Loops

Loops are structures that let you repeat Python code over and over. Learn how to read loops and write them to solve your own problems.

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Key Concepts

Review core concepts you need to learn to master this subject

break Keyword

Python List Comprehension

Python For Loop

The Python continue Keyword

Python for Loops

Python Loops with range().

Infinite Loop

Python while Loops

`break` Keyword

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

In a 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!

Python List Comprehension

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

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.

Python For Loop

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

A Python for loop can be used to iterate over a list of items and perform a set of actions on each item. The syntax of a for loop consists of assigning a temporary value to a variable on each successive iteration.

When writing a for loop, remember to properly indent each action, otherwise an IndentationError will result.

The Python `continue` Keyword

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

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 `for` Loops

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

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()`.

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

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.

Infinite Loop

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

An infinite loop is a loop that never terminates. Infinite loops result when the conditions of the loop prevent it from terminating. This could be due to a typo in the conditional statement within the loop or incorrect logic. To interrupt a Python program that is running forever, press the Ctrl and C keys together on your keyboard.

Python `while` Loops

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

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

numbers = [0, 254, 2, -1, 3]

for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

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.

Loops

Lesson 1 of 2

1
Introduction
Suppose 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(…
Start
2
Create a For Loop
In 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…
Start
3
Using Range in Loops
Previously, 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…
Start
4
Infinite Loops
We’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…
Start
5
Break
We 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…
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6
Continue
When 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 …
Start
7
While Loops
We 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…
Start
8
Nested Loops
We 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 …
Start
9
List Comprehensions
Let’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…
Start
10
More List Comprehensions
Let’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…
Start
11
Review
Good 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…
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1
Introduction
This 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…
Start
2
Divisible by Ten
divisible_by_ten(nums)
Start
3
Greetings
add_greetings(names)
Start
4
Delete Starting Even Numbers
delete_starting_evens(lst)
Start
5
Odd Indices
odd_indices(lst)
Start
6
Exponents
exponents(bases, powers)
Start
7
Larger Sum
larger_sum(lst1, lst2)
Start
8
Over 9000
over_nine_thousand(lst)
Start
9
Max Num
max_num(nums)
Start
10
Same Values
same_values(lst1, lst2)
Start
11
Reversed List
reversed_list(lst1, lst2)
Start

What you'll create

Portfolio projects that showcase your new skills

Carly's Clippers
Use your newfound knowledge of loops to discover trends in the operation of the newest hair salon in town, Carly's Clippers.

How you'll master it

Stress-test your knowledge with quizzes that help commit syntax to memory

Loops in Python
What would be the output of the following code:

Loops

Start

Loops

Key Concepts

`break` Keyword

Python List Comprehension

Python For Loop

The Python `continue` Keyword

Python `for` Loops

Python Loops with `range()`.

Infinite Loop

Python `while` Loops

Python Nested Loops

What you'll create

Carly's Clippers

How you'll master it

Loops in Python

Loops

Subjects

Languages

break Keyword

Python List Comprehension

Python For Loop

The Python continue Keyword

Python for Loops

Python Loops with range().

Infinite Loop

Python while Loops

Python Nested Loops

Carly's Clippers

Loops in Python

`break` Keyword

The Python `continue` Keyword

Python `for` Loops

Python Loops with `range()`.

Python `while` Loops