Loops

A loop is a control structure that can execute a statement or group of statements repeatedly. Python has three types of loops: while loops, for loops, and nested loops.

While Loops

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.

while condition:
  # Code inside

This loop will only run 1 time:

hungry = True

while hungry:
  print("Time to eat!")
  hungry = False

This loop will run 5 times:

i = 1

while i < 6:
  print(i)
  i = i + 1

For Loop

A for loop 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.

dog_breeds = ["boxer", "bulldog", "shiba inu"]

# Print each breed:
for breed in dog_breeds:
  print(breed)

Nested Loops

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.

groups = [["Jobs", "Gates"], ["Newton", "Euclid"], ["Einstein", "Feynman"]]

# This outer loop will iterate over each list in the groups list
for group in groups:
  # This inner loop will go through each name in each list
  for name in group:
    print(name)

Break Keyword

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.

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

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

In this example, the output would be:

0
254
2
Negative number detected!

Continue Keyword

The continue keyword is used inside a loop to skip the remaining code inside the loop code block and begin the next loop iteration.

big_number_list = [1, 2, -1, 4, -5, 5, 2, -9]

# Print only positive numbers:
for i in big_number_list:
  if i < 0:
    continue
  print(i)

Video Walkthrough

In this video, you will learn how to use the for and while loops in a Python script.