Key Concepts
Review core concepts you need to learn to master this subject
Python for Loops
Python Loops with range().
The Python break Keyword
The Python continue Keyword
Python While Loops
Python Nested Loops
Python List Comprehension
Python for Loops
Python for Loops
Python for loops can be used to iterate over and perform an action one time for each element in a list.
Proper for loop syntax assigns a temporary value, the current item of the list, to a variable on each successive iteration: for <temporary value> in <a list>:
for loop bodies must be indented to avoid an IndentationError.
Python Loops with range().
Python Loops with range().
In Python, a for loop can be used to perform an action a specific number of times in a row.
The range() function can be used to create a list that can be used to specify the number of iterations in a for loop.
The Python break Keyword
The Python break Keyword
In a Python loop, the break keyword escapes the loop, regardless of the iteration number. Once break executes, the program will continue to execute after the loop.
In this example, the output would be:
- 0
- 254
- 2
- Negative number detected!
- Finished looping
The Python continue Keyword
The Python continue Keyword
In Python, the continue keyword is used inside a loop to skip the remaining code inside the loop code block and begin the next loop iteration.
Python While Loops
Python While Loops
In Python, a while loop will repeatedly execute a code block as long as a condition evaluates to True.
The condition of a while loop is always checked first before the block of code runs. If the condition is not met initially, then the code block will never run.
Python Nested Loops
Python Nested Loops
In Python, loops can be nested inside other loops. Nested loops can be used to access items of lists which are inside other lists. The item selected from the outer loop can be used as the list for the inner loop to iterate over.
Python List Comprehension
Python List Comprehension
Python list comprehensions provide a concise way for creating lists. It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses: [EXPRESSION for ITEM in LIST <if CONDITIONAL>].
The expressions can be anything - any kind of object can go into a list.
A list comprehension always returns a list.
- 1Suppose we want to print() each item from a list of dog_breeds. We would need to use the following code snippet: dog_breeds = [‘french_bulldog’, ‘dalmatian’, ‘shihtzu’, ‘poodle’, ‘collie’] print(…
- 2In the previous exercise, we saw that we can print each item in a list using a for loop. A for loop lets us perform an action on each item in a list. Using each element of a list is known as _i…
- 3Previously, we iterated through an existing list. Often we won’t be iterating through a specific list, we’ll just want to do a certain action multiple times. For example, if we wanted to print out…
- 4We’ve iterated through lists that have a discrete beginning and end. However, let’s consider this example: my_favorite_numbers = [4, 8, 15, 16, 42] for number in my_favorite_numbers: my_favorit…
- 5We often want to use a for loop to search through a list for some value: items_on_sale = [“blue_shirt”, “striped_socks”, “knit_dress”, “red_headband”, “dinosaur_onesie”] # we want to check if the…
- 6When we’re iterating through lists, we may want to skip some values. Let’s say we want to print out all of the numbers in a list, unless they’re negative. We can use continue to move to the next i …
- 7We now have seen and used a lot of examples of for loops. There is another type of loop we can also use, called a while loop. The while loop performs a set of code until some condition is reach…
- 8We have seen how we can go through the elements of a list. What if we have a list made up of multiple lists? How can we loop through all of the individual elements? Suppose we are in charge of a …
- 9Let’s say we have scraped a certain website and gotten these words: words = [“@coolguy35”, “#nofilter”, “@kewldawg54”, “reply”, “timestamp”, “@matchamom”, “follow”, “#updog”] We want to make a…
- 10Let’s say we’re working with the usernames list from the last exercise: >>> print(usernames) [“@coolguy35”, “@kewldawg54”, “@matchamom”] We want to create a new list with the string “ please fol…
- 11Good job! In this lesson, you learned - how to write a for loop - how to use range in a loop - what infinite loops are and how to avoid them - how to skip values in a loop - how to write a while l…
- 1This lesson will help you review Python functions by providing some challenge exercises involving loops. As a refresher, function syntax looks like this: def some_function(some_input1, some_input…
- 2divisible_by_ten(nums)
- 3add_greetings(names)
- 4delete_starting_evens(lst)
- 5odd_indices(lst)
- 6exponents(bases, powers)
- 7larger_sum(lst1, lst2)
- 8over_nine_thousand(lst)
- 9max_num(nums)
- 10same_values(lst1, lst2)
- 11reversed_list(lst1, lst2)
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