program, we’ll receive the following output:
1 a b c 2 a b c 3 a b c
The output illustrates that the program completes the first iteration of the outer loop by printing 1, which then triggers completion of the inner loop, printing a, b, c consecutively. Once the inner loop has completed, the program returns to the top of the outer loop, prints 2, then again prints the inner loop in its entirety (a, b, c), etc.
Nested for loops can be useful for iterating through items within lists composed of lists. In a list composed of lists, if we employ just one for loop, the program will output each internal list as an item:
list_of_lists = [['hammerhead', 'great white', 'dogfish'],[0, 1, 2],[9.9, 8.8, 7.7]] for list in list_of_lists: print(list)
['hammerhead', 'great white', 'dogfish'] [0, 1, 2] [9.9, 8.8, 7.7]
In order to access each individual item of the internal lists, we’ll implement a nested for loop:
list_of_lists = [['hammerhead', 'great white', 'dogfish'],[0, 1, 2],[9.9, 8.8, 7.7]] for list in list_of_lists: for item in list: print(item)
hammerhead great white dogfish 0 1 2 9.9 8.8 7.7
When we utilize a nested for loop we are able to iterate over the individual items contained in the lists.
Anatidae paleo gastropro authentic flexitarian fixie tote bag, scenester Chordate sweater irony locavore. Gentrify Aves Ayelsbury craft beer aesthetic Echo Park. Polaroid Marfa cray dreamcatcher, chambray ennui hella Godard mallards fap Terry Richardson XOXO. Sartorial vinyl passion, fap kogi Austin meh Terry Richardson art party hella. Master cleanse Eider trust fund, grummage craft beer biodiesel Wes Anderson hashtag Intelligentsia hoodie. Ethnic DIY 8-bit, paddles ennui narwhal Tumblr platyrhynchos Helvetica asymmetrical fap twee viral 90's aesthetic. Blue Bottle kale chips hashtag, jean shorts disrupt semiotics wayfarers pop-up aesthetic retro kogi Tumblr put a bird up it.
Write a program that uses 1 level of nesting.
Write a program that uses 2 levels of nesting.
Write a program that uses 3 levels of nesting.
Thanks [name] from [whichclass][location], Mr McG will get your message…"
[message]"…and will get back to you ASAP via [email]. Catch you soon.
Have a great day!!
K McGuinness - 2018
No personal data will be shared with any third party. The only data processed via this website is anonymous browsing data to best manage your user experience.