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"]
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"]]
for group in groups:
  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]
for i in big_number_list:
  if i < 0:
    continue
  print(i)

Pass Keyword

The pass keyword is used as a placeholder statement to allow empty loops, functions or classes to be included in an executable code block without throwing an error. This is common when structuring future implementations.

for i in range(3):
  for j in range(3):
    if i == j:
      pass
    else:
      print(f"i: {i}, j:{j}")

Video Walkthrough

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